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Egg retrieval is an ancient behaviour displayed by birds in which a displaced egg is transferred back into the nest. Egg rejection, in contrast, is a recently evolved adaptation by hosts of brood parasites in which an egg is removed from a nest. These conflicting behaviours are both adaptive because they prevent reproductive losses, but egg retrieval may benefit brood parasites if hosts retrieve parasitic eggs into their nests. We examined these conflicting cognitive processes in the prothonotary warbler (Protonotaria citrea), a cavity‐nesting host of the brown‐headed cowbird (Molothrus ater). Eggs and nestlings are occasionally found on the rims of warbler nest cavities. We determined how this happened using video cameras and tested whether warblers retrieved their own eggs more often than cowbird eggs. We also determined if shape affects retrieval and rejection by placing cylinders inside nest cups or onto nest rims. We found that eggs were displaced by rapidly departing warblers whose nests are constantly prospected by nest competitors and brood parasites. Warblers retrieved their eggs (29%) and cowbird eggs (19%) at similar rates, and never rejected cowbird eggs after retrieval. Warblers removed most cylinders from nest rims and cups (89% and 78%, respectively), and never retrieved them. Warblers have retained egg retrieval at a low level and its expression is costly because they only retrieve a low level of their own eggs and also retrieve cowbird eggs, which benefits the brood parasite. [ABSTRACT FROM AUTHOR]

Most animals live and move in a landscape in which the risk of predation varies spatially and temporally, that is, a “landscape of fear” which may strongly affect their ecology and behaviour. Cavity‐nesting birds prospecting for nest sites are often forced to investigate unfamiliar, dark holes that may be dangerous to enter because they may conceal an aggressive nesting bird or a predator. The Fear of Feathers Hypothesis posits that some birds such as blue tits Cyanistes caeruleus, add large, conspicuous feathers on top of their own nest to exploit the fear of potential nest usurpers or brood parasites. Feathers may trigger fear in prospecting birds either because the feathers obstruct the view of the cavity interior, and/or because the feathers may be perceived as recent prey remains and suggest the predator will return. Here I studied the prospecting behaviour of unmated male pied flycatchers Ficedula hypoleuca by letting them choose between a dyad of nest boxes where one contained a nest of a great tit Parus major and one a nest of a blue tit. The objective was to test whether the presence of feathers would have a stronger repelling effect than the species that had built the nest. Great tits are larger than blue tits and may represent a greater threat to intruding flycatchers. However, blue tits but not great tits, often decorate their nest with large, conspicuous feathers. Consistent with the Fear of Feathers Hypothesis, flycatchers hesitated longer to enter blue tit than great tit nests but only if the blue tit nest contained feathers. The study has relevance to many species of birds that decorate their nest with feathers. It shows that a landscape may include aspects of fear that are important to animal behaviour although they may seem subtle and are easily overlooked. [ABSTRACT FROM AUTHOR]

Avian nest design varies depending on environmental factors but may also be influenced by between‐species interactions. In the brood parasitism context, hosts may evolve nest architectures that may limit parasite access to the nest cup, reduce parasite laying success or hinder parasite chick success. Therefore, nest characteristics may reduce the likelihood or minimise the costs of being parasitised. The common redstart Phoenicurus phoenicurus is a regular host of the common cuckoo Cuculus canorus, for which cuckoo eggs are often laid outside the nest cup, resulting in low effective parasitism rates. This allowed us to evaluate variation in host nest design and test whether nest design characteristics correlate with brood parasitism likelihood and cuckoo laying success (i.e. cuckoo egg laid in the nest cup versus outside the nest cup). While recording brood parasitism events in two distant redstart populations, we documented nest cup characteristics, such as internal dimensions, materials used and nest cup position, along with the nest‐box dimensions. Cuckoo parasitism likelihood was lower for redstart nests in cavities with smaller entrances, for redstart nests with smaller nest cups and with nest cups that were built level to the rim material. For parasitised nests, cuckoo laying success was lower at redstart nests with nest cups placed further from the cavity entrance. Our results suggest a conditional process, where the cavity entrance size first prevents brood parasites access, then the cup size and the cup level in reference to the rim material affect the cuckoo choice, and finally, the nest cup position hinders cuckoo's laying success. The use of multiple nest design strategies may explain the current low effective parasitism rates in this system. Host nest design may serve as a frontline defence that could shape parasite's preferences, and consequently host nest characteristics. [ABSTRACT FROM AUTHOR]

Urbanization reflects a major form of environmental change impacting wild birds globally. Whereas urban habitats may provide increased availability of water, some food items, and reduced predation levels compared to rural, they can also present novel stressors including increased light at night, ambient noise, and reduced nutrient availability. Urbanization can also alter levels of brood parasitism, with some host species experiencing elevated levels of brood parasitism in urban areas compared to rural areas. Though the demographic and behavioral consequences of urbanization and brood parasitism have received considerable attention, their consequences for cellular‐level processes are less understood. Telomeres provide an opportunity to understand the cellular consequences of different environments as they are a well‐established metric of biological state that can be associated with residual lifespan, disease risk, and behavior, and are known to be sensitive to environmental conditions. Here we examine the relationships between urbanization, brood parasitism, and blood telomere lengths in adult and nestling song sparrows Melospiza melodia. Song sparrows are a North American songbird found in both urban and rural habitats that experience high rates of brood parasitism by brown‐headed cowbirds Molothrus ater in the urban, but not the rural, sites in our study system. Among adults and nestlings from non‐parasitized nests, we found no differences in relative telomere lengths between urban and rural habitats. However, among urban nestlings, the presence of a brood parasite in the nest was associated with significantly shorter relative telomere lengths compared to when a brood parasite was absent. Our results suggest a novel, indirect, impact of urbanization on nestling songbirds through the physiological impacts of brood parasitism. [ABSTRACT FROM AUTHOR]

Conspecific and interspecific brood parasitism are alternate reproductive strategies more pervasive in waterfowl than in any other group of birds. While previous research has measured costs incurred by nest hosts incubating parasitized clutches, few studies have focused on the relative success of parasites. Here, we evaluated the success of wood duck (Aix sponsa) and hooded merganser (Lophodytes cucullatus) eggs laid parasitically in Louisiana and Mississippi. We monitored nest boxes, assigned eggs in each nest as host or parasitic, and determined the number of eggs that hatched and failed. Across all study areas (1994–1999 and 2020–2023), we monitored 1750 wood duck and 377 hooded merganser nests; ~13% of wood duck and ~24% of hooded merganser nests were interspecifically parasitized. We modeled egg survival of 2925 host and 691 parasitic eggs from 197 successful nests (≥1 hatched egg, regardless of species). Wood duck eggs laid in hooded merganser nests had lower survival [0.293, CI = 95% credible intervals (after, CI) = 0.176, 0.439] than hooded merganser eggs (0.762, CI = 0.704, 0.810) laid in wood duck nests. Clutch size negatively influenced parasitic wood duck egg survival (β = −.24, CI = −0.39, −0.10) but had a slight positive influence on parasitic hooded merganser eggs (β =.08, CI = 0.04, 0.12). Our results revealed that hooded merganser eggs experience higher success when laid parasitically in wood duck nests, whereas wood duck eggs experience lower success when laid parasitically in hooded merganser nests. Our results reveal new complexity in waterfowl interspecific brood parasitism, where the success of parasitic eggs is species‐, host‐, and context‐specific. [ABSTRACT FROM AUTHOR]

Obligate brood parasites pass all their parental duties to foster parents of a host species. While best understood in birds and hymenopteran insects, obligate brood parasitism has evolved independently at least 59 times across many lineages. The ancestors of brood parasites often provided no parental care to their offspring. Instead, a trophic association with their eventual hosts commonly appears to precede the origin of a brood parasitic strategy. Here, we used a game theoretical model to explore the conditions under which brood parasitism can evolve from predation and be maintained in the population. Our model was inspired by the relationship between the cuckoo catfish (Synodontis multipunctatus) parasitizing mouthbrooding cichlid fishes in the African Lake Tanganyika. Our model demonstrates the facilitatory role of host egg predation on the origin and evolutionary maintenance of brood parasitism through the exploitation of the host response to egg predation by brood parasites. We found no conditions under which brood parasitism as a pure strategy is evolutionarily stable, but we describe a range of evolutionarily stable equilibria when predators and parasites coexist. While our model is tailored to the cuckoo catfish, it generally applies to other systems where brood parasitism has evolved from other antagonistic behavior. [ABSTRACT FROM AUTHOR]

In theory, emancipation from parental care is expected to favor promiscuous mating systems. However, in avian brood parasites, monogamy is surprisingly widespread, and it has been proposed that this may be favored by factors such as low population density and territoriality. Correspondingly, our previous research revealed that brood parasitic Horsfield's bronze-cuckoos (Chalcites basalis) , which occur at low population densities and defend territories, are monogamous. Here, we contrast this study with the mating system of the congeneric little bronze-cuckoo (C. minutillus) , an obligate brood parasite that exploits more concentrated hosts and is, therefore, likely to occur at higher population densities. We use single nucleotide polymorphisms to characterize the reproductive patterns of unsampled adults by inferring sibling relationships among 30 offspring. We show that (1) little bronze-cuckoos occurred at high densities, (2) polygamy was the most common mating pattern found in this study in both sexes, and (3) where multiple cuckoo eggs are laid in the same nest, they were unrelated. These results indicate that females do not defend exclusive territories, and males do not defend multiple females (polygyny). Instead, little bronze-cuckoos appear to have a non-territorial, promiscuous mating system. Our results are consistent with theoretical predictions that polygamy is more likely to evolve in species that are emancipated from parental care, where there are plenty of available mates, and where home ranges are not defended. [ABSTRACT FROM AUTHOR]

Brood parasitism and nest predation are among the main causes of breeding failures in passerine birds. Brood parasites threaten to the nest, while predatory birds threaten to both the nest and the parents. The objective of this study was to experimentally evaluate whether male and female Chestnut-capped Blackbirds, Chrysomus ruficapillus, recognize and respond to different nest threats during the breeding stages. For this purpose, we presented taxidermy models of a brood parasite, Molothrus bonariensis; a nest predator, Caracara plancus; and a non-predator species, Colaptes campestris, on the nest of C. ruficapillus. Additionally, we filmed nests without the display models to compare them with the nests exposed to taxidermy mounts. We assessed whether parents returned to the nest for each sex based on nest attention, visit frequency, and visit duration through the filming of a total of 44 nests. The parental return to the nest in the experimental trials varied between the sexes and breeding stages. During the incubation stage, males increased their nest attention and frequency of visits in the presence of the non-predator and brood parasite models. Females also increased their frequency of visits in the presence of the non-predator model in relation to the predator and brood parasite models. During the nestling stage, males extended the duration of their visits in the presence of the predator model compared to nests without the display model. Meanwhile, females showed no differences in their behavior during this stage. Overall, the Chestnut-capped Blackbirds parents demonstrated the ability to recognize the presented models and responded differently to different threats at different stages of the breeding cycle. The flexible responses they exhibited may be consistent with the 'threat-sensitive predator avoidance´ hypothesis. Significance statement: Passerine birds face different threats to themselves and their offspring during the breeding cycle. Recognizing and responding to these threats allows them to assess risk and is critical to their breeding success. Using taxidermy models of predators, brood parasites, and non-parasites, we evaluated whether the male and female Chestnut-capped Blackbirds Chrysomus ruficapillus can recognize and respond to these diverse threats during different stages of breeding. The findings of this study highlight the ability to recognize and respond differently to different threats to their nests during the incubation and nestling stages. These results contribute to our understanding of avian parental care strategies in the face of predation and brood parasitism. Understanding how birds respond to different threats during various reproductive stages can provide valuable insights into their adaptive behaviors and strategies for breeding success. [ABSTRACT FROM AUTHOR]

Changes in the timing and duration of life cycles are distinctive fingerprints of environmental change. Yet, the biotic and abiotic cues underpinning phenology and voltinism, i.e., number of generations per year, are poorly understood. Here, I experimentally test how temperature and provision size influence voltinism and survival to emergence in a solitary bee Colletes validus, and how temperature influences voltinism in the brood parasite Tricrania sanguinipennis. Within the same population, univoltine individuals emerge after 1 year (1-year form), whereas semivoltine individuals enter prolonged dormancy and emerge after 2 years (2-year form). I reared field-collected bees under 2 × 2 factorial experiments with cool (18.5 °C ± 0.5 °C) vs. warm (24 °C ± 0.5 °C) temperature treatments (bees and beetles) and no supplement vs. supplemental food treatments (+ 20% ± 5% pollen provision by mass); beetles were reared under temperature treatments only. Cool temperatures consistently increased the proportion of 2-year bees regardless of provision size, a finding that was consistent with three years of field observations. There was a demographic cost to prolonged dormancy in that both 1- and 2-year bees survived to emergence as adults, but survival of 2-year bees was approximately 50% lower than 1-year bees. Two-year beetles were produced under cooler temperatures, but unlike bees, beetles had nearly perfect survival to emergence in all treatments. This experiment advances our mechanistic understanding of the environmental drivers of voltinism in diverse insect taxa and underscores the importance of considering cryptic life stages when interpreting responses to environmental change. [ABSTRACT FROM AUTHOR]

Birds select breeding territories by assessing territory quality and considering predation pressure. However, in many small passerines, brood parasitism may also play an important role in territory occupancy. Settlement decisions are made under time pressure, especially for late migrants. An example of late migrants is the red-backed shrike Lanius collurio, a species wintering in sub-tropical Africa, which serves as a host for the common cuckoo Cuculus canorus and arrives in territories a few days later than their brood parasite. We expected that arriving red-backed shrikes avoided territories with common cuckoo presence. Then, we tested how experimentally provided artificial playback calls of the common cuckoo and woodpigeon (treated as control) affected the probability of red-backed shrike settlement. In the breeding season 2021, we observed 279 red-backed shrike territories to assess the effect of playback of common cuckoo calls on their host presence. Moreover, we adopt information on shrike territories collected in previous years to establish territory quality, measured by occupancy pattern over years. Each of these places was also checked for the presence of corvids, which are predators of shrike nests. We did not find difference with lower preference of shrikes towards cuckoo calls and to pigeon calls Columba columbus, what suggest the human presence and manipulation in the territories during experiments with recorders should be incorporated as an important factor to understanding shrikes' settlement behaviour. What is interesting we found no negative effect of corvids (jay Garrulus glandarius) in control territories. However, shrike territories where cuckoo calls were emitted and where the presence of corvids was found were less likely to be inhabited by shrikes. Therefore, territory quality, human presence enhanced by nest predators, is an important factor affecting host–parasite interactions. [ABSTRACT FROM AUTHOR]

Video recordings of Brown-headed Cowbirds (Molothrus ater) at the nests of grassland birds are of interest because relatively few such observations exist. We documented 7 events where cowbirds removed nest contents at 6 active nests and 1 empty nest. Among 6 attended nests, cowbirds arrived at the nest an average of 3.8 min (range 0.7–11.6 min) after the host species left. Cowbirds removed nest contents during the early morning (mean = 3.9 h after sunrise, SE = 0.3 h, n = 4) or late afternoon (mean = 13.1 h after sunrise, SE = 0.4 h, n = 3), taking an average of 0.7 min (range 0.4–1.1 min) to remove nest contents. Cowbirds sequentially moved nest contents away from the nest in 4 of the 7 events taking an average of 3 trips (range 2–4) to do so. Among 6 attended nests, cowbirds left the nest an average of 3.4 min (range 0.5–10.7 min) before the host species arrived back at the nest. Of the 6 active nests, cowbird damage of nest contents directly resulted in the failure of only 1 nest. Three nests went on to fledge the remaining host young, while 2 failed due to other causes. Our observations add to a growing number of video-recorded behaviors of brood parasites that can provide data for better testing hypotheses about nest destruction behavior. [ABSTRACT FROM AUTHOR]

Avian brood parasites depend upon locating host nests to lay their eggs. However, how brood parasites locate host nests and select the nests for parasitism remains poorly studied. Here, we examined how a non‐evicting brood parasite, the Asian koel (Eudynamys scolopaceus) were able to locate host nests in locally novel nest sites. We provided a set of novel nest sites (i.e., nestboxes, n = 100) to the common myna (Acridotheres tristis), a regular host of the Asian koel, at a field site in Central Bangladesh. We found that common myna quickly utilized the novel breeding sites (n = 99 nests across 59 boxes) and similarly, 21.2% of these nests were parasitized by the Asian koel. Unsurprisingly, none of the inactive nest boxes were parasitized, neither were empty nests of common myna (i.e., either predated or fledged clutches, thus lack of parental activity, n = 83). Our results provide an experimental demonstration of the Asian koel ability to rapidly locate nests in locally novel nest sites, potentially facilitated by close monitoring of host activity and behavior around the nest site. However, the underlying mechanisms for this rapid adjustment to novel nest sites remain unresolved. [ABSTRACT FROM AUTHOR]

Sex allocation theory predicts that mothers should bias investment in offspring toward the sex that yields higher fitness returns; one such bias may be a skewed offspring sex ratio. Sex allocation is well-studied in birds with cooperative breeding systems, with theory on local resource enhancement and production of helpers at the nest, but little theoretical or empirical work has focused on birds with brood parasitic breeding systems. Wood ducks (Aix sponsa) are a conspecific brood parasite, and rates of parasitism appear to increase with density. Because female wood ducks show high natal philopatry and nest sites are often limiting, local resource competition (LRC) theory predicts that females should overproduce male offspring—the dispersing sex—when competition (density) is high. However, the unique features of conspecific brood parasitism generate alternative predictions from other sex allocation theory, which we develop and test here. We experimentally manipulated nesting density of female wood ducks in 4 populations from 2013 to 2016, and analyzed the resulting sex allocation of >2000 ducklings. In contrast to predictions we did not find overproduction of male offspring by females in high-density populations, females in better condition, or parasitic females; modest support for LRC was found in overproduction of only female parasitic offspring with higher nest box availability. The lack of evidence for sex ratio biases, as expected for LRC and some aspects of brood parasitism, could reflect conflicting selection pressures from nest competition and brood parasitism, or that mechanisms of adaptive sex ratio bias are not possible. [ABSTRACT FROM AUTHOR]

When host nests are scarce, avian brood parasites would benefit from behaviours that increase the availability of suitable nests. Several studies reported ejection of host nestlings from nests by brood parasites; however, whether brood parasites do so to induce the host to re‐nest and thus increase opportunities for future parasitism (i.e. 'farming' behaviour) remains unclear. Here, we report observational evidence of farming behaviour by a common cuckoo Cuculus canorus female in a Daurian redstart Phoenicurus auroreus population: (1) the cuckoo destroyed a host nest by ejecting all nestlings, (2) the host then produced a new nest and (3) the cuckoo successfully parasitized the replacement nest. We suggest that farming behaviour may be more common, but often goes undetected because it requires intense nest monitoring. [ABSTRACT FROM AUTHOR]

Adaptations are driven by specific natural selection pressures throughout biological evolution. However, these cannot inherently align with future shifts in selection dynamics, thus manifesting in opposing directions. We performed field experiments on cuckoo hosts to investigate the coexistence and conflict between two evolutionarily successive but opposing behavioral adaptations—egg retrieval and rejection. Our findings provide key insights. (1) Egg rejection against brood parasites in hosts reshapes egg retrieval to flexible reactions—retrieval, ignoring, or outright rejection of foreign eggs outside the nest cup, departing from instinctual retrieval. (2) Parasitism pressure and egg mimicry by parasites remarkably alter the proportions of the three host reactions. Host species with higher parasitism pressure exhibit frequent and rapid rejection of non-mimetic foreign eggs and reduced ignoring or retrieval responses. Conversely, heightened egg mimicry enhances retrieval behaviors while diminishing ignoring responses. (3) Cuckoos employ consistent mechanisms for rejecting foreign eggs inside or outside the nest cup. Direct rejection of eggs outside the nest cup shows that rejection precedes retrieval, indicating prioritization of specific adaptation over instinct. (4) Cuckoo hosts navigate the conflict between the intentions and motivations associated with egg rejection and retrieval by ignoring foreign eggs, a specific outcome of the rejection–retrieval tradeoff. This study revealed that cuckoo hosts coordinated two evolutionarily successive but opposing behaviours by reshaping the instinctive egg retrieval reaction to different flexible reactions during their coevolution with parasitic cuckoos. [ABSTRACT FROM AUTHOR]

Egg retrieval is regarded as an ancestral behaviour that evolved in birds as an adaptation to eggs accidentally rolling out of nests and a fixed pattern of instinctive behaviour. By contrast, egg recognition is a later and specific adaptation that arises under selection pressure from brood parasitism, through which eggs likely belonging to brood parasites are rejected from the nest. Furthermore, egg recognition is flexible and can be enhanced through learning. We performed an empirical study on 12 populations of seven species in the tit family (Paridae), which revealed that mimetic eggs were retrieved more frequently than nonmimetic eggs across species; however, the rejection patterns were completely contrasting. Furthermore, species with a stronger recognition capacity of nonmimetic eggs retrieved eggs with lower probability; however, they rejected them with higher probability. This study also revealed that although egg retrieval is possibly a fixed pattern of instinctive behaviour, it can be modified to become a flexible reaction that adapts to subsequent evolution. Moreover, egg recognition, a specific adaptation to brood parasitism, gradually reshapes or modifies egg retrieval behaviour during evolution in a predictable manner, resulting in birds with higher levels of egg recognition being less willing to retrieve an egg from outside their nests. • Evolution of antiparasite behaviour reshapes egg retrieval, an ancestral behaviour. • Species good at egg recognition are less willing to retrieve eggs outside the nest. • Conspecific eggs are retrieved more frequently than foreign eggs across host species. • Foreign eggs are rejected more frequently than conspecific eggs across host species. [ABSTRACT FROM AUTHOR]

The role of species interactions, as well as genetic and environmental factors, all likely contribute to the composition and structure of the gut microbiome; however, disentangling these independent factors under field conditions represents a challenge for a functional understanding of gut microbial ecology. Avian brood parasites provide unique opportunities to investigate these questions, as brood parasitism results in parasite and host nestlings being raised in the same nest, by the same parents. Here we utilized obligate brood parasite brown‐headed cowbird nestlings (BHCO; Molothrus ater) raised by several different host passerine species to better understand, via 16S rRNA sequencing, the microbial ecology of brood parasitism. First, we compared faecal microbial communities of prothonotary warbler nestlings (PROW; Protonotaria citrea) that were either parasitized or non‐parasitized by BHCO and communities among BHCO nestlings from PROW nests. We found that parasitism by BHCO significantly altered both the community membership and community structure of the PROW nestling microbiota, perhaps due to the stressful nest environment generated by brood parasitism. In a second dataset, we compared faecal microbiotas from BHCO nestlings raised by six different host passerine species. Here, we found that the microbiota of BHCO nestlings was significantly influenced by the parental host species and the presence of an inter‐specific nestmate. Thus, early rearing environment is important in determining the microbiota of brood parasite nestlings and their companion nestlings. Future work may aim to understand the functional effects of this microbiota variability on nestling performance and fitness. [ABSTRACT FROM AUTHOR]

Rejection of foreign eggs is an effective defence against brood parasitism in birds. Colourful egg stimuli are among the most important cues for recognition, but varying ambient light conditions can potentially affect the decision-making process. Birds may compensate for varied illumination through colour constancy, but this remains untested in brood parasite hosts. This ability may aid in recognising foreign eggs in the nest by making the decision process more robust and reliable. We examined if ambient light colour impacts the rate at which the common redstart (Phoenicurus phoenicurus) rejects white egg models while keeping foreignhost egg contrast consistent. The cavity-nesting redstart is host to the common cuckoo (Cuculus canorus) and exhibits colour-biased egg rejection behaviour under natural light, where they preferentially reject eggs that they perceive as browner than their own eggs but, in contrast, accept eggs that they perceive as bluer than their own. Under the colour constancy hypothesis, we predicted that their responses towards the white egg would be similar between different light conditions. On the other hand, if redstarts lack colour constancy, their responses will differ between light conditions. No difference was found among rejection rates, suggesting that redstarts most likely base decisions on perceived differences between foreign and their own eggs while compensating for changes in illumination. These results imply that perceptual mechanisms like chromatic adaptation do not drive redstart colour-biased rejections. Further study on colour constancy in open and closed-nesting species will clarify illumination effects on parasite-host coevolution. [ABSTRACT FROM AUTHOR]

A recent study by Antonson et al. claims to have found evidence of a niche construction strategy in brown-headed cowbirds. The authors suggest that cowbird nestlings selectively manipulate host brood reduction to favor the presence of two warbler nestlings in the nest. However, the study selectively cites publications and raises questions about the size of the parasitic nestling relative to its host nestmates and the evolution of such a complex adaptation in a generalist brood parasite. The authors also do not address the effect of difference in size between host and brood parasite nestlings or the rate of multiparasitism. Until these issues are resolved, the results only support the begging assistance hypothesis and may be a consequence of the provisioning capacity of the host species. [Extracted from the article]

In socially monogamous bird species, extrapair paternity is common, but its level varies considerably within and among species. For hosts of brood parasites, the risk of brood parasitism may influence the costs of engaging in extrapair copulations (e.g. leaving the nest undefended) and the benefits of having extrapair young (e.g. spreading the risk of nest predation). However, whether brood parasitism risk affects extrapair behavior in host species has rarely been tested. We investigated whether variation in the risk of brood parasitism affected levels of extrapair paternity in Daurian redstarts, a socially monogamous passerine. As a common host of the common cuckoo, the redstart is unique in that its first clutch of the year takes place before the arrival of cuckoos. As a consequence, redstarts experience large seasonal variation in cuckoo parasitism risk, with no parasitism in the first egg-laying period and a high risk in the second period. We genotyped 995 nestlings (and unhatched eggs) from 181 broods and found moderate levels of extrapair paternity, with 34.8% of broods containing at least one extrapair young and 11.7% of all offspring being sired by an extrapair male. However, levels of extrapair paternity did not differ between the first and the second egg-laying period. Also, experimentally simulating parasitism risk during the first egg-laying period by broadcasting cuckoo vocalizations and presenting taxidermic models of cuckoos did not affect levels of extrapair paternity in redstarts. Our results, therefore, suggest that the risk of brood parasitism does not affect host extrapair paternity. [ABSTRACT FROM AUTHOR]

In the sexual selection framework, nuptial gifts are materials a donor provides to a receiver that can increase the donor's fitness. In specific cases, sharing crucial information may be a nonmaterial nuptial gift. To investigate this hypothesis, we focused on the common cuckoo, Cuculus canorus , an obligate avian brood parasite whose reproduction costs of females are mainly related to finding host nests needed to lay their eggs. Nest searching is assumed to be conducted only by females. We hypothesized that males could contribute by transferring information on nest locations to females as a nonmaterial nuptial gift. Here, we show the results of a first step in this direction, in which we identified any behaviour potentially conveying information on nest abundance in the surrounding area, that is, behaviours whose frequency varied with host nest density. We conducted our investigation in a marshland area within the Po Plain (Italy), where we recorded both visual displays of cuckoos at perching sites, by using camera traps, and nest abundance of two of the most parasitized cuckoo host species, the reed warbler, Acrocephalus scirpaceus , and great reed warbler, Acrocephalus arundinaceus , by systematic nest monitoring. We found that male cuckoos adopted a certain posture, wing drooping, and tended to keep their tails up more frequently in areas with the highest versus lowest host nest densities. This is consistent with these behaviours acting as potential signals codifying information on nest abundance in the area. We finally discuss the implications of our findings for the mating choices of female cuckoos and the study directions warranted to reveal whether these displays and information transfer may be included as new elements of the sexual selection framework. • Cuckoo males may help females in nest searching by sharing information on nests. • We identified some behaviours preferentially performed in areas rich in host nests. • Under specific circumstances, information may act as a nuptial gift. • Further studies are needed to assess if these behaviours affect female mate choice. [ABSTRACT FROM AUTHOR]

Host–parasite dynamics involve coevolutionary arms races, which may lead to host specialization and ensuing diversification. Our general understanding of the evolution of host specialization in brood parasites is compromised by a restricted focus on bird and insect lineages. The cuckoo catfish (Synodontis multipunctatus) is an obligate parasite of parental care of mouthbrooding cichlids in Lake Tanganyika. Given the ecological and taxonomic diversity of mouthbrooding cichlids in the lake, we hypothesized the existence of sympatric host‐specific lineages in the cuckoo catfish. In a sample of 779 broods from 20 cichlid species, we found four species parasitized by cuckoo catfish (with prevalence of parasitism of 2%–18%). All parasitized cichlids were from the tribe Tropheini, maternal mouthbrooders that spawn over a substrate (rather than in open water). Phylogenetic analysis based on genomic (ddRAD sequencing) and mitochondrial (Dloop) data from cuckoo catfish embryos showed an absence of host‐specific lineages. This was corroborated by analyses of genetic structure and co‐ancestry matrix. Within host species, parasitism was not associated with any individual characteristic we recorded (parent size, water depth), but was costly as parasitized parents carried smaller clutches of their own offspring. We conclude that the cuckoo catfish is an intermediate generalist and discuss costs, benefits and constraints of host specialization in this species and brood parasites in general. [ABSTRACT FROM AUTHOR]

Hosts and brood parasites are a classic example of conflict. Parasites typically provide no offspring care after laying eggs, imposing costs on hosts. Female subsocial wasps (Ammophila pubescens) alternate between initiating their own nests and using an "intruder" tactic of replacing eggs in nests of unrelated conspecifics. Hosts can respond by substituting new eggs of their own, with up to eight reciprocal replacements. Remarkably, intruders usually provision offspring in host nests, often alongside hosts. We used field data to investigate why intruders provision and to understand the basis of interactions. We found that intruders could not increase their fitness payoffs by using the typical brood parasite tactic of not provisioning offspring. Intruders using the typical tactic would benefit when hosts provisioned in their stead, but their offspring would starve when hosts failed to provision. Although some hosts obtained positive payoffs when intruders mistakenly provisioned their offspring, on average utilizing a conspecific nest represents parasitism: hosts pay costs while intruders benefit. Hosts and intruders used the same tactic of egg replacement, but intruders more often laid the final egg. Selection should favor better discrimination of offspring, which could lead to repeated cycles of costly egg replacement. [ABSTRACT FROM AUTHOR]

Brood parasites are expected to lay only one egg per parasitized nest, as the existence of several parasitic nestlings in a brood increases competition and can lead the starvation of some of them. However, multiparasitism (laying of two or more eggs by one or more parasitic females in a single host nest) is surprisingly frequent. Here, we study multiparasitism by different females or by the same female (repeated parasitism) in the great spotted cuckoo Clamator glandarius, a non‐evictor brood parasite that mainly parasitizes the magpie Pica pica, and whose chicks may be raised together with host nestlings in the same nest. We used a total of 262 magpie nests found during four breeding seasons. Multiparasitism and repeated parasitism are very frequent because this brood parasite is less virulent than other cuckoo species and magpie hosts can successfully raise more than one parasitic nestling per nest. The total number of cuckoo chicks fledged was higher in multiparasitized nests than in single‐ or double‐parasitized magpie nests. Magpie breeding success (i.e. the proportion of eggs that produce young that leave the nest) did not differ between single‐, double‐, and multiparasitized magpie nests. These results suggest that multiparasitism is an adaptation in the great spotted cuckoo. The intensity of parasitism (number of cuckoo eggs per nest), after controlling for the potential effect of year, did not affect nest desertion or nest predation rate, neither during the incubation nor the nestling periods. This implies that nest concealment does not affect the susceptibility of one nest being parasitized and predated, as nest predation rate was similar regardless of the intensity of parasitism. Predation rate during the nestling phase did not vary according to intensity of parasitism, which does not support either the 'mutualism' hypothesis or the 'predation cost of begging' hypothesis. [ABSTRACT FROM AUTHOR]

Perceived predation and brood parasitism risks strongly influence nesting habitat selection in several bird species. Here, we report on a playback experiment evaluating whether perceived predation or brood parasitism risk can reduce Red-winged Blackbird (Agelaius phoeniceus) abundances in agricultural nesting habitat. We broadcast Cooper's Hawk vocalizations (Accipiter cooperii, a predator of adult blackbirds and nests), Brown-headed Cowbird vocalizations (Molothrus ater, a brood parasite of many passerine species, including blackbirds), and the "Sonic Net" as treatments, the latter of which is broadcast of frequencies that overlap with blackbird vocalizations and prevent blackbirds from accessing intraspecific communication informing of predator and brood parasite risks. Neither the hawk, cowbird, nor Sonic Net treatments reduced blackbird abundances at sites early in the breeding season (April to May), when blackbirds were selecting nesting habitat. In contrast, late in the breeding season (July to August), hawk vocalizations and the Sonic Net reduced blackbird abundances at sites, but cowbird vocalizations did not. Our latebreeding season results suggest that blackbirds may flexibly change responses to perceived predation risk based on their stage of reproductive investment. Perceived predation risk could potentially be used to manage pest birds that nest in agricultural landscapes, at least for crops that are vulnerable to birds late in the breeding season. [ABSTRACT FROM AUTHOR]

Mimicry is a widespread phenomenon whereby predatory or parasitic individuals can access unsuspecting prey or hosts for the former's benefit. For example, brood parasitic common cuckoos (Cuculus canorus) evolved several adaptations to trick hosts, including host-mimetic eggs, a barred chest plumage resembling the predatory Eurasian sparrowhawk (Accipiter nisus), and the female cuckoos' bubbling calls considered as highly similar to this raptor's calls. The sparrowhawk-like call mimicry is thought to threaten hosts while female cuckoos lay their eggs, although the similarity is restricted to the call's fundamental frequency and not to its harmonics. However, these calls are also used in conspecific contexts, for example for mate attraction. If vocal mimicry is highly adapted toward the heterospecific function, it might cause reduced effectiveness in conspecific communication. We played cuckoo bubbling and sparrowhawk calls to territorial male cuckoos to test whether conspecific receivers process the mimetic calls accurately. All male cuckoos approached the speaker when female cuckoo calls were played, but rarely (7%) approached it when sparrowhawk or green woodpecker (Picus viridis; control) calls were played. When we excised the harmonic overtones from the sparrowhawk stimulus files, making these calls structurally more similar to the pure-tone female cuckoo calls, nearly half (43%) of the cuckoo males approached the speaker. Consequently, the difference in calls' harmonic structure may explain one component of the inaccurate state of this acoustic mimicry. This imperfect mimicry by female cuckoos' bubbling call to sparrowhawk call may ensure the multipurpose functions of this vocalization, including both intra- and interspecific contexts. Significance statement: Common cuckoos are obligate brood parasites, laying their eggs into other species' nests. During laying, female cuckoos utter sparrowhawk-like vocalizations (bubbling calls), to threaten hosts and to evade their defenses. However, they also use this same call to communicate with other female and male cuckoos. If the bubbling call mimics closely sparrowhawks' calls, cuckoos may also recognize and respond to the sparrowhawk call, the call of a predator, as a cuckoo call. We used playback experiments to show that cuckoos distinguished between bubbling and sparrowhawk calls in nearly all cases. When we manipulated sparrowhawk calls to be more similar acoustically to bubbling calls by deleting overtones, recognition errors increased from 7 to 43%. We conclude that mimicry of sparrowhawk calls by female cuckoo calls is imperfect, which allows it to function in cuckoo-cuckoo communication effectively. [ABSTRACT FROM AUTHOR]

Based on emergence records from trap nests and naturally occurring nests, we determined the brood parasitism behaviour of Irenangelus punctipleuris (Pompilidae: Ceropalinae) for the first time and that of Dipogon (Nipponodipogon) nagasei (Pompilidae: Pepsinae) as a complement to a previous study. The former species was found to parasitise Auplopus carbonarius and A. pygialis (Pompilidae: Pepsinae), while the latter species was found to parasitise Di. (Deuteragenia) sperconsus (Pepsinae). When using the well-established system of categories of brood parasitism in Pompilidae, Irenangelus corresponds to type A brood parasitism ('highwayman') and Nipponodipogon corresponds to type B brood parasitism ('sneak thief'). In comparison with brood parasitism in birds, we propose new concepts, namely 'generalists' and 'specialists' of brood parasitism, which correspond to type A and type B brood parasites, respectively. We discuss the differences in behavioural patterns and morphological features between the two types. We also confirm the proposed concepts by their molecular phylogenetic positions. [ABSTRACT FROM AUTHOR]

Many studies have found that if hosts observe a brood parasite at their nest, they use it as a cue to reject parasitic eggs. However, most previous work has simulated brood parasitic events by exposing a stuffed parasite near a host's nest. Responses to the presence of a real parasite have not yet been adequately studied under natural conditions. We therefore investigated whether great reed warblers (Acrocephalus arundinaceus) are more likely to reject a parasitic egg if they see a parasitizing common cuckoo (Cuculus canorus) at their nest than if the parasite approaches the host nest unnoticed. Using video recordings of 70 nests, we showed that spotting a cuckoo at the nest did not increase the rejection rate of parasitic eggs, even if hosts saw the cuckoo repeatedly. Hosts instead used the level of mimicry in the background color for cuckoo egg rejection. Since not every visit by the cuckoo leads to parasitism, seeing the brood parasite may not be a reliable enough cue for the host. Therefore, our results suggest that the sight of a cuckoo at the nest may not have as severe consequences for it as previously thought. [ABSTRACT FROM AUTHOR]

Brood parasitism involves the exploitation of host parental care rather than the extraction of resources directly from hosts. We identify defining characteristics of this strategy and consider its position along continua with adjacent behaviors but focus on canonical brood parasites, where parasitism is obligate and hosts are noneusocial (thereby distinguishing from social parasitism). A systematic literature survey revealed 59 independently derived brood parasitic lineages with most origins (49) in insects, particularly among bees and wasps, and other origins in birds (seven) and fish (three). Insects account for more than 98% of brood parasitic species, with much of that diversity reflecting ancient (≥100-million-year-old) brood parasitic lineages. Brood parasites usually, but not always, evolve from forms that show parental care. In insects, brood parasitism often first evolves through exploitation of a closely related species, following Emery's rule, but this is less typical in birds, which we discuss. We conducted lineage-level comparisons between brood parasitic clades and their sister groups, finding mixed results but an overall neutral to negative effect of brood parasitism on species richness and diversification. Our review of brood parasites reveals many unanswered questions requiring new research, including further modeling of the coevolutionary dynamics of brood parasites and their hosts. [ABSTRACT FROM AUTHOR]

Animals across diverse lineages use referential calls to warn of and respond to specific threats, and the ability to understand these calls may be dependent on experience with the threat being referenced. Yellow warblers (Setophaga petechia) produce referential 'seet' calls towards brood parasitic brown-headed cowbirds (Molothrus ater), which threaten the warblers' reproductive success. Seet calls are produced frequently in populations sympatric with cowbirds, but rarely in allopatric populations, even when those populations are genetically similar, begging the question of the role of personal experience in anti-parasitic responsiveness in aggression and propensity to seet call towards brood parasites. Here we tested for seet call responses from a yellow warbler population on the Galapagos Islands (subspecies aureola), which has been geographically isolated from the mainland and obligate brood parasites for ~ 300,000 years. We presented playbacks of brown-headed cowbird calls (allopatric brood parasite), seet calls (North American yellow warbler's referential anti-parasitic call), chip calls (yellow warbler's general alarm call), sympatric predator calls, and harmless allopatric and sympatric control songs to breeding yellow warblers, and compared behavioral and vocal responses between treatments. We found that in response to playbacks signaling brood parasitic risk (seet and cowbird calls), Galapagos yellow warblers showed aggression comparable to controls, and much lower compared to chip or predator playbacks. Galapagos yellow warblers never produced any seet calls in response to the playbacks. Our results suggest that in geographic isolation from cowbirds, Galapagos yellow warblers do not produce or respond to referential alarm calls indicating mainland brood parasitic nest threats. Significance Statement: Communication signals that denote specific objects in the environment, known as referential signals, are shaped by several ecologically important drivers, such as the extent of geographic overlap between signalers and referents, social learning, and direct experience with the referent. The yellow warbler is a useful focal species to explore questions about the contexts in which referential alarm calls occur because of the specificity for production of its anti-parasitic "seet" calls and because multiple populations of yellow warblers exist with varying exposure to obligate brood parasites. Our study explores referential alarm calling in a context without personal or social experience/learning (due to ~ 300,000 year insular separation from mainland brood parasites), and the findings are starkly different, as no anti-parasitic calls were produced at all on the Galapagos Islands, compared to mainland warblers allopatric from brood parasites for only ~ 6000 years, which are still able to produce referential anti-parasitic calls. [ABSTRACT FROM AUTHOR]

Two nests of Ancistrocerus oviventris (Wesmael, 1836) are described. One nest from Poland was attached to a concrete fence and consisted of 10 mud cells. The second nest from the Crimea was attached to a stone and consisted of three mud cells covered with an additional layer of mud and gravel particles. Females provisioned their cells with moth caterpillars of the families Crambidae and Depressariidae; 5–13 caterpillars were stored per cell. A chalcid wasp Monodontomerus aeneus (Fonscolombe, 1832) was recorded as a parasitoid while a cuckoo wasp Chrysis rutiliventris Abeille, 1879 and a sarcophagid fly Amobia sp. were recorded as kleptoparasites. [ABSTRACT FROM AUTHOR]

Many animal species protect their eggs against predators, and other species may profit from this by adding their eggs to those of the protecting species.We studied two tiny species of predatory mites that share a food source, are engaged in intraguild predation, and which eggs are attacked by the same egg predator.One of these predatory mites (Neoseiulus californicus) adds its eggs to those of the other species (Gynaeseius liturivorus) which guards its eggs from egg predators, resulting in reduced predation of the eggs of the nonguarding species. The guarding species experiences costs in the form of intraguild predation of her offspring, and the addition of eggs to the guarding species therefore qualifies as brood parasitism.In the presence of egg predators, the brood parasite preferentially adds its eggs to those of the guarding host species, not to those of another, nonguarding mite species.This cuckoo behaviour comes with a cost for the parasite when egg predators are absent, and therefore only occurs when egg predators are present.Our findings emphasize the importance of the risk of egg predation as a factor driving facultative brood parasitism. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

The abundance of a widespread brood parasite, the Brown-headed Cowbird (Molothrus ater), has decreased by ∼30% in North America over the past 5 decades. Within a community, brood parasite abundance may be expected to positively correlate with host brood parasitism frequency and intensity, but evidence for this correlation is mixed. Few studies have examined if long-term changes in brood parasite abundance have resulted in changes to host parasitism frequency. We measured cowbird abundance, brood parasitism frequency and intensity of 4 riparian songbird species, and host abundance and richness in 2001–2004 and 2012–2014 in riparian vegetation of the south Okanagan Valley of British Columbia, Canada. We compared our data to historical data for the same parameters previously collected between ∼1960 and the early 1990s in the same area. We found that cowbird abundance decreased by ∼80% over 2 decades in the Okanagan Valley, mirroring or exceeding regional-scale trends. Host abundance and richness increased as cowbird abundance decreased. However, songbird brood parasitism frequency and intensity either increased or remained relatively high over more than 4 decades. We discuss possible explanations for this apparent disconnect between brood parasite abundance and host parasitism frequency and intensity, which offer opportunity for further study. Temporal changes in brood parasite abundance, such as the decline of Brown-headed Cowbirds in North America and the Common Cuckoo (Cuculus canorus) in Europe, should not be assumed to lead to correlated changes to host parasitism frequency and intensity. La abundancia de un parásito de nidadas ampliamente distribuido, el tordo Molothrus ater, ha declinado un ∼30% en Norteamérica a lo largo de las últimas 5 décadas. Dentro de una comunidad, se podría esperar que la abundancia de los parásitos de puestas se correlacione positivamente con la frecuencia e intensidad de parasitismo de puestas del hospedero, pero la evidencia al respecto es mixta. Pocos estudios han examinado si a largo plazo los cambios de abundancia de parásitos de puestas han resultado en cambios de frecuencia de parasitismo del hospedero. Medimos la abundancia del tordo Molothrus ater, la frecuencia e intensidad de parasitismo de puestas de 4 aves canoras riparias, y la abundancia y riqueza del hospedero en 2001–2004 y 2021–2014 en vegetación riparia del sur de Okanagan Valley en British Columbia, Canadá. Comparamos nuestros datos con datos históricos para los mismos parámetros colectados previamente entre ∼1960 y los inicios de 1990s en la misma zona. Encontramos que la abundancia del tordo Molothrus ater disminuyó un ∼80% a lo largo de 2 décadas en Okanagan Valley, reflejando o superando tendencias a escala regional. La abundancia y riqueza del hospedero aumentó conforme la abundancia del tordo Molothrus ater disminuyó. Sin embargo, la frecuencia e intensidad del parasitismo de puestas de las aves canoras se incrementó o permaneció relativamente alto por más de 4 décadas. Discutimos posibles explicaciones acerca de la aparente desconexión entre la abundancia del parásito de puestas y la frecuencia e intensidad del parasitismo del hospedero, que abre oportunidades de realizar futuros estudios. Cambios temporales en abundancia de parásito de puestas, como el declive del tordo Molothrus ater en Norteamérica y del cuco común (Cuculus canorus) en Europa no deben suponer cambios correlacionados en la frecuencia e intensidad del hospedero del parásito. Palabras clave: British Columbia, dinámica de poblaciones, interferencia competitiva, Okanagan Valley, parasitismo múltiple, parásitos de puestas, tasa de parasitismo. [ABSTRACT FROM AUTHOR]

Bumblebees (genus Bombus) have received increased academic and conservation attention with the recognition of a global pollinator crisis. However, the Bombus subgenus Psithyrus (cuckoo bumblebees), has been overlooked. Psithyrus species parasitise social Bombus species and are frequently viewed as being detrimental to their hosts. This lack of attention, combined with the taxon's relatively low abundances and enhanced vulnerability to climate change due to its phenological dependence on Bombus, means there is an urgent conservation challenge facing Psithyrus. We found that academic literature focused on Psithyrus comprised less than 2% of the total works on Bombus, despite forming approximately 11% of Bombus species. We argue that species of the Psithyrus subgenus have key roles in the ecosystem and have been potentially overlooked bioindicator of Bombus diversity. We present evidence suggesting that the close phenological ties between Psithyrus and its hosts are being impacted by climate change. Going forward, research on Psithyrus distributions and phenology will be essential for conserving this subgenus. Additionally, addressing these areas provides broader opportunities to improve our understanding of ecosystem dynamics under climate change and their associated links with ecosystem services. Implications for Insect Conservation: This article aims to increase awareness of an underappreciated group of insects (the Bombus subgenus Psithyrus), and to encourage vital research to support targeted conservation efforts. [ABSTRACT FROM AUTHOR]

Obligate brood parasites lay their eggs in nests of other species, with host parents bearing the cost of raising their offspring. These costs imposed on hosts select for the evolution of host defenses against parasitism at all stages of the reproductive cycle. The most effective defense is egg rejection at early stages of the breeding cycle, with later-stage defenses (nestling and fledgling discrimination) being less common. In this study, we tested whether the hoopoe (Upupa epops), a potential host of the great spotted cuckoo (Clamator glandarius) without egg rejection ability, presents defenses after the egg stage. We experimentally parasitized hoopoe nests with great spotted cuckoo nestlings creating mixed broods (with hoopoe and cuckoo nestlings) and broods with only cuckoo nestlings and measured parental feeding behavior and survival of nestlings and fledglings of both species. Cuckoo fledglings were fed fewer often than hoopoe fledglings in mixed broods, and adults approached more often to feed hoopoe fledglings than cuckoo fledglings. Consequently, the survival of cuckoo fledglings in both mixed and only-cuckoo-broods, was significantly lower than that of hoopoe fledglings. These results suggest that hoopoes would discriminate great spotted cuckoo fledglings, with or without direct comparison with their own fledglings. However, the survival of some cuckoos suggests that hoopoes have not reached highly efficient defenses so, other life history traits hindering parasitism by cuckoos may explain low parasitism rates and low levels of defenses in this species. Significance statement: Brood parasites lay their eggs in nests of other species, tricking hosts into raising their parasitic offspring. However, hosts may fight back impeding successful parasitism by developing defences at any of the stages of their breeding cycle. We investigated why the hoopoe is not parasitized by the great spotted cuckoo despite this potential host apparently does not show such anti-parasitic defenses. We found that hoopoes have evolved the less common host defense: discrimination of parasite fledglings, even in the absence of their own fledgling for comparison. Our study supports the idea that discrimination during the later stages of the nesting cycle (i.e. nestling and fledgling periods) may be more common that previously assumed. [ABSTRACT FROM AUTHOR]

In the coevolutionary interactions between avian brood parasites and their hosts, egg recognition based on color and/or pattern is a common and effective defense to counter parasitism. However, for egg recognition based on size, only a few studies have found affirmative results, and they do not provide unambiguous evidence that egg size recognition in hosts has evolved as an important and specific anti-parasite adaptation against parasite eggs. We studied the brood parasite system between the Asian emerald cuckoo Chrysococcyx maculatus and its host, the chestnut-crowned warbler Phylloscopus castaniceps. The cuckoo parasitizes the warbler using non-mimetic and larger eggs at a parasitism rate of 12.9%. The warbler nests used in this experiment were built in a dark environment with the nest illuminance near 0 lux. Experiments with 2 types of model eggs with colors and patterns resembling cuckoo eggs of different sizes (cuckoo egg size or host egg size) showed that the warblers were able to reject 63.6% of cuckoo model eggs under these dim light conditions. However, model eggs with the same color and pattern similar to the warbler egg size were always accepted. This study provides strong evidence supporting the theory that egg size recognition can be evolved in hosts as a specific anti-parasite adaptation against cuckoos. We suggest that the egg size recognition of the warbler is an outcome of the tradeoff between the costs of violating the parental investment rule and suffering cuckoo parasitism. [ABSTRACT FROM AUTHOR]

Nest‐sharer avian brood parasites do not evict or otherwise kill host chicks, but instead inflict a range of negative effects on their nestmates that are mediated by interactions between the parasite and host life history traits. Although many of the negative fitness effects of avian brood parasitism are well documented across diverse host species, there remains a paucity of studies that have examined the impacts of parasitism across the entirety of host ontogeny (i.e., from when an egg is laid until independence). More specifically, few studies have examined the impact of brood parasitism on the pre‐ and post‐fledging development, physiology, behavior, and survival of host offspring. To help fill this knowledge gap, we assessed the effects of brood parasitism by Brown‐headed Cowbirds (Molothrus ater) across the ontogeny (incubation, nestling, and post‐fledging period) of nine sympatrically breeding host species in central Illinois, USA; due to sample sizes, impacts on the post‐fledging period were only examined in two of the nine species. Specifically, we examined the impact of brood parasitism on ontogenetic markers including the embryonic heart rate, hatching rate, nestling period length, nest survival, and offspring growth and development. Additionally, in species in which we found negative impacts of cowbird parasitism on host nestmate ontogeny, we examined whether the difference in adult size between parasites and their hosts and their hatching asynchrony positively predicted variation in host costs across these focal taxa. We found that costs of cowbird parasitism were most severe during early nesting stages (reduction in the host clutch or brood size) and were predicted negatively by host size and positively by incubation length. In contrast, we only found limited costs of cowbird parasitism on other stages of host ontogeny; critically, post‐fledging survival did not differ between host offspring that fledged alongside cowbirds and those that did not. Our findings (i) highlight the direct costs of cowbird parasitism on host fitness, (ii) provide evidence for when (the stage) those costs are manifested, and (iii) may help to explain why many anti‐cowbird defenses of hosts have evolved for protection from parasitism during the laying and incubation stages. [ABSTRACT FROM AUTHOR]

The arms race between brood parasites and their hosts provides a classic model to study coevolution. Hosts often reject the parasitic egg, and brood parasites should therefore select host nests in which the colour of the eggs best matches that of their own. Although this hypothesis has received some support, direct experimental evidence is still lacking. Here, we report on a study of Daurian redstarts, which show a distinct egg-colour dimorphism, with females laying either blue or pink eggs. Redstarts are often parasitized by common cuckoos, which lay light blue eggs. First, we showed that cuckoo eggs were more similar in spectral reflectance to the blue than to the pink redstart egg morph. Second, we report that the natural parasitism rate was higher in blue than in pink host clutches. Third, we performed a field experiment in which we presented a dummy clutch of each colour morph adjacent to active redstart nests. In this set-up, cuckoos almost always chose to parasitize a blue clutch. Our results demonstrate that cuckoos actively choose redstart nests in which the egg colour matches the colour of their own eggs. Our study thus provides direct experimental evidence in support of the egg matching hypothesis. [ABSTRACT FROM AUTHOR]

In functionally referential communication systems, the signaler's message intended for a conspecific receiver may be intercepted and used by a heterospecific eavesdropper for its own benefit. For example, yellow warblers (Setophaga petechia) produce seet calls to warn conspecifics of nearby brood parasitic brown‐headed cowbirds (Molothrus ater), and red‐winged blackbirds (Agelaius phoeniceus) eavesdrop on and recruit to seet calls to mob the brood parasites. Prior work found that warblers nesting closer to blackbirds were less likely to be parasitized, suggesting that blackbirds may even be the target of warbler's seet calls to assist with antiparasitic defense. Here we discovered for the reverse to apply too: blackbirds nesting closer to yellow warblers also experienced lower probability of brood parasitism. Concurrently, we also found that blackbirds nesting closer to other blackbirds also experience lower parasitism rates. Although these are strictly correlational results, they nonetheless suggest that blackbirds are better able to defend their nest against cowbirds when also listening to nearby warblers' referential alarm calls. [ABSTRACT FROM AUTHOR]

Parasites face a trade-off if the highest quality hosts are also most resistant to exploitation. For brood parasites, well-defended host nests may be both harder to parasitize and harder to predate, leading to better survival of parasitic chicks. This trade-off could be accentuated if brood-parasitic adaptations to reduce front-line defences of hosts, such as mimicry of hawks by Cuculus cuckoos, do not deter hosts which aggressively mob raptors. Here we investigate the costs and benefits to the African cuckoo (Cuculus gularis) of specializing on a highly aggressive host species, the fork-tailed drongo (Dicrurus adsimilis). Field experiments showed that drongos strongly attacked and mobbed both cuckoo and hawk models, implying that hawk mimicry does not deter front-line defences against African cuckoos. Attacks on cuckoo and hawk models generally declined after the egg stage but attacks on snake models sharply increased, suggesting drongos may treat hawks more like cuckoos than predators. We suggest that the cost to cuckoos of parasitizing an aggressive host may be alleviated by subsequent benefits to their offspring, since drongo nests survived better than nests of other species with similar nesting ecology. These results are indicative of a trade-off between host quality and susceptibility for a brood parasite. [ABSTRACT FROM AUTHOR]

One of the most effective defenses of avian hosts against obligate brood parasites is the ejection of parasitic eggs from the nests. Despite the clear fitness benefits of this behavior, individuals within so‐called "egg‐rejecter" host species still show substantial variation in their propensity to eliminate foreign eggs from the nest. We argue that this variation can be further understood by studying the physiological mechanisms of host responses to brood parasitic egg stimuli: independent lines of research increasingly support the hypothesis that stress‐related physiological response to parasitic eggs may trigger egg rejection. The "stress‐mediated egg rejection" hypothesis requires that hosts activate the stress‐response when responding to parasitic egg stimuli. We tested this prediction by asking whether hosts showed differential stress response when exposed to host‐like (mimetic) or parasite‐like (non‐mimetic) eggs. We experimentally parasitized incubating American robins Turdus migratorius, a robust egg‐rejecter host to obligate brood parasitic brown‐headed cowbirds Molothrus ater, with mimetic or non‐mimetic model eggs. To assess the stress response, we measured the heart rate in incubating females immediately after experimental parasitism. We also measured plasma corticosterone and, in a subset of birds, used RNA‐sequencing to analyze the expression of proopiomelanocortin (POMC), a precursor of adrenocorticotropic hormone, 2 h after experimental parasitism. We found that egg type had no effect on heart rate. Two hours following experimental parasitism, plasma corticosterone did not differ between the differently‐colored model egg treatments or between rejecter and accepter females within the non‐mimetic treatment. However, females exposed to non‐mimetic eggs showed an upregulation of POMC gene expression (before FDR correction) in the pituitary compared with females treated with mimetic eggs. Our findings suggest that in an egg‐rejecter host species, non‐mimetic parasitic eggs may increase the activity of the stress‐related hypothalamic–pituitary–adrenal axis compared with mimetic eggs, although the temporal dynamics of this response are not yet understood. [ABSTRACT FROM AUTHOR]

Decades of research have shown that the coevolutionary arms race between avian brood parasites and their hosts can promote phenotypic diversification in hosts and brood parasites. However, relatively little is known about the role of brood parasitism in promoting phenotypic diversification of nestlings.We review field data collected over four decades in Australia, New Caledonia and New Zealand to assess potential for coevolutionary interactions between the shining bronze‐cuckoo (Chalcites lucidus) and its hosts, and how diversification at the nestling stage may be generating different subspecies.The shining bronze‐cuckoo is a specialist parasite of a few hosts in the family Acanthizidae. It has diversified into subspecies, of which the nestlings closely mimic the respective host nestlings in each region. Additionally, some cuckoo subspecies have polymorphic nestlings.The Acanthizidae hosts have similar breeding and nesting habits and only moderately effective frontline defences against parasitism at cuckoo egg laying or at the egg stages. However, some hosts have developed highly effective defences at the nestling stage by recognising and ejecting cuckoo nestlings from the nest. As with the cuckoo nestlings, some hosts have polymorphic nestlings.The coevolutionary interactions in each region suggest different evolutionary stages of the arms race in which either the parasite or the host is currently in the lead. The presence of moderately effective defences at the egg laying and egg stages might explain why some hosts do not have defences at the nestling stage.The south‐Pacific cuckoo – host systems are excellent models to explore the evolutionary mechanisms driving the diversification at the nestling stage in the coevolutionary arms race between avian brood parasites and their hosts. [ABSTRACT FROM AUTHOR]

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Nest defense is an effective strategy of hosts against parasites. Typically, hosts will aggressively attack brood parasites that approach or visit their nests, which can prevent the parasites from laying eggs or may even lead to the death of the parasites. Few previous studies have specifically reported such fatal cases involving brood parasites and have attributed the cause of death to either drowning or hypothermia after falling into the water following an attack from hosts. In this study, we recorded the process of multiple host individuals of the Oriental reed warbler (Acrocephalus orientalis) mobbing and attacking a female common cuckoo (Cuculus canorus) in the field. We discovered that the immediate cause of the cuckoo's death was the fatal physical damage resulting from the aggressive defense from the hosts, suggesting that frantic pecking and scratching by the hosts is the most proximate cause of mortality among egg‐laying female cuckoos. This finding enhances our essential understanding of the effectiveness of host attacks. [ABSTRACT FROM AUTHOR]

Despite growing interest in gut microbiomes of aculeate Hymenoptera, research so far focused on social bees, wasps, and ants, whereas non-social taxa and their brood parasites have not received much attention. Brood parasitism, however, allows to distinguish between microbiome components horizontally transmitted by spill-over from the host with such inherited through vertical transmission by mothers. Here, we studied the bacterial gut microbiome of adults in seven aculeate species in four brood parasite–host systems: two bee-mutillid (host–parasitoid) systems, one halictid bee–cuckoo bee system, and one wasp–chrysidid cuckoo wasp system. We addressed the following questions: (1) Do closely related species possess a more similar gut microbiome? (2) Do brood parasites share components of the microbiome with their host? (3) Do brood parasites have different diversity and specialization of microbiome communities compared with the hosts? Our results indicate that the bacterial gut microbiome of the studied taxa was species-specific, yet with a limited effect of host phylogenetic relatedness and a major contribution of shared microbes between hosts and parasites. However, contrasting patterns emerged between bee–parasite systems and the wasp–parasite system. We conclude that the gut microbiome in adult brood parasites is largely affected by their host–parasite relationships and the similarity of trophic food sources between hosts and parasites. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Field Ornithology is the property of Resilience Alliance and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In some populations of the host of brood parasites, more than two parasite eggs may be laid in a single nest. This phenomenon is known as multiple parasitism, representing a cost to both the host and parasite. In this study, we analysed a long-term dataset (2007–2021) focusing on multiple parasitism of the common cuckoo (Cuculus canorus) parasitizing the great reed warbler (Acrocephalus arundinaceus). The annual parasitism rate was on average 54.3% and varied between 5.8% and 92.2%, depending on the year. Out of 720 parasitized nests, double parasitism was recorded in 172 (23.9%) nests, triple in 51 (7.1%) nests, quadruple in 10 (1.3%) nests, and, exceptionally, in the years of heavy parasitism (about 90%), quintuple parasitism was recorded in three (0.4%) nests. The rate of multiple parasitism ranged from 0 to 63% inter-annually and strongly correlated with the parasitism rate and the total number of parasite eggs found. Furthermore, the number of cuckoo eggs laid per one nest increased with the decreasing daily availability of host nests that were at a suitable breeding stage for parasitism. Both genetic and egg phenotype analyses revealed that no cuckoo female laid more than one egg in the same host nest. Using data on long-term parasite–host interactions and from continuous video recording, as well as progressive methods to assign parasite offspring thus helped us better understand various aspects of multiple parasitism in hosts heavily parasitized by an evictor brood parasite. Significance statement: Laying more parasite eggs in one host nest (i.e. multiple parasitism) is common in brood parasites whose nestlings share the nest with nestmates. In the species where the parasite's nestling kills its nest mates, multiple parasitism should be rare because it is costly for the parasite. However, in host populations with high parasitism rates, multiple parasitism occurs more often than predicted. Using long-term and video-recording data, we quantified multiple parasitism in the common cuckoo across years and host egg-laying sequences. We found that the rate of multiple parasitism is positively related to the parasitism rate and that the lower the number of nests suitable for parasitism, the higher the number of parasite eggs in one nest. Based on genetic and egg phenotype analyses, we also showed that individual parasitic females avoid laying in the nests they had already parasitized. [ABSTRACT FROM AUTHOR]

Avian obligate brood parasites and their hosts exhibit coevolution, where parasites respond to the evolution of host defences with counter-adaptations, such as egg mimicry, which can then lead to the evolution of more efficient host defences. Using objective measurements and avian visual modelling, we quantified the degree of similarity between the eggs of the channel-billed cuckoo (Scythrops novaehollandiae) and the eggs of three host species from the hosts' visual perspective. We found no evidence that this cuckoo species has host-specific egg types, but it does appear to mimic one of its primary hosts, the pied currawong (Strepera graculina), in both colour and pattern. Currawongs showed significantly higher interclutch variation in colour and luminance than the other two hosts and had significantly lower intraclutch variation in pattern than interclutch variation. High interclutch and low intraclutch variation in a host could make it more difficult for a cuckoo to match closely any particular host clutch, potentially allowing a host to detect a parasitic egg more easily. This is the first empirical study using objective measurements to quantify the degree of egg mimicry in the channel-billed cuckoo and the results suggest a potential host defence to egg mimicry that merits further investigation. [ABSTRACT FROM AUTHOR]

Brood parasites are involved in coevolutionary arms races with their hosts, whereby adaptations of one partner elicit the rapid evolution of counter-adaptations in the other partner. Hosts can also mitigate fitness costs of brood parasitism by learning from individual or social experience. In brood parasites, however, the role of learning can be obscured by their stealthy behaviour. Cuckoo catfish (Synodontis multipunctatus) parasitise clutches of mouthbrooding cichlids in Lake Tanganyika and are the only non-avian obligate brood parasites among vertebrates. We experimentally demonstrate that cuckoo catfish greatly enhance their efficiency in parasitising their hosts as they learn to overcome host defences. With increasing experience, cuckoo catfish increased their parasitism success by greater efficiency through improved timing and coordination of intrusions of host spawnings. Hence, within the coevolutionary arms races, brood parasites learn to overcome host defences during their lifetime. The importance of learning for brood parasites is explored using cuckoo catfish. The catfish increase their parasitism success as they gain experience, mainly by improving their social coordination and timing of intrusions to cichlid host spawnings. [ABSTRACT FROM AUTHOR]

Avian brood parasite hosts can use distinct egg traits to recognize a parasitic egg. Previous studies suggested an important role of eggshell ultraviolet (UV) reflectance in rejection. According to the egg detectability hypothesis, natural selection might have selected for a conspicuous egg UV signal, especially in dark nest environments. We tested this hypothesis in a cavity nesting host by experimentally parasitizing a population of common redstarts breeding in Central Europe. We manipulated UV reflectance using egg models with enhanced UV and used models with UV characteristics similar to the host as a control treatment. We additionally manipulated nest light conditions by adjusting the entrance size of the nest-boxes, because dark environments have been suggested to affect detectability of an egg, particularly in UV light. Hosts rejected experimental eggs with similar probabilities in both UV treatments and also under different light conditions. Results of this study did not support the egg detectability hypothesis. Our conclusions on the role of egg UV for egg recognition and rejection emphasize how little we understand its function in cavity nesting hosts. [ABSTRACT FROM AUTHOR]