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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]

Hosts of avian brood parasites suffer a high cost of reproductive loss due to parasitism, driving them to evolve a variety of anti-parasitic defenses. These defenses comprise a series of components, including the recognition of brood parasites and the eggs laid by the parasites, cues used for recognition, and the mechanisms on which these behaviors are based. In this study, we conducted egg recognition and nest intruder experiments to examine these components of anti-parasitic behavior in the black-browed reed warbler (Acrocephalus bistrigiceps), a rare host of the common cuckoo (Cuculus canorus). We found that the host possessed strong recognition capacity, rejecting 100% of parasitic eggs, and used a template-based mechanism for egg recognition. The host birds also rejected 80% of their own eggs on which artificial markings were added to the blunt pole; however, they accepted all eggs with the same manipulation on the sharp pole, implying that the blunt pole was an important recognition cue. Furthermore, the host exhibited stronger aggression to cuckoos than to harmless controls; a behavior specific to the incubation stage rather than the nestling stage. Therefore, the host was able to distinguish the cuckoo from other nest intruders as being a brood parasite. These results together help explain the near absence of cuckoo parasitism in black-browed reed warblers and provide new information concerning anti-parasitic defenses in this host species. [ABSTRACT FROM AUTHOR]

Communication between parents and dependent offspring is critical not only during provisioning, but also in antipredator contexts. In altricial birds, a top cause of reproductive failure is nest predation, and alarm calls both by parents and chicks can serve to alert others and increase the likelihood of offspring escaping predation. Understanding the factors that determine the strength of parental antipredator responses to different nestling alarm calls can provide insight into parent–offspring recognition. The prothonotary warbler (Protonotaria citrea), a host of the obligate brood parasite, the brown-headed cowbird (Molothrus ater), never rejects cowbird young and raises the parasite together with its own offspring. To determine whether warbler parents learn cowbird nestling alarm calls, we presented experimentally parasitized or non-parasitized parents with playbacks of conspecific warbler, parasitic cowbird, and a harmless heterospecific control, eastern bluebird (Sialis sialis), nestling alarm calls. We recorded the latency to respond and the number of chips given by members of the resident warbler pair. We found that parents were most likely to respond to warbler nestling alarm calls, least likely to respond to bluebird calls, with a statistically intermediate likelihood of responding to cowbird calls. Critically, current and past parasitism status did not affect the likelihood of response to any playback or the number of chips given, however, currently parasitized parents had greater response latencies to playbacks than non-parasitized parents. These results suggest that warbler parents do not learn cowbird alarm calls from breeding experiences and, in turn, that cowbirds may employ a generalized, bet-hedging alarm call. [ABSTRACT FROM AUTHOR]

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Generalist brood parasites that share nests with host nestlings can optimize resource acquisition from host parents by balancing the benefits that host nest-mates provide, including attracting increased provisions to the nest, against the costs of competing with the same host young over foster parental resources. However, it is unclear how parasitic chicks cope when faced with more nest-mates than are optimal for their survival upon hatching. We suggest that, in the obligate brood parasitic brown-headed cowbird (Molothrus ater), chicks use a niche construction strategy and reduce larger, more competitive host broods to maximize the parasites' survival to fledging. We experimentally altered brood sizes to test for Goldilocks principle patterns (i.e. a 'just right' intermediate brood size) of cowbird survival in nests of prothonotary warbler (Protonotaria citrea) hosts. We found that intermediate brood sizes of two host nestlings maximized cowbird fledging success relative to 0 or 4 host nest-mates at hatching. Specifically, cowbird nestlings lowered host brood sizes towards this optimum when placed in broods with more host nestlings. The results suggest that cowbirds reduce, but do not eliminate, host broods as a niche construction mechanism to improve their own probability of survival. [ABSTRACT FROM AUTHOR]

Obligate brood parasites depend entirely on other species to raise their offspring. Most avian obligate brood parasites have altricial offspring that require enormous amounts of posthatching parental care, and the large fecundity boost that comes with complete emancipation from parental care likely played a role in the independent evolution of obligate parasitism in several altricial lineages. The evolution of obligate parasitism in the black‐headed duck, however, is puzzling because its self‐feeding precocial offspring should not constrain parental fecundity of a potential brood parasite in the way that altricial offspring do. We used an experimental nest predation study to test the idea that high nest predation rates played a role in the evolution of brood parasitism in this enigmatic duck. Experimental duck eggs in untended nests suffered massive rapid predation, while eggs in tended nests of the three main hosts, all aggressive nest defenders, had very high success, illustrating the benefits of parasitizing these 'bodyguard' hosts. [ABSTRACT FROM AUTHOR]

Social learning can enable the rapid dissemination of behaviours throughout a population. Rejection of foreign eggs is a key defence used by hosts of avian brood parasites, and although learning has long been recognized as an important factor in egg rejection decisions, the role played by social cues remain less well understood. Here, we aimed to determine whether access to social information influenced egg rejection behaviours in semicolonial barn swallows, Hirundo rustica. By manipulating the social information available from a neighbouring nest, we showed that swallows with access to social information (i.e. a neighbour that recently rejected an egg) were more likely to reject a foreign egg that those that did not have access to social information (i.e. a neighbour that did not reject an egg). A subset of nests were videorecorded, and in these recordings the rejections were always performed by the incubating parent rather than a conspecific. This study provides direct evidence that egg rejection behaviours can be informed by social information in the absence of brood parasites, and in doing so highlights the dynamic nature of the defences that hosts deploy against brood parasitism. • Information about parasitism risk can inform host defences. • We tested whether social information could inform egg rejection decisions. • Barn swallows with access to social information rejected more foreign eggs. • Our results highlight the role that social information can play in coevolution. [ABSTRACT FROM AUTHOR]

Breeding habitat choice based on the attraction to other species can provide valuable social information and protection benefits. In birds, species with overlapping resources can be a cue of good quality habitats; species with shared predators and/or brood parasites can increase joint vigilance or cooperative mobbing, while raptors may provide a protective umbrella against these threats. We tested whether the migratory common redstart (Phoenicurus phoenicurus) is attracted to breed near active nests of the great tit (Parus major), a keystone-information source for migrant passerine birds, or a top predator, the northern goshawk (Accipiter gentilis). This system is unique to test these questions because the redstart is a regular host for the common cuckoo (Cuculus canorus). Therefore, we also evaluated other possible benefits coming from the heterospecific attraction, especially in terms of reducing brood parasitism risk. We monitored redstart occupancy rates, onset of breeding, reproductive investment, and followed nest outcomes in terms of brood parasitism, nest predation risk and overall reproductive success. Redstarts avoided breeding near goshawks, but showed neither attraction nor avoidance to breed next to great tits. Both neighbours neither reduced brood parasitism risk nor affected overall nesting success in redstarts. Redstarts may not use heterospecific attraction for settlement decisions, as associations with other species can only exist when some benefits are gained. Thus, environmental cues may be more important than social information for redstarts when breeding habitat choice. Other front-line defence strategies may have a better impact reducing breeding negative interactions, such brood parasitism. [ABSTRACT FROM AUTHOR]

The relationships between avian brood parasites and their hosts are widely recognised as model systems for studying coevolution. However, while most brood parasites are known to parasitise multiple species of host and hosts are often subject to parasitism by multiple brood parasite species, the examination of multispecies interactions remains rare. Here, we compile data on all known brood parasite–host relationships and find that complex brood parasite–host systems, where multiple species of brood parasites and hosts coexist and interact, are globally commonplace. By examining patterns of past research, we outline the disparity between patterns of network complexity and past research emphases and discuss factors that may be associated with these patterns. Drawing on insights gained from other systems that have embraced a multispecies framework, we highlight the potential benefits of considering brood parasite–host interactions as ecological networks and brood parasitism as a model system for studying multispecies interactions. Overall, our results provide new insights into the diversity of these relationships, highlight the stark mismatch between past research efforts and global patterns of network complexity, and draw attention to the opportunities that more complex arrangements offer for examining how species interactions shape global patterns of biodiversity. [ABSTRACT FROM AUTHOR]

Simple Summary: Obligate brood parasite birds such as the common cuckoo (Cuculus canorus) can trick their hosts via a variety of means. Cuckoos are threats to the host nests but not to the host adults themselves. To successfully parasitize the host nests, female cuckoos have been hypothesized to distract the hosts' attention from their nests by mimicking the calls of sparrowhawks (Accipiter nisus), a predator of the hosts. We performed playback experiments in two populations of the host oriental reed warbler (Acrocephalus orientalis) that have experienced different levels of parasitic pressure. We found that female cuckoo calls evoked both populations of the hosts to leave their nests more frequently than did the calls of male cuckoos or doves that do not pose threats to the hosts. This indicated that the call of the female cuckoo functions to deceive the host and thus favors host brood parasitism. However, we propose that such a deceiving effect of the female cuckoo call is due to the rapid cadence of the call rather than sparrowhawk mimicry. The common cuckoo (Cuculus canorus) is an obligate brood parasite that has evolved a series of strategies to trick its hosts. The female cuckoo has been hypothesized to mimic the appearance and sounds of several raptors to deceive the hosts into exhibiting anti-predator behavior. Such behavior would relax the protection of the host nest and thus allow the female cuckoo to approach the host nest unopposed. Many anti-parasite strategies have been found to vary among geographical populations due to different parasitic pressures from cuckoos. However, the effect of female cuckoo calls related to different levels of parasitic pressure has not been examined. Here, we studied the effect of female cuckoo calls on the oriental reed warbler (Acrocephalus orientalis), one of the major hosts of the common cuckoo, in two geographical populations experiencing different levels of parasite pressure. Four kinds of sounds were played back to the hosts: the calls from female common cuckoos, male common cuckoos, sparrowhawks (Accipiter nisus), and oriental turtle doves (Streptopelia orientalis). The results showed that the female cuckoo calls induced the hosts to leave their nests more frequently than the male cuckoo or dove calls in both populations, and two populations of the hosts reacted similarly to the female cuckoo calls, implying that the function of female cuckoo calls would not be affected by the difference in parasitism rate. This study indicates that female cuckoo calls function to distract the hosts' attention from protecting their nests. However, we propose that such a deception by the female cuckoo call may not be due to the mimicry of sparrowhawk calls, but rather that the rapid cadence of the call that causes a sense of anxiety in the hosts. [ABSTRACT FROM AUTHOR]

Brood parasites represent a substantial but often poorly studied fraction of the wider diversity of bees. Brood parasitic bees complete their life cycles by infiltrating the nests of solitary host bees thereby enabling their offspring to exploit the food provisions intended for the host's offspring. Here, we present the draft assembly of the bee Holcopasites calliopsidis, the first brood parasitic species to be the subject of detailed genomic analysis. Consistent with previous findings on the genomic signatures of parasitism more broadly, we find that H. calliopsidis has the smallest genome currently known among bees (179 Mb). This small genome does not appear to be the result of purging of repetitive DNA, with some indications of novel repetitive elements which may show signs of recent expansion. Nor does H. calliopsidis demonstrate any apparent net loss of genic content in comparison with nonparasitic species, though many individual gene families do show significant contractions. Although the basis of the small genome size of this species remains unclear, the identification of over 12,000 putative genes--with functional annotation for nearly 10,000 of these--is an important step in investigating the genomic basis of brood parasitism and provides a valuable dataset to be compared against new genomes that remain to be sequenced. [ABSTRACT FROM AUTHOR]

Visual complexity is ubiquitous in nature. Drivers of complexity include selection in coevolutionary arms races between antagonists. However, the causes and consequences of biological complexity and its perception are largely understudied, partly because complexity is difficult to quantify. Here, we address this by studying egg pattern complexity and its perception in hosts (tawny-flanked prinia Prinia subflava), which visually recognize and reject mimetic eggs of their virulent brood parasite (cuckoo finch Anomalospiza imberbis). Using field data and an optimization algorithm, we compute a complexity metric which predicts rejection of experimentally placed conspecific eggs in prinia nests. Real cuckoo finch eggs exhibit significantly lower pattern complexity than prinia eggs, suggesting that high complexity benefits hosts because it distinguishes host eggs from parasitic eggs. We show that prinias perceive complexity differences according to Weber's law of proportional processing (i.e. relative, rather than absolute, differences between stimuli are processed in discrimination, such that two eggs with simple patterns are more easily discriminable than two with complex patterns). This may influence coevolutionary trajectories of hosts and parasites. The new methods presented for quantifying complexity and its perception can help us to understand selection pressures driving the evolution of complexity and its consequences for species interactions. [ABSTRACT FROM AUTHOR]

Microbiota affect many aspects of vertebrate biology and vertebrates provide diverse ecological niches for microorganisms. In avian systems, potential habitats for microorganisms include the feathers, skin, gastrointestinal tract, and eggshells. Eggshell microbiota may seed bird microbiota as the birds hatch and may be shaped by parental and environmental influences. Brood parasites are a natural system where parental and environmental influences are obligately decoupled. Here, we sequenced the V4 region of the 16S rRNA gene to characterize the eggshell microbiota of a brood parasite, the Brown-headed Cowbird (Molothrus ater), and its host, the Ovenbird (Seiurus aurocapilla), to explore the relationships between nest environment, host phylogeny, and the eggshell microbiota. Eggshell microbiota did not differ significantly between species. The observed variation was best explained by the nest in which an egg was laid and the collection site. Our study suggests that eggshell microbiota are influenced by the nest and local environment with significantly less influence by the species of the bird. [ABSTRACT FROM AUTHOR]

Obligate avian brood parasites lay their eggs in the nest of other bird species, known as hosts. Brood parasitism often imposes severe fitness costs on hosts, selecting for the evolution of effective antiparasitic defences, such as recognition and rejection of brood parasite eggs. Glucocorticoids have been recently found to mediate host physiological and behavioral adjustments in response to brood parasite eggs; however, it remains unclear whether brood parasitism triggers a general response involving multiple physiological elements. In this study, we experimentally investigated whether a salient brood parasitic stimulus (the presence of a nonmimetic model egg in the nest) causes physiological adjustments in adult Eurasian blackbirds (Turdus merula) at immune (leukocyte profiles) and cellular (heat‐shock protein Hsp70 synthesis) level. Also, we explored whether these physiological changes are mediated by variations in corticosterone (CORT) levels. We found that experimental brood parasitism caused an increase in heterophils and a decrease in lymphocytes, leading to higher heterophils and lymphocytes ratios in parasitized birds. Nevertheless, we did not find tradeoffs between immune function and CORT levels. Hsp70 synthesis was not affected by our experimental manipulation. Our findings provide evidence that brood parasite eggs trigger a general stress response in egg‐rejecter hosts, including changes in cellular immune profiles. Highlights: Physiological effects of avian brood parasitism were assessed in adult hosts.Non‐mimetic brood parasite eggs led to increased heterophil/lymphocyte ratios, but not Hsp70 levels, in hosts.Changes on hosts' immune function were not mediated by plasma corticosterone. [ABSTRACT FROM AUTHOR]

Avian brood parasites leave parental care of their offspring to foster parents. Theory predicts that parasites should select for large host nests when they have sufficient available host nests at a given time. We developed an empirical experimental design to test cognitive ability of female cuckoos in nest size by studying nest choice of common cuckoos (Cuculus canorus) among nests of its Oriental reed warbler (Acrocephalus orientalis) hosts. We presented three groups of experimental nests: 1) nest dyads tied together including one large and one small artificial nest from reed leaves, 2) nest triads tied together used the old modified warbler's own nests including enlarged, reduced and medium-sized nests, and 3) nest dyads are similar to group 1, but not tied together to elicit parasitism by common cuckoos. We predict that cuckoos prefer larger nest than medium one, the next is smaller nest. Our findings showed that common cuckoo females generally prefer large nests over medium or small sized nests in all three experimental groups. Furthermore, cuckoo parasitism was significantly more common than in previous studies of the same warbler population, implying that larger, higher and more exposed host nests effectively increased the probability of cuckoo parasitism. [ABSTRACT FROM AUTHOR]

The Striped Cuckoo (Tapera naevia) is an obligate brood parasite with poorly understood breeding habits. Here, we describe the reproductive biology of Striped Cuckoos parasitizing Rufous-fronted Thornbird (Phacellodomus rufifrons) nests in the Cerrado of central Brazil and discuss parasite adaptations. Striped Cuckoos parasitized 38.5% of Rufous-fronted Thornbird nests in October–November. Cuckoos laid 1 white elliptical egg per thornbird nest. The mean mass of eggs was 2.64 g and the mean size was 21.0 × 15.3 mm (n = 4). Striped Cuckoo egg incubation period had a duration of 14.7 ± 1.4 d (n = 7), 2.4 d less than eggs of the Rufous-fronted Thornbird (17.1 ± 2.2 d; n = 8). Striped Cuckoo nestlings were very aggressive, pecking their nest mates to death, and the single cuckoo nestling remained alone in parasitized nests for 20.8 ± 1.2 d (n = 8), a duration similar to its host in nonparasitized nests (19.7 ± 1.6 d; n = 14). The mass gain of cuckoo nestlings was faster (3.0 g/d) than thornbird nestlings in nonparasitized nests (1.7 g/d). The predation rate during the nestling stage was similar for nests with a Striped Cuckoo nestling (20%) to that of nonparasitized nests with thornbird nestlings (26%). We propose that several adaptations allow Striped Cuckoos to successfully parasitize Rufous-fronted Thornbird nests: a highly mimetic egg, as the host's egg was also white and elliptically shaped, a smaller egg in proportion to body size, the ability of its nestlings to puncture or eject host eggs and kill host nestlings, a shorter incubation period, a faster nestling mass gain, and a flexible breeding season. O saci (Tapera naevia), é um parasita de ninho obrigatório cujos hábitos reprodutivos são pouco conhecidos. Descrevemos aqui a biologia reprodutiva do saci parasitando ninhos do joão-de-pau (Phacellodomus rufifrons) no Cerrado do Brasil central, e discutimos as adaptações do parasita. O saci parasitou 38,5% dos ninhos entre outubro e novembro. Colocou 1 ovo branco e elíptico por ninho. O peso médio dos ovos foi de 2,64 g e as medidas 21,0 × 15,3 mm (n = 4). O período de incubação do saci foi de 14,7 ± 1,4 d (n = 7), 2,4 dias a menos que o joão-de-pau (17,1 ± 2,2 d; n = 8). Os ninhegos do saci eram muito agressivos bicando seus companheiros de ninho até a morte, e assim o ninhego solitário permaneceu no ninho por 20,8 ± 1,2 d (n = 8), um período semelhante do seu hospedeiro em ninhos não parasitados, 19,7 ± 1,6 d (n = 14). O ganho de peso dos ninhegos do saci foi mais rápido (3,0 g/d) do que dos ninhegos do joão-de-pau em ninhos não parasitados (1,7 g/d). A taxa de predação dos ninhos durante o estágio de ninhegos foi semelhante entre ninhos com o saci (20%), e ninhos não parasitados com ninhegos do joão-de-pau (26%). Propomos que diversas adaptações de parasitas permitem que o saci parasite com sucesso em ninhos do joão-de-pau: um ovo altamente mimético, já que o ovo do hospedeiro também era branco e de formato elíptico, um ovo pequeno em relação ao seu tamanho corporal, a habilidade dos ninhegos de furar e ejetar ovos do hospedeiro e de matar os ninhegos do hospedeiro; um menor período de incubação, uma taxa de crescimento mais rápida dos ninhegos, e uma estação reprodutiva mais flexível. [ABSTRACT FROM AUTHOR]

Jelínek V., Šulc M., Štětková G. & Honza M. 2021. Fast and furious: host aggression modulates behaviour of brood parasites. Ibis 163:824–833. https://doi.org/10.1111/ibi.12930In the Abstract of the above article it currently reads:"We video‐recorded 168 brood parasitic events at 102 active host nests and found that Cuckoos avoided host mobbing in 62% of cases."The authors would like to change the percentage value 62% to 38%, so that it reads:"We video‐recorded 168 brood parasitic events at 102 active host nests and found that Cuckoos avoided host mobbing in 38% of cases."We apologize for this error. [Extracted from the article]

The antagonistic arms races between obligate brood parasites and their hosts provide critical insights into coevolutionary processes and constraints on the evolution of life history strategies. In avian brood parasites—a model system for examining host–parasite dynamics—research has primarily focused on the egg and nestling stage, while far less is known about the behavior and ecology of fledgling and juvenile brood parasites. To provide greater insights into the post-fledging period of generalist brood parasites, we used handheld and automated telemetry systems to examine the behavior and survival of fledgling brown-headed cowbirds (Molothrus ater). Our host community-wide analysis (data on cowbirds fledged from different host species were pooled) shows that fledgling cowbirds' follow patterns of movement and survival found across the post-fledging literature on parental passerine species. Cowbird fledgling survival was lowest during the first 3 days post-fledging, whereas daily rates of survival neared 100% after about 16 days post-fledging. Cowbird daytime post-fledging activity rates, perch heights, and distance from the natal area all increased with fledging age and young generally gained independence from host parents at 3–4 weeks post-fledging, with approximately the same latency as has been observed in studies on fledglings of cowbird host species. Our research demonstrates how automated telemetry systems can overcome past methodological limitations in post-fledging research and provides an important foundation for future studies examining adaptations that cowbirds and other brood parasites use to exploit hosts during the post-fledging period. [ABSTRACT FROM AUTHOR]

Obligate avian brood parasites usually require longer and/or more parental care than host progeny and thus may have a detrimental effect on survival of host parents. Many hosts of brood parasites are long-distance migrants, spending significant proportions of annual cycles at different sites around the world, which makes correct timing of particular events within these cycles of utmost importance. Nevertheless, conditions in a given phase of the annual cycle may influence the timing of the following phases via carry-over effects. Here, we explored whether great reed warbler (Acrocephalus arundinaceus) parents fostering a brood-parasitic common cuckoo (Cuculus canorus) were delayed on departure from the breeding site and arrival at the wintering sites compared to the parents caring for own nestlings. Additionally, we tested whether the parents with nestlings hatched later in the season set out for autumn migration and arrived at the wintering sites later than the parents with nestlings hatched earlier in the season and whether female parents lagged in time behind male parents during autumn migration. We found that the parents fostering the common cuckoo were delayed neither on departure from the breeding site nor on arrival at the wintering sites compared to the parents rearing own nestlings. Moreover, there was no effect of hatching date and parent sex on the timing of autumn migration. Future studies may rather focus on tracking female parents which could be more affected by the care for a brood parasite than male parents. Future researchers may also strive to monitor post-fledging survival of young, as increased mortality of either the brood-parasitic or host fledglings could affect the end of host parental care and thus also the timing of host autumn migration. [ABSTRACT FROM AUTHOR]

Interspecific avian brood parasites, like cuckoos and cowbirds, lay their eggs in nests of other species, the hosts, which take over the entire parental care of the parasite's eggs and chicks. This breeding strategy requires decisions that may affect the parasite's reproductive success. During the breeding season, cowbirds search for host nests and revisit them to monitor its progress and parasitize at the time host laying begins. When visiting hosts nests, they repeatedly peck the nest contents trying to destroy one or more eggs. This behaviour favours parasite's offspring by reducing the competition for food with nestmates. We evaluated if the egg-pecking behaviour of female shiny (Molothrus bonariensis) and screaming (M. rufoaxillaris) cowbirds is affected by the strength and the size of the eggs they find in the nest. We presented to wild-caught females artificial clutches with two natural eggs that differ in size and shell strength. We found that female shiny and screaming cowbirds adjusted egg-pecking behaviour based on the strength but not on the size of the eggs. When differences in strength between eggs were high, both cowbird species pecked more frequently the egg with the weaker shell, increasing the probability of a successful puncture. Our results indicate that female cowbirds can discriminate eggs through the strength of the shell, and by choosing the weaker egg to peck, they increase the probability of puncturing. [ABSTRACT FROM AUTHOR]

Simple Summary: Insects have different strategies to adapt to environment change and parasites, but the supercooling response of pollinator populations under the biological stress has not been sufficiently investigated. This study assessed the supercooling traits of the solitary bee and its brood parasite. We discovered significant differences in the supercooling points were found between the solitary bee and its brood parasite in the same sex, and also between sexes of same species. The supercooling traits (supercooling points, fresh weight, and fat content) of the two species were significantly positively correlated. Our results suggest that the supercooling points of the solitary bee increase under the biological stress of its brood parasite at a certain level. Overall, the supercooling of pollinator populations varies regularly under brood parasitism pressure. (1) Background: Many insects have evolved different strategies to adapt to subzero temperatures and parasites, but the supercooling response of pollinator populations under the brood parasitism pressure has not been sufficiently investigated. (2) Methods: This study assessed the supercooling traits (supercooling points, fresh weight and fat content) of the solitary bee Osmia excavata Alfken and its brood parasite, Sapyga coma Yasumatsu & Sugihara. We measured 4035 samples (3025 O. excavata and 1010 S. coma, one individual as one sample) and discovered the supercooling traits relations between solitary bee and brood parasite. (3) Results: Significant differences in the supercooling points were found between O. excavata (females: −24.18 (−26.02~−20.07) vs. males: −23.21 (−25.15~−18.65) °C) and S. coma (females: −22.19 (−25.46~−18.38) vs. males: −20.65 (−23.85~−16.15) °C, p < 0.0001) in the same sex, and also between sexes of same species. The two species' supercooling traits (supercooling points, fresh weight, and fat content) were significantly positively correlated. The supercooling points of the solitary bee varies regularly under brood parasitism pressure. (4) Conclusions: Our study indicates the supercooling traits relationships between a solitary bee and its brood parasite and suggests that the supercooling points of the solitary bee increase under the biological stress of its brood parasite in a certain level. [ABSTRACT FROM AUTHOR]

The decline in pollinator insect abundance and diversity is increasing on a global scale. Major threats are the byproducts of numerous negative environmental pressures acting individually or in combination. They vary throughout different geographical areas, affecting the solitary bees differently. One of the most important negative pressures are the many parasites, predators and pests representing a threat to the successful reproduction of solitary bees in artificial nests. Especially vulnerable are the managed Osmia spp. bee populations reared for commercialization and trade. The primary goals of our monitoring study were: (i) to examine the presence and the prevalence of brood parasites in the various types of bees' nesting material and in semi-field rearing conditions using the nest section analyses; (ii) to determine the presence of Nosema spp. in samples of feces and homogenized bee abdomens using a multiplex PCR method; (iii) the evaluation of the survival success level and emergence mass of healthy bees at each of the four studied bee rearing locations separately, depending on different environments and on the implementation of different managing practices. We determined the presence and prevalence of nest destructor parasites and accompanying fauna. Their presence was positively correlated with bee rearing failures. The results of this study may be used as a baseline for further solitary bee nest parasites monitoring schemes. [ABSTRACT FROM AUTHOR]

Neural plasticity in the hippocampus has been studied in a wide variety of model systems, including in avian species where the hippocampus underlies specialized spatial behaviours. Examples of such behaviours include navigating to a home roost over long distances by homing pigeons or returning to a potential nest site for egg deposit by brood parasites. The best studied example, however, is food storing in parids and the interaction between this behaviour and changes in hippocampus volume and neurogenesis. However, understanding the interaction between brain and behaviour necessitates research that includes studies with at least some form of captivity, which may itself affect hippocampal plasticity. Captivity might particularly affect spatial specialists where free-ranging movement on a large scale is especially important in daily, and seasonal, behaviours. This review examines how captivity might affect hippocampal plasticity in avian spatial specialists and specifically food-storing parids, and also considers how the effects of captivity may be mitigated by researchers studying hippocampal plasticity when the goal is understanding the relationship between behaviour and hippocampal change. [ABSTRACT FROM AUTHOR]

Obligate brood parasites rely upon unrelated hosts to provide parental care for their offspring. Specialist brood parasites use visual and vocal mimicry of host offspring to induce host parents to care for the parasite offspring. Generalist brood parasites, however, use different tactics to obtain host parental care. Here, we examine how brown-headed cowbirds (Molothrus ater) restructure host red-winged blackbird (Agelaius phoeniceus) broods and the consequent effects on the expected survival of the brood parasite nestling. Competition within the host brood is governed by an asymmetric sibling rivalry, with the size of the core brood driving the survival of both core and marginal nestlings. Competitively inferior marginal nestlings conversely have little effect on core nestlings. Cowbirds primarily removed core host nestlings when altering the size and structure of host broods. Host brood size at hatching was reduced in conjunction with parasitism by cowbirds. The resulting brood structures differed markedly from those in unparasitized broods with most notably fewer core nestlings, the most potent competitors for the cowbird nestling. Marginal brood size, however, differed only slightly. This reflects the competitive dynamics: core but not marginal brood size affected cowbird survival. The same asymmetric nestling rivalry that governs competition among host nestlings applies to these brood parasitic cowbirds as well; by altering host family structure, they enhanced the effective environment for their nestlings. Significance statement: Obligate brood parasites must purloin parental care from unrelated host species to rear offspring. Generalist brood parasites, such as cowbirds, face an especially steep challenge, as the offspring are not visual or vocal mimics of host offspring: they are inserted into the host brood and must compete against host offspring. Here, we show that changes in host brood size and structure associated with cowbird parasitism enhance the effective environment for the brood parasite nestling. Through the removal/damage of host eggs or interference with host egg hatching, host brood size is reduced, and most importantly, the number of first-hatched "core" offspring that are the most potent competitors for the brood parasite offspring. The same asymmetric sibling rivalry that exists within the host brood extends to competition between host nestlings and brood parasites. [ABSTRACT FROM AUTHOR]

Egg recognition and rejection is a common and effective anti-parasitism adaptation in bird hosts. Hosts reject alien eggs using recognition signals such as egg ground color and maculation. Green-backed tits (Parus monticolus) have a highly developed capability for recognizing parasite eggs, but the cues used for egg recognition are unclear. We combined avian visual perceptual modeling and field experiments to explore the effects of egg ground color and maculation on the recognition of parasite eggs. P. monticolus uses a multi-cue with possible hierarchical mechanism for egg recognition. The mechanism of egg recognition in P. monticolus appears to be a specific adaptation to the phenotypes of eggs laid by local brood parasites. [ABSTRACT FROM AUTHOR]

Brood parasites relegate all parental duties to unrelated hosts. Host resistance against brood parasitism is most effective during egg laying and is best countered by surreptitious oviposition. This may be aided through distraction of host attention by the male partner or a larger cooperative group. Cuckoo catfish (Synodontis multipunctatus) parasitize the broods of mouthbrooding cichlids, which collect their eggs immediately after oviposition. Cuckoo catfish must time their intrusion precisely, as the temporal window for parasitism lasts only a few seconds. As the cuckoo catfish typically intrude host spawning as a group, we tested whether groups of catfish distract spawning cichlid pairs more successfully than a single catfish pair. We found that larger catfish groups were not more effective in parasitism, as parasitism success by groups of three catfish pairs increased only proportionally to single catfish pairs. The number of cichlid eggs in host clutches decreased at high catfish abundance, apparently due to elevated cuckoo catfish predation on the eggs. Hence, group intrusions do not represent cooperative actions, but incur an increased cost to the host cichlid from greater egg predation by cuckoo catfish. [ABSTRACT FROM AUTHOR]

Modern genetic parentage methods reveal that alternative reproductive strategies are common in both males and females. Under ideal conditions, genetic methods accurately connect the parents to offspring produced by extra‐pair matings or conspecific brood parasitism. However, some breeding systems and sampling scenarios present significant complications for accurate parentage assignment. We used simulated genetic pedigrees to assess the reliability of parentage assignment for a series of challenging sampling regimes that reflect realistic conditions for many brood‐parasitic birds: absence of genetic samples from sires, absence of samples from brood parasites and female kin‐structured populations. Using 18 microsatellite markers and empirical allele frequencies from two populations of a conspecific brood parasite, the wood duck (Aix sponsa), we simulated brood parasitism and determined maternity using two widely used programs, cervus and colony. Errors in assignment were generally modest for most sampling scenarios but differed by program: cervus suffered from false assignment of parasitic offspring, whereas colony sometimes failed to assign offspring to their known mothers. Notably, colony was able to accurately infer unsampled parents. Reducing the number of markers (nine loci rather than 18) caused the assignment error to slightly worsen with colony but balloon with cervus. One potential error with important biological implications was rare in all cases—few nesting females were incorrectly excluded as the mother of their own offspring, an error that could falsely indicate brood parasitism. We consider the implications of our findings for both a retrospective assessment of previous studies and suggestions for best practices for future studies. [ABSTRACT FROM AUTHOR]

Egg rejection is the most effective and widespread defense used by host species to counteract the extreme fitness costs frequently imposed by obligate avian brood parasites. Yet, the proximate mechanisms underlying between- and within-individual variation in host responses remain poorly explored. Emerging evidence suggests that egg rejection is dependent on individual physiological states, and draws attention to the role of hormones as mediators of flexible antiparasitic responses. In this perspective article, I outline recent advances in our understanding of the proximate factors that mediate egg rejection. I also point out some areas where knowledge remains still lacking, especially those related to the development and maintenance of effective cognitive functions, the potential role of oxidative stress, immunological state, and developmental stressors. I propose new hypotheses that stimulate future research on behavioral host responses toward brood parasitism. [ABSTRACT FROM AUTHOR]

Coevolutionary interactions between avian brood parasites and their hosts often lead to the evolution of discrimination and rejection of parasite eggs or chicks by hosts based on visual cues, and the evolution of visual mimicry of host eggs or chicks by brood parasites. Hosts may also base rejection of brood parasite nestlings on vocal cues, which would in turn select for mimicry of host begging calls in brood parasite chicks. In cuckoos that exploit multiple hosts with different begging calls, call structure may be plastic, allowing nestlings to modify their calls to match those of their various hosts, or fixed, in which case we would predict either imperfect mimicry or divergence of the species into host-specific lineages. In our study of the little bronze-cuckoo (LBC) Chalcites minutillus and its primary host, the large-billed gerygone Gerygone magnirostris , we tested whether: (1) hosts use nestling vocalizations as a cue to discriminate cuckoo chicks; (2) cuckoo nestlings mimic the host begging calls throughout the nestling period; and (3) the cuckoo begging calls are plastic, thereby facilitating mimicry of the calls of different hosts. We found that the begging calls of LBCs are most similar to their gerygone hosts shortly after hatching (when rejection by hosts typically occurs) but become less similar as cuckoo chicks get older. Begging call structure may be used as a cue for rejection by hosts, and these results are consistent with gerygone defenses selecting for age-specific vocal mimicry in cuckoo chicks. We found no evidence that LBC begging calls were plastic. [ABSTRACT FROM AUTHOR]

Brood parasites such as the common cuckoo Cuculus canorus exploit the parental abilities of their hosts, hosts avoid brood parasitism and predation by showing specific behavior such as loss of feathers, emission of fear screams and contact calls, displaying wriggle behavior to avoid hosts or potential prey, pecking at hosts and prey, and expressing tonic immobility (showing behavior like feigning death or rapid escape from predators and brood parasites). These aspects of escape behavior are consistent for individuals but also among sites, seasons, and years. Escape behavior expressed in response to a broad range of cuckoo hosts and prey are consistently used against capture by humans, but also hosts and brood parasites and predators and their prey. An interspecific comparative phylogenetic analysis of escape behavior by hosts and their brood parasites and prey and their predators revealed evidence of consistent behavior when encountering potential parasites or predators. We hypothesize that personality axes such as those ranging from fearfulness to being bold, and from neophobic to curiosity response in brood parasites constitute important components of defense against brood parasitism that reduces the overall risk of parasitism. [ABSTRACT FROM AUTHOR]

Coevolutionary arms races between brood parasites and hosts provide tractable systems for understanding antagonistic coevolution in nature; however, little is known about the fate of frontline antiparasite defenses when the host "wins" the coevolutionary arms race. By recreating bygone species interactions, using artificial parasitism experiments, lingering defensive behaviors that evolved in the context of parasitism can be understood and may even be used to identify the unknown agent of parasitism past. Here we present the first study of this type by evaluating lingering "frontline" nest defenses that have evolved to prevent egg laying in a former brood parasite host. The Australian reed warbler Acrocephalus australis is currently not parasitized but is known to exhibit fine-tuned egg discrimination—a defensive behavior indicative of a past brood parasite–host arms race and common in closely related parasitized species. Here, using 3D-printed models of adult brood parasites, we examined whether the Australian reed warbler also exhibits frontline defenses to adult brood parasites, and whether we could use these defenses to identify the warbler's "ghost of parasitism past." Our findings provide evidence that the Australian reed warbler readily engages in frontline defenses that are considered adaptive specifically in the context of brood parasitism. However, individuals were unable to discriminate between adults of different brood parasite species at their nest. Overall, our results demonstrate that despite a relaxation in selection, defenses against brood parasitism can be maintained across multiple stages of the host's nesting cycle, and further suggest that, in accordance with previous findings, that learning may be important for fine-tuning frontline defense. [ABSTRACT FROM AUTHOR]

Obligate brood parasitism is associated with huge reproduction costs, forcing hosts to evolve various anti-parasitic strategies against brood parasites, among which egg recognition and rejection is the most effective defense strategy. According to the crypsis hypothesis, non-mimetic yet cryptic eggs in a nest can also deceive their hosts and eventually be accepted. To validate this hypothesis, we conducted field experiments on Oriental reed warblers (Acrocephalus orientalis), a common host for common cuckoos (Cuculus canorus). We firstly tested the egg recognition and rejection abilities of Oriental reed warblers, using black and white model eggs in natural nests. Then we designed a comparison test where the cryptic effects of the two groups of experimental eggs were different. We manipulated the nest lining color and added relatively cryptic and bright model eggs to test warblers' rejection behaviors against cryptic and bright foreign eggs. The results showed that warblers have strong egg recognition and rejection abilities. There is a significant tendency for warblers to prefer to peck and reject relatively distinguishable foreign eggs, which supports the crypsis hypothesis. These findings indicate that even in the host-parasite system of open nests, parasitic eggs that are cryptic enough are prevented from being discovered and rejected by the host, and thus obtain the possibility of successful parasitism. [ABSTRACT FROM AUTHOR]

Breeding- and nest-site choice is a behavioral strategy often used to counter negative interactions. Site choices before breeding prevent costs of predation and competition but have been neglected in the context of brood parasitism. For hosts of brood parasites, the earlier brood parasitism is prevented in the breeding cycle the lower the future costs. Suitable nest-sites for cavity-nesting common redstarts (Phoenicurus phoenicurus), a host of the common cuckoo (Cuculus canorus), are a limited resource, but their cavity-nesting strategy could potentially deter predators and brood parasites. We altered the entrance size of breeding cavities and investigated redstart nest-site choice and its consequences to nest predation and brood parasitism risk, although accounting for potential interspecific competition for nest sites. We set-up paired nest-boxes and let redstarts choose between 7 cm and 5 cm entrance sizes. Additionally, we monitored occupancy rates in nest-boxes with 3 cm, 5 cm, and 7 cm entrance sizes and recorded brood parasitism and predation events. We found that redstarts preferred to breed in 5 cm entrance size cavities, where brood parasitism was eliminated but nest predation rates were comparable to 7 cm entrance size cavities. Only in 3 cm cavities both, brood parasitism and predation rates were reduced. In contrast to the other cavity-nesting species, redstart settlement was lowest in 3 cm entrance size cavities, potentially suggesting interspecific competition for small entrance size cavities. Nest-site choice based on entrance size could be a frontline defense strategy that redstarts use to reduce brood parasitism. [ABSTRACT FROM AUTHOR]

Movement of the embryo is essential for musculoskeletal development in vertebrates, yet little is known about whether, and why, species vary. Avian brood parasites exhibit feats of strength in early life as adaptations to exploit the hosts that rear them. We hypothesized that an increase in embryonic movement could allow brood parasites to develop the required musculature for these demands. We measured embryo movement across incubation for multiple brood-parasitic and non-parasitic bird species. Using a phylogenetically controlled analysis, we found that brood parasites exhibited significantly increased muscular movement during incubation compared to non-parasites. This suggests that increased embryo movement may facilitate the development of the stronger musculoskeletal system required for the demanding tasks undertaken by young brood parasites. [ABSTRACT FROM AUTHOR]

Brood parasites lay their eggs in the nests of other females, thereby shifting the costs of offspring care onto others. Given that care is costly, potential hosts should evolve mechanisms to avoid brood parasitism. Meanwhile, brood parasites should evolve mechanisms to circumvent host defences. Here we investigate whether hosts or intraspecific brood parasites adjust their egg laying behaviour as a mechanism to reduce or increase the effectiveness of brood parasitism. We use the burying beetle Nicrophorus vespilloides as our study system, in which hosts and brood parasites lay their eggs in the soil around a carcass controlled by the host. To test whether females adjust their egg laying behaviour when breeding as a host or brood parasite, we used an experimental design with three treatments: hosts, where focal females bred alongside a smaller female; brood parasites, where focal females bred alongside a larger female; and controls, where focal females bred alone. We used focal females from a narrowly defined size range to control for potential effects of body size. We found that hosts delayed the start of egg laying, which may allow them to recognise brood parasitic offspring that arrive too early. Meanwhile, brood parasites laid their eggs over an extended period, which may increase the chances that their egg laying overlapped with the host. Our results suggest that adjusting egg laying behaviour is a mechanism used by both hosts and brood parasites that may contribute to the differences in reproductive success shown in prior studies. [ABSTRACT FROM AUTHOR]

Generalist avian brood parasites commonly serve as a model system to test for plasticity in offspring growth and behavior under various host environmental and parental regimes. While past research has provided compelling evidence that developmental rates differ among hosts, the fitness consequences of such variation remain unclear. In hosts, carryover effects from the nesting to the post-fledging stage are critical in driving differential post-fledging survival within and among species, as well as the evolution of songbird life histories. Consequently, offspring of brood parasites may be subject to the same carryover effects and life history constraints observed in host species. Applying a pre- to post-fledging carryover effect framework to avian brood parasites may therefore provide novel insights into parasite life histories as well as parasite-host interactions. We assessed the potential influence of host carryover effects on the development and survival of juvenile cowbirds using empirical data from 10 realized host species in east-central Illinois, USA. We found that life history variation among juvenile cowbirds reflects variation in tradeoffs between mortality risk and nestling period length as well as pre- to post-fledging carryover effects found across host species. Cowbirds from host nests with higher mortality rates had shorter nestling periods, fledged with less developed wings, and exhibited higher rates of post-fledging mortality. Cowbird mass at fledging also predicted post-fledging survival, but was not associated with other life history traits. Our results provide novel links between juvenile growth, development, and mortality that help to explain differential survival in parasitic young across host species. Significance statement: Offspring of avian brood parasites develop at different rates based on the host environment in which they are raised, but the fitness consequences of such plasticity are poorly understood. Parasitic offspring may be subject to carryover effects from the nesting to the post-fledging stages and other life history constraints operating in host systems. We assessed the potential influence of host carryover effects on the development and survival of juvenile brown-headed cowbirds (Molothrus ater) using empirical data from 10 realized host species. Reflecting pre- to post-fledging carryover effects found across host species, cowbirds from host nests with higher mortality rates had shorter nestling periods, fledged with less developed wings, and exhibited higher rates of post-fledging mortality. Our results provide novel links between juvenile growth, development, and mortality that help to explain differential survival in parasitic young across host species. [ABSTRACT FROM AUTHOR]

The Asian koel (Eudynamys scolopacea) is a well-known brood parasitic bird that visits Pakistan every summer from India. This species produces at least five loud call types that are presumed to be used in territorial and mating behaviour. We noticed vocal differences among individuals in the wild and hypothesised that Asian koels produce individually distinct vocalizations. To test this, we recorded type 1 cooee calls, coe calls, type 1 coegh calls, type 2 coegh call and wurroo calls from 46 adult birds in the wild, and measured 15 different acoustic parameters, both spectral and temporal, for each vocalization type. We also measured the Potential of Individual Coding (PIC). Parameters having PIC values ≥1.1 were further analysed through stepwise cross-validated discriminant function analysis (DFA). The DFA accurately categorised 86.5% of the type 1 cooee calls, 95.7% of coe calls, 91.5% of the type 1 coegh calls, 82.2% of type 2 coegh calls and 83.3% of wurroo calls to the individuals from which these calls were recorded. Thus, consistent with our hypothesis, our results indicate that Asian koels produce individually distinct calls. [ABSTRACT FROM AUTHOR]

Obligate avian brood parasites depend on hosts for parental care, which in turn suffer fitness losses as a result of parasitism. Mechanisms by which brood parasitic cowbirds (Molothrus spp.) reduce host breeding success include the puncture (M. rufoaxillaris and M. bonariensis) or removal (M. ater) of the eggs of the host. Our working hypothesis is that the host eggs' mechanical strength and their size and shape in species with higher frequency of parasitism covaries with the cowbird's strategy to reduce host clutch size. Our results, obtained through phylogenetic analyses based on egg 2D geometric morphometry and eggshell mechanical and ultrastructural measurements, suggest that egg-puncturer behaviour has led to an increase in the strength of the host's eggshell, which might make them more difficult to be pierced. We also characterized larger, more rounded and asymmetrical eggs in frequent hosts of M. ater , which might be more difficult to be removed. These interspecific host egg and shell traits were also positively affected by the frequency of parasitism, indicating that species-specific patterns of parasitic costs select for respective anti-parasitic defences in hosts. [ABSTRACT FROM AUTHOR]

Avian brood parasitism is costly for the host, in many cases leading to the evolution of defenses like discrimination of parasitic eggs. The parasite, in turn, may evolve mimetic eggs as a counter-adaptation to host egg rejection. Some generalist parasites have evolved host-specific races (gentes) that may mimic the eggs of their main hosts, while others have evolved 'jack-of-all-trades' egg phenotypes that mimic key features of the eggs of several different host species. The Asian koel (Eudynamys scolopaceus) is a widely distributed generalist brood parasite that exploits a wide range of host species. Based on human vision, previous studies have described Asian koel eggs as resembling those of its main host, the house crow (Corvus splendens). Using measurements of egg length and breadth, digital image analysis, reflectance spectrophotometry and avian visual modelling, we examined Asian koel egg variation and potential mimicry in egg size and shape, and eggshell pattern and color in three sympatrically occurring host species in Bangladesh: the common myna (Acridotheres tristis), house crow, and long-tailed shrike (Lanius schach). We found some differences among Asian koel eggs laid in different host nests: a) Asian koel eggs in long-tailed shrike nests were larger than those laid in common myna and house crow nests, and b) Asian koel eggs in house crow nests were less elongated than those in common myna nests. However, these changes in Asian koel egg volume and shape were in the opposite direction with respect to their corresponding host egg characteristics. Thus, our study found no evidence for Asian koel host-specific egg mimicry in three sympatrically occurring host species. [ABSTRACT FROM AUTHOR]

Multiple parasitism in obligate avian brood parasites occurs when several brood parasitic females lay their eggs in the nest of the same host. While multiple parasitism is common in the highly social, nonevicting cowbird species (Molothrus sp.), in which multiple parasitic nestlings can be raised simultaneously by the same hosts, it is less common in the case of cuckoo species (Cuculus sp.). The first cuckoo nestling to hatch from the egg evicts all nestmates; therefore, it is costly for cuckoo females to lay eggs in already parasitized nests. However, this can occur in sites with very high parasitism rates, and it can even increase the breeding success of the brood parasites, as the presence of multiple parasitic eggs in the nest of the host decreases rejection rates. Here, we present a case of a quintuple brood parasitism of a great reed warbler (Acrocephalus arundinaceus) nest, an extreme form of multiple brood parasitism. [ABSTRACT FROM AUTHOR]

Brood parasitism is the introduction of unrelated progeny into the nest or colony of a host that then raises the foreign young. This reproductive strategy has evolved independently and repeatedly among diverse animal taxa, and brood parasite–host interactions have become models for understanding coevolutionary arms races. Yet brood parasites have remained largely overlooked in previous syntheses of natural enemy ecology. Here, we argue that brood parasites are a heterogeneous and versatile class of natural enemies, blending traits characteristic of predators and trophic parasites. The functional distinctness of brood parasites reinforces the idea that natural enemies exist along a continuum rather than as a dichotomy. Brood parasite–host interactions can serve as valuable case studies to unify parasite–host and predator–prey theories. Adult brood parasites usurp the parental care of their hosts, which suffer fitness losses when raising brood parasitic progeny. Brood parasites have been largely overlooked in previous theoretical and empirical syntheses of natural enemy ecology of trophic parasites versus predators. We characterize brood parasites as a diverse and heterogeneous group of organisms that blend traits characteristic of both predators and trophic parasites. Brood parasites are thus functionally distinct from predators and trophic parasites with respect to a broad array of their own life-history traits and impacts on hosts. Brood parasites can serve to further unify natural enemy ecology and promote integration of parasite–host and predator–prey theories. [ABSTRACT FROM AUTHOR]

Understanding egg-laying behaviour of brood parasites in space and time can improve our knowledge of interactions between hosts and parasites. However, no studies have combined information on the laying activity of an obligate brood parasite with detailed information on the distribution of host nests within an area and time period. Here, we used molecular methods and analysis of egg phenotypes to determine maternal identity of common cuckoo, Cuculus canorus , eggs and chicks found in the nests of four species of Acrocephalus warblers in consecutive years. The median size of a cuckoo female laying area (calculated as a minimum convex polygon) was correlated negatively with the density of host nests and positively with the number of eggs assigned to a particular female. Cuckoo female laying areas overlapped to a large extent and their size and location did not change between years. Cuckoo females preferentially parasitized host nests located close to their previously parasitized nests and were mostly host specific except for two that parasitized two host species. Future studies should focus on sympatric host and parasite communities with variable densities across different brood-parasitic systems to investigate how population density of hosts affects fitness and evolution of brood parasites. For instance, it remains unknown whether female parasites moving to new sites need to meet a threshold density of a potential host. In addition, young females may be more limited in their egg laying, particularly with respect to the activity of other parasites and hosts, than older females. • Cuckoo egg-laying areas varied in size and overlapped to a large extent. • Cuckoo egg-laying areas were smaller when density of host nests was higher. • Size and location of laying areas remained similar between years. • Cuckoos laid eggs in host nests closest to previously parasitized nests. • Majority of cuckoos parasitized only one host species. [ABSTRACT FROM AUTHOR]

Avian brood parasites pose a serious threat to hosts, substantially reducing their fitness, which selects for the evolution of host defences. A classic example of a host frontline defence is mobbing, which frequently includes contact attacking of brood parasites. Here, we investigated how the nest defence of a very aggressive host, the Great Reed Warbler Acrocephalus arundinaceus, influences the speed of egg‐laying and egg‐removing behaviour of its brood parasite – the Common Cuckoo Cuculus canorus. We video‐recorded 168 brood parasitic events at 102 active host nests and found that Cuckoos avoided host mobbing in 62% of cases. If hosts spotted the Cuckoo at their nests, they almost always attacked it (91 of 104 cases; 88%); however, such attacks only rarely and temporarily prevented Cuckoos from parasitizing (11 additional cases). When attacked, Cuckoos parasitized host nests significantly faster and left them immediately after laying. However, when not attacked, Cuckoos frequently stayed at or near the nest, suggesting that host aggression, rather than the risk of being spotted, influences the speed of brood parasitism in this species. Further, we found that Cuckoos performed egg‐removing behaviour in all parasitic events without regard to host aggression. As a result, Cuckoos removed at least one egg during all brood parasitism events except those when the egg slipped from their beaks when they were attacked by hosts and, thus, remained in the nest (in nine of 82 cases when they were attacked; 10.9%). This indicates that egg‐removing behaviour is not costly for the Common Cuckoo and is an essential part of its parasitism strategy, widening our understanding of this elusive behaviour. [ABSTRACT FROM AUTHOR]

In host–parasite arms races, hosts can evolve signatures of identity to enhance the detection of parasite mimics. In theory, signatures are most effective when within-individual variation is low ('consistency'), and between-individual variation is high ('distinctiveness'). However, empirical support for positive covariation in signature consistency and distinctiveness across species is mixed. Here, we attempt to resolve this puzzle by partitioning distinctiveness according to how it is achieved: (i) greater variation within each trait, contributing to elevated 'absolute distinctiveness' or (ii) combining phenotypic traits in unpredictable combinations ('combinatorial distinctiveness'). We tested how consistency covaries with each type of distinctiveness by measuring variation in egg colour and pattern in two African bird families (Cisticolidae and Ploceidae) that experience mimetic brood parasitism. Contrary to predictions, parasitized species, but not unparasitized species, exhibited a negative relationship between consistency and combinatorial distinctiveness. Moreover, regardless of parasitism status, consistency was negatively correlated with absolute distinctiveness across species. Together, these results suggest that (i) selection from parasites acts on how traits combine rather than absolute variation in traits, (ii) consistency and distinctiveness are alternative rather than complementary elements of signatures and (iii) mechanistic constraints may explain the negative relationship between consistency and absolute distinctiveness across species. [ABSTRACT FROM AUTHOR]

Obligate avian brood parasites lay their eggs in the nest of other species and impose reproductive costs upon their hosts. In response, many hosts have evolved a variety of defenses to avoid or reduce the costs associated with parasitism, including the rejection of parasitic eggs. Here, we review the different sensory and contextual cues involved in egg rejection by the American robin (Turdus migratorius), one of the handful of rejecter host species of the brood parasitic brown‐headed cowbird (Molothrus ater) in North America. Robins are an open‐cup nesting species whose eggs dramatically differ in appearance from the cowbirds' eggs; therefore, we expect that visual cues would be the primary sensory modality used when rejecting foreign eggs. The results of our comprehensive review confirm that visual cues, including eggshell color and maculation, are strong predictors of robins' egg rejection responses. Egg size, egg shape, and other contextual traits (including clutch composition, maternal status, and sympatry with the parasite) also affect egg rejection in this host species. Future research into this species' egg rejection mechanisms would benefit from comparisons to other strongly and moderately egg rejecter host species of brown‐headed cowbirds, as well as from seeking to understand further why so many hosts of this particular parasitic lineage accept foreign eggs. We also discuss the implications of this review for other egg rejecter and non‐rejecter hosts of diverse avian brood parasites. [ABSTRACT FROM AUTHOR]

Conspecific brood parasitism (CBP) has been observed in approximately half of all species of waterfowl, a philopatric group in which breeding females are frequently locally related. It has been suggested that kin selection can facilitate the evolution of CBP in waterfowl via fitness benefits for the host and parasite. One model demonstrates that discrimination of related and unrelated parasites by the host must be sufficient for kinship to promote CBP, provided that costs of brood parasitism to host fitness are sufficiently low. We parameterized the model using demographic data and behavioural observations from a population of colonial Red-breasted Mergansers (Mergus serrator Linnaeus, 1758) in which 47% of nests were parasitized by conspecifics. The costs of 1–3 foreign eggs to host hatching success were generally small (decline of 1.8% per additional egg). Nevertheless, model outputs revealed that brood parasites maximize their inclusive fitness by avoiding nests of relatives, primarily because of constraints on a host's ability to detect parasites at the nest. Indeed, hosts spent <8% of the diurnal period at the nest during egg laying, a period when parasite activity is greatest. It is thus highly unlikely that relatedness and kin selection promote brood parasitism in this population. [ABSTRACT FROM AUTHOR]

Nest predation and avian brood parasitism are the main sources of nest failure in many passerine birds. Large predators threaten both brood and parents, whereas brood parasites pose only a danger to eggs or nestlings. The fan-tailed gerygone Gerygone flavolateralis from New Caledonia is subjected to high rates of nest predation by the New Caledonian crow Corvus moneduloides (responsible for about 20–40% of predation) and moderate rates of brood parasitism by the shining bronze-cuckoo Chalcites lucidus (parasitizing about 18% of nests), which also depredates nests that are too advanced for parasitism (13% of nests). To test if fan-tailed gerygones are able to discriminate predators from brood parasites, we presented 3 bird models at active gerygone nests: a brood parasite/small nest predator (shining bronze-cuckoo), a large nest predator (crow), and a small non-native bird (common chaffinch Fringilla coelebs), which is unknown to the gerygone, as a control. We assessed the response of adult gerygones to the presentation of each model by measuring the minimum approach distance, number of alarm calls, number of attacks, and time to first nest visit after the presentation (latency). Adult gerygones often attacked the cuckoo, approached but never attacked the chaffinch and always avoided the crow. Latency was shorter after an attack response and during brooding, but similar among models. We did not find any link between the cuckoo model presentation and later ejection of cuckoo nestlings. We conclude that adult fan-tailed gerygones discriminate between different models and respond accordingly to the level of threat, but do not show awareness of parasitism risk and increase of nestling ejection rates following exposure to the cuckoo model. [ABSTRACT FROM AUTHOR]