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and elsewhere. A lighter prose is encouraged and no summary is required. Formal research papers, however short, will not be considered.Like many ecologists we were intrigued by Tomas Grim’s original and thought-provoking evaluation of professional achievement amongst his Bohemian ecologist colleagues in your sister journal, Oikos (Grim 2008). In his paper, Grim argues and infers from a correlational analysis that publication success is negatively affected by beer drinking – an assumed correlate of ‘social activity’ – due to nega -tive effects of alcohol on cognitive performance. Here we question Grim’s conclusions. We criticize his focus on a single hypothesis, without consideration of reasonable al-ternatives, and note that his approach provides a valuable illustration of a more general flaw in ecological inference.Grim correlated publication success of Czech ecologists to their self-reported beer consumption. We note short-falls in this approach. We could, for example, quibble with Grim’s measures of publication success (Lortie et al. 2007), note the problems of using self-reported drinking as a measure of true alcohol consumption (Nevitt and Lundak 2005), question his neglect of gender differences (Bailly et al. 1991) and debate whether heavier drinkers really are involved in more social or anti-social behaviour (Ogle and Miller 2004). We might also note the paper focuses on just 10% of the variation in publication success – that part not explained by each ecologist’s age and duration of publica -tion career. However, this would be nit-picking. Our main concern is with Grim’s pessimism-by-default regarding the impending obscurity of drinking ecologists. As we justify below, there are good reasons to be more optimistic.

Climate change causes altitudinal shifts of animal distributions and this effect can potentially be accentuated or altered due to human agricultural activities. Because of the availability of uniquely long-term monitoring data, we chose the white stork (Ciconia ciconia) as a model species. We analyzed large data sets (record cards from 1191 nests) covering the long-term period from 1875 to 2005 in the Czech Republic, central Europe. We analyzed nest altitude during years of founding of the nest, temperature, land use variables and types of nests (natural vs. human provided nest pads). Consistent with findings from previous studies we predicted a temporal increase in the altitude of white stork nests. Surprisingly, we found that the altitude of nests was decreasing, despite an increase in local mean spring temperatures. The altitude of nests was higher when the proportion of arable land, water areas and developed land was lower and when the proportion of grass and forest cover was greater. The decrease in altitude was significant in natural nests (built by storks) but non-significant in nests on nest platforms (built by humans). Thus, human agricultural activities might potentially override the opposing effects of global climate change on animal altitudinal distribution shifts.

Brood parasite–host interactions during the incubation and nestling stages have been well studied, but the post‐fledging period remains virtually unknown. Using radiotracking, we provide the first detailed data on post‐fledging interactions between the Common Cuckoo Cuculus canorus and its only regular cavity‐nesting host, the Common Redstart Phoenicurus phoenicurus. Cuckoos raised alone (‘solitary’) fledged at higher mass, with higher wing and tarsus length and started to fly at a younger age than Cuckoos raised alongside young Redstarts (‘mixed’). However, a further 23 fledging and post‐fledging parameters measured at five pre‐determined times (fledging, first‐flight, predation, starvation, independence) did not differ between solitary and mixed Cuckoos. In addition, none of the parameters measured during the post‐fledging period (growth, dispersal distances, number of flights) differed between solitary and mixed Cuckoos. Redstart fledglings from non‐parasitized broods (‘solitary’) showed generally similar fledging and post‐fledging parameters to fledglings reared alongside a Cuckoo (‘mixed’). Surprisingly, there were no significant differences in post‐fledging predation rate, starvation or overall survival rates between mixed and solitary Cuckoos or mixed and solitary Redstarts. Thus, during the post‐fledging period, mixed Cuckoo fledglings successfully compensated for the poorer performance experienced during the nestling stage whereas mixed and solitary Redstarts did not differ in any measured parameters. This suggests that the regular occurrence of mixed broods in this host–parasite system – which is unique among the many Cuckoo hosts – is evolutionarily stable for both hosts and parasites.

Birds are hosts for several zoonotic pathogens. Because of their high mobility, especially of longdistance migrants, birds can disperse these pathogens, affecting their distribution and phylogeography. We focused on Borrelia burgdorferi sensu lato, which includes the causative agents of Lyme borreliosis, as an example for tick-borne pathogens, to address the role of birds as propagation hosts of zoonotic agents at a large geographical scale. We collected ticks from passerine birds in 11 European countries. B. burgdorferi s.l. prevalence in Ixodes spp. was 37% and increased with latitude. The fieldfare Turdus pilaris and the blackbird T. merula carried ticks with the highest Borrelia prevalence (92 and 58%, respectively), whereas robin Erithacus rubecula ticks were the least infected (3.8%). Borrelia garinii was the most prevalent genospecies (61%), followed by B. valaisiana (24%), B. afzelii (9%), B. turdi (5%) and B. lusitaniae (0.5%). A novel Borrelia genospecies “Candidatus Borrelia aligera” was also detected. Multilocus sequence typing (MLST) analysis of B. garinii isolates together with the global collection of B. garinii genotypes obtained from the Borrelia MLST public database revealed that: (a) there was little overlap among genotypes from different continents, (b) there was no geographical structuring within Europe, and (c) there was no evident association pattern detectable among B. garinii genotypes from ticks feeding on birds, questing ticks or human isolates. These findings strengthen the hypothesis that the population structure and evolutionary biology of tick-borne pathogens are shaped by their host associations and the movement patterns of these hosts., This study received financial support from Fundação para a Ciência e a Tecnologia by the strategic program of MARE (MARE ‐ UID/MAR/04292/2013) and the fellowship to Ana Cláudia Norte (SFRH/BPD/108197/2015), and the Portuguese National Institute of Health. Raivo Mänd, Tomi Trilar, Tapio Eeva, Tomas Grim and Dieter Heylen were supported by the Estonian Research Council (research grant # IUT34‐8), the Slovenian Research Agency ‐programme “Communities, relations and communications in the ecosystems” (No. P1‐0255), the Academy of Finland (project 265859), the Internal Grant Agency of Palacky University (PrF_2014_018, PrF_2015_018, PrF_2013_018) and the Marie Sklodowska‐Curie Actions (EU‐Horizon 2020, Individual Global Fellowship, project no 799609), respectively.

In contrast to the thoroughly studied incubation and nestling periods, the postfledging period of the Common Cuckoo (Cuculus canorus) remains virtually unknown. Here, we report detailed observations of a cuckoo fledgling attended by a male Chaffinch (Fringilla coelebs). Molecular data (nuclear DNA) showed the fledgling was a male belonging to the Cuculus c. canorus/C. saturatus clade while mitochondrial DNA data confirmed that it did not belong to blue egg gens that parasitizes Redstarts (Phoenicurus phoenicurus), which is the most common local host and the only regular Common Cuckoo host. During one week of observations, feeding rates did not change, body mass decreased (by 10%), and wing length increased (by 16%). Video recordings showed that the provided diet consisted mostly of larvae and that the fledgling also self-fed on lichens. A radio transmitter fitted on the fledgling revealed that daily movement distances ranged from 0 to 650 m and significantly increased with age. We suggest that fu...

Research on brood parasitism has focused primarily on specific host anti-parasite behaviours and parasite counter-adaptations, and little is known about other aspects of their behaviours such as consistent behavioural differences between individuals. Therefore, we examined consistency in behaviour of nestlings of common cuckoos (Cuculus canorus) raised by great reed warblers (Acrocephalus arundinaceus). Cuckoo chicks showed high repeatability of both aggressive behaviour and breath rate, and both traits were strongly correlated with each other. This represents the first evidence for consistent differences in behaviour among avian brood parasites. Males were consistently more aggressive and less stressed than females. Nestlings of both sexes that hatched later in the season exhibited higher levels of aggression and lower stress responses than nestlings hatched earlier. This suggests that rearing conditions (e.g., food availability and quality) may modulate stress and aggressive phenotypes of brood parasites. We discuss potential effects of the observed patterns on host-parasite dynamics.

Avian brood parasitism provides a tractable system within which to study diverse aspects of animal ecology and evolution. Yet, parasite–host research has focused on specific adaptations and counter-adaptations, such as egg rejection by hosts and egg mimicry by parasites, leaving other aspects of these relationships poorly studied, including general life history traits of hosts that did not evolve as specific antiparasite defenses. In particular, the diet and fledging parameters (age, mass, success) of parasitic nestlings are poorly known, although they are central to our understanding of host selection and the potential for coevolution in parasite–host relationships. We focused on the diet composition and fledging parameters of parasitic Common Cuckoo (Cuculus canorus) nestlings raised by their only regular cavity-nesting host, the Common Redstart (Phoenicurus phoenicurus). In addition to invertebrates, both Common Cuckoo and Common Redstart nestlings were fed fruits and lizards by some host pair...

Conspecific brood parasitism (CP) is a facultative breeding tactic whereby females lay their eggs in the nests of conspecifics. In some species, potential hosts have evolved the ability to identify and reject foreign eggs from their nest. Previous studies suggest that the ubiquitous House Sparrow Passer domesticus in Spain and South Africa employs both CP and egg rejection, while a population in China does not. Given the species' invasive range expansions, the House Sparrow represents a potentially excellent global model system for parasitic egg rejection across variable ecological conditions. We examined the responses of House Sparrows to experimental parasitism at three geographically distinct locations (in Israel, North America, and New Zealand) to provide a robust test of how general the findings of the previous studies are. In all three geographic regions egg rejection rates were negligible and not statistically different from background rates of disappearance of control eggs, suggesting that the House Sparrow is not a suitable model species for egg rejection experiments on a global scale. This article is protected by copyright. All rights reserved.

Generalist brood parasites, like Common Cuckoos Cuculus canorus, target many host species. Why other sympatric hosts are not used, or actively avoided, remains one of the main gaps in our understanding of parasite-host coevolution. Cavity nesting passerines always represented a text-book example of unsuitable hosts but recent evidence casts multiple doubts on this traditional view. In general, any species can become an unsuitable host for a parasite at laying, incubation, or nestling stages with the last one being much less studied than the others. Therefore we examined Cuckoo chick performance in five cavity nesting host species, including one regular Cuckoo host — the Common Redstart Phoenicurus phoenicurus and four non-hosts: the Pied Flycatcher Ficedula hypoleuca, Spotted Flycatcher Muscicapa striata, Great Tit Parus major, and Coal Tit Periparus ater. Natural nests of non-hosts, as opposed to artificial nest boxes with small entrance holes, are often placed in cavities that show both entrance and inner cavity sizes large enough for female Cuckoos to lay and Cuckoo chicks to fledge. We did not find any evidence for chick discrimination in non-hosts, i.e., no chicks were rejected, attacked, or neglected. Cuckoo chicks grew similarly in nests of all four species of non-hosts, similarly to chicks in host Redstart nests, and generally better than in nests of the most numerous Cuckoo host, the Reed Warbler Acrocephalus scirpaceus. Although Cuckoo chick fledging mass was highly host speciesspecific (i.e., showed high statistical repeatability across various host species), we did not find any evidence for the hypothesis that host body size (mass) positively affects parasite chick growth (fledging mass or age). These findings provide impetus to further study apparently unsuitable hosts and perhaps even reconsider traditional classifications of host suitability in the context of brood parasite-host coevolution.

One of the most effective defenses against avian brood parasitism is the rejection of the foreign egg from the host’s nest. Until recently, most studies have tested whether hosts discriminate between own and foreign eggs based on their absolute differences in avian-perceivable eggshell coloration and maculation. However, recent studies suggest that hosts may instead contrast egg appearances across a directional eggshell color gradient. We assessed which discrimination rule best explained egg rejection by great reed warblers Acrocephalus arundinaceus , a frequent host to an egg-mimetic race of common cuckoos Cuculus canorus . We deployed 3D-printed model eggs varying in blue-green to brown coloration and in the presence of maculation. Using visual modeling, we calculated the absolute chromatic and achromatic just-noticeable differences (JNDs), as well as directional JNDs across a blue-green to brown egg color gradient, between host and model eggs. While most model eggs were rejected by great reed warblers, browner eggs were rejected with higher probability than more blue-green eggs, and the rejection probability did not depend on maculation. Directional egg color discrimination shown here and in a suite of recent studies on other host species may shape the cognitive decision rules that hosts use to recognize foreign eggs and affect the course of evolution in parasitic egg mimicry.

Common cuckoos Cuculus canorus are obligate brood parasites that lay their eggs in the nests of other (host) species. To increase the likelihood of successful parasitism, common cuckoos lay eggs with thicker and structurally stronger eggshells than those of their hosts and non-parasitic relatives. Although hatching from thicker eggshells requires greater effort and may impose physiological costs on cuckoo embryos during hatching, it is unclear whether cuckoo eggshells are indeed thicker at the time of hatching. This is because avian embryos decalcify the innermost eggshell layer (mammillary layer) for organ development during embryogenesis, reducing eggshell thickness and making hatching easier. Therefore, common cuckoo eggshells may undergo a greater degree of decalcification during embryonic development to facilitate hatching from an initially thicker shelled egg. We used scanning electron microscopy to test this hypothesis by comparing the thickness and degree of decalcification of eggshells collected either before incubation or after hatching. We found that cuckoo eggshells undergo similar degrees of decalcification during embryonic development as the thinner eggshells of a host that lays similarly sized eggs, the great reed warbler Acrocephalus arundinaceus. Cuckoo eggshells hence remain thicker than eggshells of this host throughout embryogenesis, supporting the predicted trade-off between the benefits of laying puncture-resistant eggs and the physiological costs associated with hatching from thick shelled eggs.

Evolutionary hypotheses regarding the function of eggshell phenotypes, from solar protection through mimicry, have implicitly assumed that eggshell appearance remains static throughout the laying and incubation periods. However, recent research demonstrates that egg coloration changes over relatively short, biologically relevant timescales. Here, we provide the first evidence that such changes impact brood parasite–host eggshell color mimicry during the incubation stage. First, we use long‐term data to establish how rapidly the Acrocephalus arundinaceus Linnaeus (great reed warbler) responded to natural parasitic eggs laid by the Cuculus canorus Linnaeus (common cuckoo). Most hosts rejected parasitic eggs just prior to clutch completion, but the host response period extended well into incubation (~10 days after clutch completion). Using reflectance spectrometry and visual modeling, we demonstrate that eggshell coloration in the great reed warbler and its brood parasite, the common cuckoo, changes rapidly, and the extent of eggshell color mimicry shifts dynamically over the host response period. Specifically, 4 days after being laid, the host should notice achromatic color changes to both cuckoo and warbler eggs, while chromatic color changes would be noticeable after 8 days. Furthermore, we demonstrate that the perceived match between host and cuckoo eggshell color worsened over the incubation period. These findings have important implications for parasite–host coevolution dynamics, because host egg discrimination may be aided by disparate temporal color changes in host and parasite eggs.

Virulent brood parasites refrain from arduous parental care, often kill host progeny and inflict rearing costs upon their hosts. Quantifying the magnitude of such costs across the whole period of care (from incubation through to parasite fledgling independence) is essential for understanding the selection pressures on hosts to evolve antiparasitic defences. Despite the central importance of such costs for our understanding of coevolutionary dynamics, they have not yet been comprehensively quantified in any host of any avian brood parasite. We quantified parasite-rearing costs in common redstarts Phoenicurus phoenicurus raising either parasitic common cuckoo Cuculus canorus or their own chicks throughout the complete breeding cycle, and used multiple cost parameters for each breeding stage: incubation, brooding and feeding effort; length of parental/host care; parent/host body condition; and heterophil/lymphocyte ratio (stress-level indicator). Contrary to traditional assumptions, rearing the parasite per se was not associated with overall higher physiological or physical costs to hosts above the natural levels imposed by efforts to rear their own progeny. The low parasite-rearing costs imposed on hosts may, in part, explain the low levels of known host counter-defences in this unusually frequently parasitized cuckoo host.

Animals often show correlated suites of consistent behavioural traits, i.e., personality or behavioural syndromes. Does this conflict with potential phenotypic plasticity which should be adaptive for animals facing various contexts and situations? This fundamental question has been tested predominantly in studies which were done in non-breeding contexts and under laboratory conditions. Therefore, in the present study we examined the temporal stability of behavioural correlations in a breeding context and under natural conditions. We found that in the great reed warbler (Acrocephalus arundinaceus) females, the intensity of their nest defence formed a behavioural syndrome with two other traits: their aggression during handling (self-defence) and stress responses during handling (breath rate). This syndrome was stable across the nesting cycle: each of the three behavioural traits was highly statistically repeatable between egg and nestling stages and the traits were strongly correlated with each other during both the egg stage and the nestling stage. Despite this consistency (i.e., rank order between stages) the individual behaviours changed their absolute values significantly during the same period. This shows that stable behavioural syndromes might be based on behaviours that are themselves unstable. Thus, syndromes do not inevitably constrain phenotypic plasticity. We suggest that the observed behavioural syndrome is the product of interactions between behavioural and life history trade-offs and that crucial proximate mechanisms for the plasticity and correlations between individual behaviours are hormonally-regulated.

Common Cuckoos (Cuculus canorus) are obligate brood parasites that lay their eggs in nests of other species and use these hosts to raise their parasitic offspring. Two key adaptations that increase their reproductive success are (1) the capacity for cuckoos to lay large numbers of eggs and thereby parasitize many nests per year, and (2) the ability of cuckoo eggs to hatch before those of hosts, enabling cuckoo nestlings to evict host eggs and eliminate competition for food. Producing more eggs is generally associated with reduced investment of nutrients and energy reserves per egg, which in turn is associated with shorter incubation periods both within and between species. We hypothesized that Common Cuckoos deposit reduced energy reserves into their eggs than do their hosts to facilitate both (1) and (2). To test these hypotheses, we compared the concentration of yolk lipids (per wet yolk mass) between eggs of 3 cuckoo gentes and their respective host species: Great Reed Warblers (Acrocephalus a...

Three species of brood parasites are increasingly being recorded as transoceanic vagrants in the Northern Hemisphere, including two Cuculus cuckoos from Asia to North America and a Molothrus cowbird from North America to Eurasia. Vagrancy patterns suggest that their establishment on new continents is feasible, possibly as a consequence of recent range increases in response to a warming climate. The impacts of invasive brood parasites are predicted to differ between continents because many host species of cowbirds in North America lack egg rejection defenses against native and presumably also against invasive parasites, whereas many hosts of Eurasian cuckoos frequently reject non-mimetic, and even some mimetic, parasitic eggs from their nests. During the 2014 breeding season, we tested the responses of native egg-rejecter songbirds to model eggs matching in size and color the eggs of two potentially invasive brood parasites. American Robins (Turdus migratorius) are among the few rejecters of the eggs of Brown-headed Cowbirds (M. ater), sympatric brood parasites. In our experiments, robins rejected one type of model eggs of a Common Cuckoo (C. canorus) host-race, but accepted model eggs of a second cuckoo host-race as well as robin-mimetic control eggs. Common Redstarts (Phoenicurus phoenicurus), frequent hosts of Common Cuckoos in Eurasia, rejected ∼50% of model Brown-headed Cowbird eggs and accepted most redstart-mimetic control eggs. Our results suggest that even though some hosts have evolved egg-rejection defenses against native brood parasites, the invasion of brood parasites into new continents may negatively impact both naive accepter and coevolved rejecter songbirds in the Northern Hemisphere.

Obligate avian brood parasitic species impose the costs of incubating foreign eggs and raising young upon their unrelated hosts. The most common host defence is the rejection of parasitic eggs from the nest. Both egg colours and spot patterns influence egg rejection decisions in many host species, yet no studies have explicitly examined the role of variation in spot coloration. We studied the American robin Turdus migratorius, a blue-green unspotted egg-laying host of the brown-headed cowbird Molothrus ater, a brood parasite that lays non-mimetic spotted eggs. We examined host responses to model eggs with variable spot coloration against a constant robin-mimetic ground colour to identify patterns of rejection associated with perceived contrast between spot and ground colours. By using avian visual modelling, we found that robins were more likely to reject eggs whose spots had greater chromatic (hue) but not achromatic (brightness) contrast. Therefore, egg rejection decision rules in the American robin may depend on the colour contrast between parasite eggshell spot and host ground coloration. Our study also suggests that egg recognition in relation to spot coloration, like ground colour recognition, is tuned to the natural variation of avian eggshell spot colours but not to unnatural spot colours.

The great majority of brood parasitism studies focused on a single ontogenetic stage, typically egg stage, and on open-nesting hosts, especially those of the common cuckoo. Using extensive data from the cuckoo’s only known regular cavity-nesting host, the common redstart, we highlight the importance of a comprehensive approach when all ontogenetic stages are studied. In contrast to open-nesting hosts, only minority of the cuckoo eggs are a threat to redstart hosts: most are laid outside the host nest cup and perish. Contrary to previous claims, we found that the impact of parasitism per host nest was virtually the same between this only regular cuckoo cavity-nesting host and a typical open-nesting host (the reed warbler): in both species, fitness of an average non-parasitized host nest was by an order of magnitude higher than fitness of an average parasitized host nest. This was partly because of uniquely low eviction success of cuckoo chicks and resulting cohabitation of parasite and host progeny in mixed broods. Data from post-fledging period, which remains the least known stage of parasite–host coevolution in any study system globally, were crucial because they showed that data from nestling period greatly overestimated cuckoo fitness. We suggest that metareplication of these approaches (i.e. integrative study of laying, incubation, nestling, fledgling and migration stages) across various parasite–host systems is the most important task for future coevolutionary studies in the context of brood parasite–host coevolution.

Fitness costs associated with brood parasitism have led host species to evolve several lines of defence. The first two lines of defence, aggression against adult parasites and egg rejection, are present at varying levels in almost all hosts. However, it remains unclear how these two fundamental defences covary at host individual level, with previous studies suggesting both positive and negative correlations. A theoretically critical yet empirically untested scenario is that variation in host antiparasite behaviour may relate to individual variation in host behavioural types or personalities. Here we examined whether host aggression against adult brood parasites and egg rejection behaviour were correlated with host behaviours displayed outside the context of brood parasitism. We selected the great reed warbler, Acrocephalus arundinaceus, a favourite cuckoo, Cuculus canorus, host as a suitable model. Only females reject foreign eggs and show high individual repeatability of both aggression towards cuckoos and nest guarding. We found that female behaviours in different situations (nest guarding, nest defence, handling in the net) were strongly correlated with each other. This is the first empirical evidence on correlation between individually consistent antiparasite adaptation (female nest defence) and behaviours that are not directly related to brood parasitism. In contrast, egg rejection/acceptance responses and latency to these responses did not correlate with any of the female defence/guarding behaviours and behaviour during handling. Proximately, this may be because nest defence and egg recognition represent cognitively and behaviourally completely different tasks. These patterns were not affected by female mating status in this polygynous cuckoo host. We hypothesize that differences in host behavioural types, rather than host egg discrimination ability, may predict host nest defence behaviour against adult brood parasites in general.

There is growing evidence that birds are able to discriminate different types of nest intruders and adjust their nest defence behaviour according to intruder dangerousness and distance from the nest (the dynamic risk assessment hypothesis). Here, we tested whether birds’ decisions about nest defence may additionally be affected by an increasing familiarity with a particular nest predator. We tested nest defence responses of great reed warblers Acrocephalus arundinaceus to a nest predator, the little bittern Ixobrychus minutus. Great reed warbler nests located close (≤7 m) to synchronously breeding little bitterns were “neighbour”, other nests were “solitary”. Great reed warbler specific aggression towards a little bittern dummy was much lower (~5-times) at neighbour than solitary nests. In contrast, generalised responses to a control innocuous intruder (the turtle dove, Streptopelia turtur) were statistically identical at neighbour and solitary nests. These patterns are in line with dynamic risk assessment hypothesis. We hypothesise that decreased great reed warbler aggression at neighbour nests also represents a specific behavioural adaptation to nesting in association with the little bittern. Little bitterns breeding closer to great reed warblers showed decreased risks of failure due to predation. However, further research is needed to experimentally test the causal links behind these patterns.

Life-history theory posits that the evolutionary responses of hosts to avian brood parasitism will be shaped by the extent of the fitness costs of parasitism. Previous modelling work predicted that hosts of more virulent parasites should eject foreign eggs, irrespective of clutch size, whereas hosts of less virulent parasites, with smaller clutch sizes, should desert (abandon) parasitized clutches and, with larger clutch sizes, should eject foreign eggs. Egg rejection behaviour of European blackbirds (Turdus merula) and song thrush (T. philomelos) in their introduced range in New Zealand was induced by manipulating the colour of one of the birds’ own eggs. We also used parallel experimental manipulations in the common redstart (Phoenicurus phoenicurus), a regular host species with a large clutch size which pays a moderate cost when parasitized by the common cuckoo (Cuculus canorus). In all three species, eggs coloured entirely black were more often rejected than eggs coloured with black spots but with the rest of the background colour left visible. Rejections of black eggs occurred mainly through nest desertion in blackbirds, which have smaller clutch sizes, and mainly through egg ejection in song thrush, which have larger clutch sizes. As predicted, redstarts mostly ejected black eggs. Alternative egg rejection behaviours may have evolved in response to differently virulent brood parasitism across these species. For example, in the absence of interspecific parasitism in both their native and introduced ranges, selection by low-cost intraspecific brood parasitism may explain the experimentally-induced behavioural differences in egg rejection in blackbirds, with smaller clutch sizes, versus song thrushes, with larger clutch sizes. Such experimental approaches, informed by life-history theory

Theory of parental care in biparental species predicts that a decrease in 1 mate’s parental effort should trigger a partial increase of care by the other mate. Previous studies investigating compensatory behavior used nestling provisioning as the measure of parental effort. However, nest defense is also a costly component of parental care because defenders risk injury or death caused by predators. Here for the first time, we test the compensation hypothesis in the context of nest defense. We experimentally widowed (by temporarily removing the other mate) female or male great reed warblers Acrocephalus arundinaceus and faced them with a predator near the nest (the stuffed Eurasian sparrowhawk Accipiter nisus). Female responses were independent of their partner’s presence or absence. In contrast, lone males did not compensate for the absence of their mates; they even behaved more aggressively when their partner was present, contradicting the partial compensation hypothesis. We discuss potential determinants of between-species variation in sex-specific compensatory behavior. We predict that a lack of compensation might be found in species with different renesting and remating potentials between males and females, for example, where males are unwilling/unable to raise the brood when unassisted by females and therefore, avoid an investment that cannot increase their fitness.

Animal populations, with a known history of introduction events, provide opportunities to study the dynamics of how rapid shifts in ecological context affect behavioral (e.g., responses to brood parasitism) and life-history (e.g., clutch and egg parameters) traits. We studied the European Greenfinch ( Carduelis chloris) introduced to New Zealand, regarding foreign-egg rejection behaviors and also compared their clutch characteristics with data from the source populations in the United Kingdom. Although previously this species had been considered an unsuitable host for the Common Cuckoo (Cuculus canorus), and not impacted by selection pressure associated with brood parasit- ism, we found that Greenfinches in our study population were able to eject experimental eggs at low frequencies. In contrast, nest desertion rates were similar in experimentally parasitized and control unmanipulated nests, implying that nest desertion is not an antiparasite adaptation in this species. Contrary to previous studies, we did not find significant differences in clutch and egg sizes between introduced and source populations. This study emphasizes (1) the importance of using control treat - ments in studies of host responses to experimental parasitism, (2) including apparently unsuitable hosts of brood parasites, and (3) meta-replicating prior studies to further the process of gaining and validating scientific knowledge.

Being an obligate parasite, juvenile common cuckoos Cuculus canorus are thought to reach their African wintering grounds from Palearctic breeding grounds without guidance from experienced conspecifics but this has not been documented. We used satellite tracking to study naïve migrating common cuckoos. Juvenile cuckoos left breeding sites in Finland moving slowly and less consistently directed than adult cuckoos. Migration of the juveniles (N = 5) was initiated later than adults (N = 20), was directed toward the southwest–significantly different from the initial southeast direction of adults–and included strikingly long Baltic Sea crossings (N = 3). After initial migration of juvenile cuckoos toward Poland, the migration direction changed and proceeded due south, directly toward the winter grounds, as revealed by a single tag transmitting until arrival in Northwest Angola where northern adult cuckoos regularly winter. Compared to adults, the juvenile travelled straighter and faster, potentially correcting for wind drift along the route. That both migration route and timing differed from adults indicates that juvenile cuckoos are able to reach proper wintering grounds independently, guided only by their innate migration programme. © 2016 Vega et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Hosts of brood parasites may vary geographically in their ability to resist parasitism. In contrast, geographic variation in defenses, such as egg rejection, is not expected to be present or vary geographically in unsuitable hosts. We examined spatial patterns of resistance in the great tit Parus major, a passerine that is a textbook example of an unsuitable host for brood parasites because of its hole-nesting habits. We experimentally tested for spatial variation in foreign egg rejection in 395 nests across latitudinal gradients in China (5 populations) and Europe (7 populations). In China, egg rejection rates were very high but showed a latitudinal gradient from 100% in the south to 52% in the north. In Europe, rejection rates were very low (on average only 4%) and did not vary latitudinally. The egg ejection rate patterns matched geographic patterns of parasitism risk with rejection probabilities decreasing with latitude (a surrogate measure of the diversity of brood parasites). The present study for the first time challenges the idea that hole-nesting birds did not evolve resistance mechanisms against brood parasites and highlights the importance of large-scale geographic comparisons in ecological research. © The Author 2016; all rights reserved. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. This is the authors' accepted and refereed manuscript to the article.

Host specialization evolved in many parasite-host systems. Evolution and maintenance of host specificity may be influenced by host life-history traits, active host selection by the parasite, and host anti-parasite strategies. The relative importance of these factors is poorly understood in situations that offer parasites a choice between hosts with similar habitat requirements. The common cuckoo Cuculus canorus is a generalist parasite on the species level, but individual females prefer particular host species. In reed beds of the Yellow River Delta, China, two potential hosts with similar nest characteristics, Oriental reed warblers Acrocephalus orientalis and reed parrotbills Paradoxornis heudei, breed in sympatry. We found that warblers were parasitized at much higher rates than parrotbills. Both hosts recognized and rejected non-mimetic model eggs well, indicating that they have been involved in an arms-race with cuckoos. Cuckoo eggs closely resembled warbler eggs, and such eggs were mostly accepted by warblers but rejected by parrotbills. Only warblers recognized adult cuckoos as a specific threat. Both hosts were equally good at raising cuckoo chicks. Low nest density, partial isolation by breeding time, small scale differences in nest and nest site characteristics, and high rejection rates of natural cuckoo eggs are likely cumulatively responsible for the low current parasitism rate in parrotbills. This study emphasizes the importance of integrating the study of general host life-history characteristics and specific anti-parasitism strategies of hosts across all breeding stages to understand the evolution of host specificity.

Obligate brood parasitic birds lay their eggs in nests of other species and parasite eggs typically have evolved greater structural strength relative to host eggs. Increased mechanical strength of the parasite eggshell is an adaptation that can interfere with puncture ejection behaviours of discriminating hosts. We investigated whether hardness of eggshells is related to differences between physical and chemical traits from three different races of the parasitic common cuckoo Cuculus canorus , and their respective hosts. Using tools developed for materials science, we discovered a novel correlate of increased strength of parasite eggs: the common cuckoo's egg exhibits a greater microhardness, especially in the inner region of the shell matrix, relative to its host and sympatric non-host species. We then tested predictions of four potential mechanisms of shell strength: (i) increased relative thickness overall, (ii) greater proportion of the structurally harder shell layers, (iii) higher concentration of inorganic components in the shell matrix, and (iv) elevated deposition of a high density compound, MgCO 3 , in the shell matrix. We confirmed support only for hypothesis (i). Eggshell characteristics did not differ between parasite eggs sampled from different host nests in distant geographical sites, suggesting an evolutionarily shared microstructural mechanism of stronger parasite eggshells across diverse host-races of brood parasitic cuckoos.

1. Why are some common and apparently suitable resources avoided by potential users? This interesting ecological and evolutionary conundrum is vividly illustrated by obligate brood parasites. Parasitic birds lay their eggs into nests of a wide range of host species, including many rare ones, but do not parasitize some commonly co-occurring potential hosts. 2. Attempts to explain the absence of parasitism in common potential hosts are limited and typically focused on single-factor explanations while ignoring other potential factors. We tested why thrushes Turdus spp. are extremely rarely parasitized by common cuckoos Cuculus canorus despite breeding commonly in sympatry and building the most conspicuous nests among forest-breeding passerines. 3. No single examined factor explained cuckoo avoidance of thrushes. Life-history traits of all six European thrush species and the 10 most frequently used cuckoo hosts in Europe were similar except body/egg size, nest design and nestling diet. 4. Experiments (n = 1211) in several populations across Europe showed that host defences at egg-laying and incubation stages did not account for the lack of cuckoo parasitism in thrushes. However, cross-fostering experiments disclosed that various factors during the nestling period prevent cuckoos from successfully parasitizing thrushes. Specifically, in some thrush species, the nest cup design forced cuckoo chicks to compete with host chicks with fatal consequences for the parasite. Other species were reluctant to care even for lone cuckoo chicks. 5. Importantly, in an apparently phylogenetically homogenous group of hosts, there were interspecific differences in factors responsible for the absence of cuckoo parasitism. 6. This study highlights the importance of considering multiple potential factors and their interactions for understanding absence of parasitism in potential hosts of parasitic birds. In the present study, comparative and experimental procedures are integrated, which represent a novel approach that should prove useful for the understanding of interspecific ecological relationships in general.

The study of avian eggshell structure, including composition, pigmentation, thickness, and strength,hasimportantecologicalandeconomicimplications.Previousinvestigatorshaveusedavarietyoftechniques to derive either direct measures or indirect estimates of eggshell thickness. Assessing the repeatability and method agreement of different techniques is necessary to permit comparison of eggshell thickness values from different studies on various genetic stocks, populations, and species. We recorded and analyzed measurements of eggshell thickness using two methods, micrometers and scanning electron microscopy (SEM), for several Palaeognathae and Neognathae taxa, including nonpasserines and passerines. Applying a tolerance-interval approach, we found that repeatability of measurements for eggs with thinner shells (

L. Polacˇikova´ (correspondence), and T. Grim, Dept. of Zool. and Lab. Ornithol., Palacky´ Univ., trˇ. Svobody 26, CZ 771 46 Olomouc, CzechRepublic. E-mail: lenka.polacikova@upol.czEggshell colour patterns play a crucial role in avian host parasite coevolution. In contrast to many experimentsinvestigating general host egg discrimination abilities, studies testing where specific recognition cues are located on theeggshells (on blunt, sharp or both egg poles) are lacking. Previous studies suggested that discrimination cues might belocated at the blunt egg pole, where the shell patterning is typically concentrated. We tested this hypothesisexperimentally in species subject to interspecific (great reed warblers Acrocephalus arundinaceus, reed warblersA. scirpaceus), and also intraspecific parasitism (song thrushes Turdus philomelos, blackbirds T. merula). We examinedhost responses towards two types of intraspecific eggs painted non-mimetic immaculate blue either at blunt or sharppoles. All four species rejected eggs manipulated at the blunt pole at significantly higher rates, indicating that theyperceive the critical recognition cues in the same egg part. Thus, the presence of egg recognition cues at the blunt egg polemay be a general phenomenon in birds parasitized by both intraspecific and interspecific parasites.

The known chemical basis of diverse avian eggshell coloration is generated by the same two classes of tetrapyrrole pigments in most living birds. We aimed to extend the evolutionary scope of these patterns by detecting pigments from extinct birds’ eggs. In our samples biliverdin was successfully extracted from subfossil shell fragments of the blue-green egg-laying upland moa Megalapteryx didinus, while protoporphyrin was extracted from the beige eggs of two other extinct moa species. Our data on pigment detection from eggshells of other extant paleognath birds, together with published information on other modern lineages, confirm tetrapyrroles as ubiquitous and conserved pigments contributing to diverse eggshell colours throughout avian evolution.

Chicks of some avian brood parasites show high virulence by eliminating all host progeny in the nest whereas others develop in the presence of host nestmates. Common cuckoo (Cuculus canorus) chicks are typically highly virulent parasites as they attempt to evict all host eggs and chicks soon after hatching. However, several features of nest design, including steep walls and ⁄ or cavity nests, may effectively prevent cuckoo hatchlings from evicting nestmates. A previous observational study showed low success of cuckoo chicks in evicting progeny of a cavity nester host, the redstart (Phoenicurus phoenicurus) but cuckoo chicks showed low survival both when reared alone or in mixed broods with host nestmates. Whether poor cuckoo performance was caused by eviction costs and ⁄ or by the effect of presence of host chicks per se remains unclear. We experimentally cancelled any potential eviction costs by removing host eggs immediately after the cuckoo hatched and creating mixed broods 5 days later when the eviction instinct of the cuckoo already ceased. Cuckoos that were forced to compete with host nestlings experienced lower provisioning rates, poorer growth, and lower fledging success than control lone cuckoos. Cuckoos in mixed broods that survived until fledging fledged later, and at lower masses, than those in the sole cuckoo group. Thus, the cuckoo gens specializing on redstarts is similar to other cuckoo gentes, whose chicks are more successful in evicting host nestmates, and it does not benefit from the presence of host brood. Cohabitation with host nestlings then should be viewed as a maladaptive by-product of host cavity nest design.

Avian chicks use different begging strategies when soliciting parental care. A novel begging strategy was recently observed in Horsfield’s hawk-cuckoo Hierococcyx hyperythrus (=Cuculus fugax). Chicks of this brood-parasitic species raise and shake their wings and display to fosterers a gape-coloured patch on the undersides of their wings. Although the gape-coloured wing-patch may be a unique trait of Horsfield’s hawk-cuckoo, wing-shaking in the context of begging is virtually universal in both brood parasites and their hosts. A simple qualitative comparison across different avian taxa suggests that wing-shake begging is most probably an ancestral feature of cuckoos and perhaps all altricial birds. The wing-shaking may be an honest signal of chick quality. It could also reduce the risk of predation if wing-shaking was coupled with reduced loudness of begging. Horsfield’s hawk-cuckoo chicks could have exploited the universal pre-existing host responsiveness to wing-shake begging. Evolution could have then further proceeded by making the wing-shaking more conspicuous by addition of another stimulus—the unique colourful wing-patch. I also hypothesize that wing-shake begging may have evolved from pre-fledging restlessness and is used secondarily in courtship displays, threatening postures, and distraction displays by adults. Further discussions and tests of these hypotheses may facilitate research into the so far unstudied phylogenetic history of avian chick-begging strategies.

Impressive variation in egg colouration among birds has puzzled evolutionary biologists for a long time. The most frequently studied selective forces moulding egg colouration—predation and brood parasitism—have either received little empirical support or may play a role in only a minority of species. A novel hypothesis has suggested that egg colour may be significantly influenced by sexual selection. Females may deposit a blue-green pigment biliverdin into eggshells instead of using it for themselves as a powerful antioxidant. By this handicap, females may signal their quality to males, which are then hypothesized to increase their paternal effort. We experimentally tested the hypothesis in the collared flycatcher (Ficedula albicollis), a species laying blue-green eggs. We cross-fostered clutches between nests to disentangle effects of female/territory quality and egg colour on paternal effort and nestling quality. The results supported two assumptions of sexual signalling through egg colour hypothesis: Blue pigment seems to be a limited resource for females, and female quality is positively correlated with the intensity of the blue-green colour. However, we did not find support for the main prediction of the hypothesis, as male parental effort parameters (feeding frequencies to nestlings and intensity of nest defence) were unrelated to egg colour. We discuss possible reasons for the discrepancy between our results and previous correlative analyses that supported the hypothesis that blue egg colour may be a postmating, sexually selected signal in females.

Brood parasitic birds lay their eggs in other birds' nests, leaving hosts to raise their offspring. To understand parasite-host coevolutionary arms races, many studies have examined host responses to experimentally introduced eggs. However, attending parents often need to be flushed from their nests to add experimental eggs. If these birds witness parasitism events, they may recognize and reject foreign eggs more readily than parents who did not. We found that, after being flushed, female blackbirds, Turdus merula, remained close to their nests. Flushed females were more likely to eject foreign eggs and did so more quickly than females that were not flushed during experimentation. In contrast, flushing did not predict responses and latency to responses to parasitism by song thrush, Turdus philomelos, which flew farther from their nests and likely did not witness experimental parasitism. When statistically considering flushing, previously published conclusions regarding both species' response to experimental parasitism did not change. Nevertheless, we recommend that researchers record and statistically control for whether hosts were flushed prior to experimental parasitism. Our results have broad implications because more vigilant and/or bolder parents can gain more information about parasitism events and therefore have better chances of successfully defending against brood parasitism.

The role of pigments in generating the colour and maculation of birds' eggs is well characterized, whereas the effects of the eggshell's nanostructure on the visual appearance of eggs are little studied. Here, we examined the nanostructural basis of glossiness of tinamou eggs. Tinamou eggs are well known for their glossy appearance, but the underlying mechanism responsible for this optical effect is unclear. Using experimental manipulations in conjunction with angle-resolved spectrophotometry, scanning electron microscopy, atomic force microscopy and chemical analyses, we show that the glossy appearance of tinamou eggshells is produced by an extremely smooth cuticle, composed of calcium carbonate, calcium phosphate and, potentially, organic compounds such as proteins and pigments. Optical calculations corroborate surface smoothness as the main factor producing gloss. Furthermore, we reveal the presence of weak iridescence on eggs of the great tinamou (Tinamus major), an optical effect never previously documented for bird eggs. These data highlight the need for further exploration into the nanostructural mechanisms for the production of colour and other optical effects of avian eggshells.

Recognition is considered a critical basis for discriminatory behaviours in animals. Theoretically, recognition and discrimination of parasitic chicks are not predicted to evolve in hosts of brood parasitic birds that evict nest-mates. Yet, an earlier study showed that host reed warblers ( Acrocephalus scirpaceus ) of an evicting parasite, the common cuckoo ( Cuculus canorus ), can avoid the costs of prolonged care for unrelated young by deserting the cuckoo chick before it fledges. Desertion was not based on specific recognition of the parasite because hosts accept any chick cross-fostered into their nests. Thus, the mechanism of this adaptive host response remains enigmatic. Here, I show experimentally that the cue triggering this ‘discrimination without recognition’ behaviour is the duration of parental care. Neither the intensity of brood care nor the presence of a single-chick in the nest could explain desertions. Hosts responded similarly to foreign chicks, whether heterospecific or experimental conspecifics. The proposed mechanism of discrimination strikingly differs from those found in other parasite–host systems because hosts do not need an internal recognition template of the parasite's appearance to effectively discriminate. Thus, host defences against parasitic chicks may be based upon mechanisms qualitatively different from those operating against parasitic eggs. I also demonstrate that this discriminatory mechanism is non-costly in terms of recognition errors. Comparative data strongly suggest that parasites cannot counter-evolve any adaptation to mitigate effects of this host defence. These findings have crucial implications for the process and end-result of host–parasite arms races and our understanding of the cognitive basis of discriminatory mechanisms in general.

The quality and quantity of food delivered to young are among the major determinants of fitness. A parental provisioning capacity is known to increase with body size. Therefore, brood parasitism provides an opportunity to test the effects of varying provisioning abilities of different-sized hosts on parasitic chick growth and fledging success. Knowledge of growth patterns of common cuckoo, Cuculus canorus, chicks in nests of common hosts is very poor. Moreover, no study to date has focused on any currently unused hosts (i.e., suitable cuckoo host species in which parasitism is currently rare or absent). Here, I compare the growth performance of cuckoo chicks in nests of a common host (the reed warbler, Acrocephalus scirpaceus) and two unparasitized hosts (the song thrush, Turdus philomelos, and the blackbird, Turdus merula). Parasitic chicks were sole occupants of the observed nests, thus eliminating the confounding effect of competition with host chicks. Experiments revealed striking differences in parasitic chick growth in the two closely related Turdus hosts. Cuckoo chicks cross-fostered to song thrush nests grew much quicker and attained much higher mass at fledging than those in nests of their common reed warbler host. Alternatively, parasitic chicks in blackbird nests grew poorly and did not survive until fledging. I discuss these observations with respect to host selection by parasitic cuckoos.

Mimicry is one of the most conspicuous and puzzling phenomena in nature. The best-known examples come from insects and brood parasitic birds. Unfortunately, the term ‘mimicry’ is used indiscriminately and inconsistently in the brood parasitic literature despite the obvious fact that similarities of eggs, nestlings and adults of brood parasites to their hosts could result from many different processes (phylogenetic constraint, predation, intraspecific arms-races, vocal imitation, exploitation of pre-existing preferences, etc.). In this note I wish to plead for a more careful use of the term. I review various processes leading to a similarity between propagules (both eggs and nestlings) of brood parasites and their hosts and stress that: (1) mimetic and non-mimetic similarities should be differentiated, (2) a mere similarity of host and parasite propagules provides no evidence for mimicry, (3) mimicry is more usefully understood as a (coevolutionary) process rather than an appearance, and (4) mimicry terminology should reflect the process which led to mimetic similarity. Accepting the mimicry hypothesis requires both the experimental approach and rejection of alternative hypotheses explaining similarities of host and parasite propagules. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 69–78.

Capsule Reed Warblers in a regularly parasitized population do not recognize Cuckoo Cuculus canorus as a special enemy and do not change their behaviour at nest immediately after being parasitized. Aims To assess if an intruder near the nest influences the behaviour of the Cuckoo host. Methods Host responses to Cuckoo, control Pigeon dummies and human intruder were observed. Host behaviour at 71 nests was video-recorded for 30 minutes at four experimental groups of nests: Cuckoo dummy, Cuckoo dummy + Cuckoo egg, Pigeon dummy, human intruder. Results Reed Warblers did not respond differently to the Cuckoo and the control species. The experimental procedure had no significant effect on the behaviour of hosts during the study period. We were unable to find any differences in the time spent at the nest, clutch inspection behaviour and nest defence behaviour between morning and afternoon experimental groups. Our results do not support the hypothesis that afternoon laying by the Cuckoo is maintained by a selection pressure from the host. We observed no ejection or egg-pecking during the 30-min period after the experimental parasitism. Conclusions Low aggression and non-specificity of host responses in our study area are in line with the fact that the Reed Warbler is an intermediate rejecter of Cuckoo eggs as expected from the spatial habitat structure hypothesis.

Background: Why have birds evolved the ability to reject eggs? Typically, foreign egg discrimination is interpreted as evidence that interspecific brood parasitism (IP) has selected for the host’s ability to recognize and eliminate foreign eggs. Fewer studies explore the alternative hypothesis that rejection of interspecific eggs is a by-product of host defenses, evolved against conspecific parasitism (CP). We performed a large scale study with replication across taxa (two congeneric Turdus thrushes), space (populations), time (breeding seasons), and treatments (three types of experimental eggs), using a consistent design of egg rejection experiments (n = 1057 nests; including controls), in areas with potential IP either present (Europe; native populations) or absent (New Zealand; introduced populations). These comparisons benefited from the known length of allopatry (one and a half centuries), with no gene flow between native and introduced populations, which is rarely available in host-parasite systems. Results: Hosts rejected CP at unusually high rates for passerines (up to 60%). CP rejection rates were higher in populations with higher conspecific breeding densities and no risks of IP, supporting the CP hypothesis. IP rejection rates did not covary geographically with IP risk, contradicting the IP hypothesis. High egg rejection rates were maintained in the relatively long-term isolation from IP despite non-trivial rejection costs and errors. Conclusions: These egg rejection patterns, combined with recent findings that these thrushes are currently unsuitable hosts of the obligate parasitic common cuckoo (Cuculus canorus), are in agreement with the hypothesis that the rejection of IP is a by-product of fine-tuned egg discrimination evolved due to CP. Our study highlights the importance of considering both IP and CP simultaneously as potential drivers in the evolution of egg discrimination, and illustrates how populations introduced to novel ecological contexts can provide critical insights into brood parasite-host coevolution.

Obligate avian brood parasites lay their eggs in hosts' nests and play no role in the provisioning of the progeny. Many parasites, including Cuculus cuckoos, hatch before their hosts and the altricial chick evicts hosts eggs and nestlings. A hypothesized, but so far untested parasite adaptation is that the embryos of cuckoos develop more quickly than the hosts' because the higher porosity of the parasite's eggshell allows greater gaseous exchange, potentially supporting more rapid development. We compared the water vapour conductance (GH2O) of common cuckoo (Cuculus canorus) eggshells and those of several passerines, including various cuckoo host species, and non-passerine species. Contrary to the prediction, the cuckoo eggs had lower GH2O than eggs of their hosts, and lower GH2O than predicted for their egg size and phylogeny. A potential advantage for the cuckoo egg of having a lower GH2O may be that the yolk is depleted at a slower rate, allowing more reserves to remain at the end of incubation, assisting the embryo with the energetically demanding tasks of hatching from a thicker eggshell, and evicting host eggs and nestmates.

Brood parasitism represents a unique mode of avian reproduction that requires a number of adaptations. For example, to reduce chances of puncture ejection of their eggs by small hosts, brood parasites may have been selected for laying eggs of unusually great structural strength. However, great structural strength of eggshells should hinder hatching. The goals of our study were to establish if chicks of the Common Cuckoo Cuculus canorus have more difficulty with hatching out of their strong eggs than chicks of species with eggs of similar size, and whether they possess any mechanisms facilitating hatching. To achieve these goals, we compared hatching pattern and selected body characteristics of chicks of the Common Cuckoo with those of another altricial species with eggs of a similar size, the Great Reed Warbler Acrocephalus arundinaceus. Although the rate of pecking was similar in the two species, the Common Cuckoo chicks started pecking earlier in relation to their emergence and consequently required more time and a greater cumulative number of pecks for breaking open their eggs than did young Great Reed Warblers. The two species also differed with respect to the pattern of opening their shells; in contrast to the warbler chicks, which enlarged the original pip circularly, the cuckoo chicks opened the egg by systematically creating a long narrow slit until they emerged. Finally, our study of hatched young revealed several differences; the Cuckoo hatchlings were significantly heavier, had a longer forearm, and their egg tooth was located significantly farther from the tip of the beak. The edge used for cutting through the shell was also significantly longer than that of hatchling Great Reed Warblers. To conclude, our data suggest that hatching is more difficult for a Cuckoo than for a Great Reed Warbler and that Cuckoos possess several mechanisms to overcome the problems of hatching from a structurally strong egg.

How do birds tell the colours of their own and foreign eggs apart? We demonstrate that perceptual modelling of avian visual discrimination can predict behavioural rejection responses to foreign eggs in the nest of wild birds. We use a photoreceptor noise-limited colour opponent model of visual perception to evaluate its accuracy as a predictor of behavioural rates of experimental egg discrimination in the song thrush Turdus philomelos . The visual modelling of experimental and natural eggshell colours suggests that photon capture from the ultraviolet and short wavelength-sensitive cones elicits egg rejection decisions in song thrushes, while inter-clutch variation of egg coloration provides sufficient contrasts for detecting conspecific parasitism in this species. Biologically realistic sensory models provide an important tool for relating variability of behavioural responses to perceived phenotypic variation.

How do potential hosts escape detrimental interactions with brood parasites? Current consensus is that hole-nesting and granivorous birds avoid brood parasites, like common cuckoos Cuculus canorus , by their inaccessible nest-sites and food unsuitable for parasites, respectively. Any open-nesting insectivorous hosts are believed to remain open to brood parasite exploitation which leads to the evolution of costly host defences like egg or chick discrimination. In contrast to this coevolutionary scenario, we show for the fi rst time that a previously not studied but seemingly suitable host species escapes brood parasites. Th e Asian verditer fl ycatcher Eumyias thalassinus , feed newly hatched chicks entirely with beetles and grasshoppers. Th ese are poor quality and hard to digest diet items that are rarely fed to own or cuckoo chicks by regular hosts. Indeed, chick cross-fostering experiments showed that these food items remained undigested by either cuckoos or other sympatric passerines causing them to die quickly. Egg discrimination experiments showed that the fl ycatcher accepts any foreign eggs. Although most but not all other potential explanations can be safely excluded at present, the most parsimonious historical explanation for these patterns is that the fl ycatcher exploits a trophic niche that no other sympatric bird can exploit, and that any cuckoo lineages that switch from their original hosts to the fl ycatcher have no possibilities for establishing viable populations. Th us, the current classifi cation of host suitability based on diet composition may need revision, raising an important cautionary tale for comparative studies and the interpretation of apparent host rejection of parasitic chicks. This is the peer reviewed version of the following article: Host selection in parasitic birds: are open-cup nesting insectivorous passerines always suitable cuckoo hosts?, which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1111/j.1600-048X.2013.00123.x/abstract. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Background Co-evolutionary struggles between dangerous enemies (e.g., brood parasites) and their victims (hosts) lead to the emergence of sophisticated adaptations and counter-adaptations. Salient host tricks to reduce parasitism costs include, as front line defence, adult enemy discrimination. In contrast to the well studied egg stage, investigations addressing the specific cues for adult enemy recognition are rare. Previous studies have suggested barred underparts and yellow eyes may provide cues for the recognition of cuckoos Cuculus canorus by their hosts; however, no study to date has examined the role of the two cues simultaneously under a consistent experimental paradigm. Methodology/Principal Findings We modify and extend previous work using a novel experimental approach – custom-made dummies with various combinations of hypothesized recognition cues. The salient recognition cue turned out to be the yellow eye. Barred underparts, the only trait examined previously, had a statistically significant but small effect on host aggression highlighting the importance of effect size vs. statistical significance. Conclusion Relative importance of eye vs. underpart phenotypes may reflect ecological context of host-parasite interaction: yellow eyes are conspicuous from the typical direction of host arrival (from above), whereas barred underparts are poorly visible (being visually blocked by the upper part of the cuckoo's body). This visual constraint may reduce usefulness of barred underparts as a reliable recognition cue under a typical situation near host nests. We propose a novel hypothesis that recognition cues for enemy detection can vary in a context-dependent manner (e.g., depending on whether the enemy is approached from below or from above). Further we suggest a particular cue can trigger fear reactions (escape) in some hosts/populations whereas the same cue can trigger aggression (attack) in other hosts/populations depending on presence/absence of dangerous enemies that are phenotypically similar to brood parasites and costs and benefits associated with particular host responses.

Pigment-based coloration is a common trait found in a variety of organisms across the tree of life. For example, calcareous avian eggs are natural structures that vary greatly in color, yet just a handful of tetrapyrrole pigment compounds are responsible for generating this myriad of colors. To fully understand the diversity and constraints shaping nature's palette, it is imperative to characterize the similarities and differences in the types of compounds involved in color production across diverse lineages. Pigment composition was investigated in eggshells of eleven paleognath bird taxa, covering several extinct and extant lineages, and shells of four extant species of mollusks. Birds and mollusks are two distantly related, calcareous shell-building groups, thus characterization of pigments in their calcareous structures would provide insights to whether similar compounds are found in different phyla (Chordata and Mollusca). An ethylenediaminetetraacetic acid (EDTA) extraction protocol was used to analyze the presence and concentration of biliverdin and protoporphyrin, two known and ubiquitous tetrapyrrole avian eggshell pigments, in all avian and molluscan samples. Biliverdin was solely detected in birds, including the colorful eggshells of four tinamou species. In contrast, protoporphyrin was detected in both the eggshells of several avian species and in the shells of all mollusks. These findings support previous hypotheses about the ubiquitous deposition of tetrapyrroles in the eggshells of various bird lineages and provide evidence for its presence also across distantly related animal taxa.

Avian brood parasites lay their eggs in other birds' nests and impose considerable fitness costs on their hosts. Historically and scientifically, the best studied example of circumventing host defences is the mimicry of host eggshell colour by the common cuckoo ( Cuculus canorus ). Yet the chemical basis of eggshell colour similarity, which impacts hosts' tolerance towards parasitic eggs, remains unknown. We tested the alternative scenarios that (i) cuckoos replicate host egg pigment chemistry, or (ii) cuckoos use alternative mechanisms to produce a similar perceptual effect to mimic host egg appearance. In parallel with patterns of similarity in avian-perceived colour mimicry, the concentrations of the two key eggshell pigments, biliverdin and protoporphyrin, were most similar between the cuckoo host-races and their respective hosts. Thus, the chemical basis of avian host–parasite egg colour mimicry is evolutionarily conserved, but also intraspecifically flexible. These analyses of pigment composition reveal a novel proximate dimension of coevolutionary interactions between avian brood parasites and hosts, and imply that alternative phenotypes may arise by the modifications of already existing biochemical and physiological mechanisms and pathways.

BackgroundChicks of virulent brood parasitic birds eliminate their nestmates and avoid costly competition for foster parental care. Yet, efforts to evict nest contents by the blind and naked common cuckoo Cuculus canorus hatchling are counterintuitive as both adult parasites and large older cuckoo chicks appear to be better suited to tossing the eggs and young of the foster parents.Methodology/principal findingsHere we show experimentally that egg tossing imposed a recoverable growth cost of mass gain in common cuckoo chicks during the nestling period in nests of great reed warbler Acrocephalus arundinaceus hosts. Growth rates of skeletal traits and morphological variables involved in the solicitation of foster parental care remained similar between evictor and non-evictor chicks throughout development. We also detected no increase in predation rates for evicting nests, suggesting that egg tossing behavior by common cuckoo hatchlings does not increase the conspicuousness of nests.ConclusionThe temporary growth cost of egg eviction by common cuckoo hatchlings is the result of constraints imposed by rejecter host adults and competitive nestmates on the timing and mechanism of parasite virulence.

One of the great evolutionary puzzles is why hosts of parasitic birds discriminate finely against alien eggs, but almost never discriminate against parasitic chicks. A theoretical model has shown that an adaptive host response to alien eggs can be based on learning. However, learned nestling discrimination is too costly to be favoured by selection in hosts of evicting parasites, such as the European cuckoo (Cuculus canorus). Indeed, parasitic chick rejection has never been reported for any European cuckoo host species. As learned nestling discrimination is maladaptive, one can expect that a viable alternative for hosts would be to use discrimination mechanisms not involving learning and/or recognition. We suggest that hosts may starve and desert cuckoo chicks that require higher amounts of food than an average host brood at fledging (i.e. feeding rates to a parasite are outside the normal range of host behaviour in unparasitized nests). Our observations of the reed warbler (Acrocephalus scirpaceus) at parasitized nests indicate that such behaviour could possibly work in this host species.