Egg shape mimicry in parasitic cuckoos

Parasitic cuckoos lay their eggs in nests of host species. Rejection of cuckoo eggs by hosts has led to the evolution of egg mimicry by cuckoos, whereby their eggs mimic the colour and pattern of their host eggs to avoid egg recognition and rejection. There is also evidence of mimicry in egg size in some cuckoo–host systems, but currently it is unknown whether cuckoos can also mimic the egg shape of their hosts. In this study, we test whether there is evidence of mimicry in egg form (shape and size) in three species of Australian cuckoos: the fan‐tailed cuckoo Cacomantis flabelliformis, which exploits dome nesting hosts, the brush cuckoo Cacomantis variolosus, which exploits both dome and cup nesting hosts, and the pallid cuckoo Cuculus pallidus, which exploits cup nesting hosts. We found evidence of size mimicry and, for the first time, evidence of egg shape mimicry in two Australian cuckoo species (pallid cuckoo and brush cuckoo). Moreover, cuckoo–host egg similarity was higher for hosts with open nests than for hosts with closed nests. This finding fits well with theory, as it has been suggested that hosts with closed nests have more difficulty recognizing parasitic eggs than open nests, have lower rejection rates and thus exert lower selection for mimicry in cuckoos. This is the first evidence of mimicry in egg shape in a cuckoo–host system, suggesting that mimicry at different levels (size, shape, colour pattern) is evolving in concert. We also confirm the existence of egg size mimicry in cuckoo–host systems.     

Egg color variation,but not egg rejection behavior,changes in a cuckoo host breeding in the absence of brood parasitism

Interactions between parasitic cuckoos and their songbird hosts form a classical reciprocal “arms race,” and are an excellent model for understanding the process of coevolution. Changes in host egg coloration via the evolution of interclutch variation in egg color or intraclutch consistency in egg color are hypothesized counter adaptations that facilitate egg recognition and thus limit brood parasitism. Whether these antiparasitism strategies are maintained when the selective pressure of parasitism is relaxed remains debated. However, introduced species provide unique opportunities for testing the direction and extent of natural selection on phenotypic trait maintenance and variation. Here, we investigated egg rejection behavior and egg color polymorphism in the red‐billed leiothrix (Leiothrix lutea), a common cuckoo (Cuculus canorus) host, in a population introduced to Hawaii 100 years ago (breeding without cuckoos) and a native population in China (breeding with cuckoos). We found that egg rejection ability was equally strong in both the native and the introduced populations, but levels of interclutch variation and intraclutch consistency in egg color in the native population were higher than in the introduced population. This suggests that egg rejection behavior in hosts can be maintained in the absence of brood parasitism and that egg appearance is maintained by natural selection as a counter adaptation to brood parasitism. This study provides rare evidence that host antiparasitism strategies can change under parasite‐relaxed conditions and reduced selection pressure.     

The evolution of egg size in the brood parasitic cuckoos

We compared genera of nonparasitic cuckoos and two groups ofparasitic cuckoos: those raised together with host young ("nonejectors")and those in which the newly hatched cuckoo either ejects thehost eggs or chicks, or kills the host young ("ejectors"). Nonejectorsare similar to their hosts in body size and parasitize largerhosts than do ejectors, which parasitize hosts much smallerthan themselves. In both types of parasite, the cuckoo's eggtends to match the host eggs in size. To achieve this, nonejectorshave evolved a smaller egg for their body size than have nonparasiticcuckoos, and ejectors have evolved an even smaller egg. Amongejector cuckoo genera, larger cuckoos have larger eggs relativeto the eggs of their hosts, and the relationship between cuckooegg volume (mass of the newly-hatched cuckoo) and host egg volume(mass to be ejected) did not differ from that predicted by weight-liftingallometry. However, comparing among Cuculus cuckoo species,the allometric slope differed from the predicted, so it is notclear that egg size is related to the need to give the cuckoochick sufficient strength for ejection. Comparing the two mostspeciose ejector genera, Chrysococcyx cuckoos (smaller and parasitizedome-nesting hosts) lay eggs more similar in size to their host'seggs than do Cuculus cuckoos (larger and parasitize open cup–nestinghosts). Closer size-matching of host eggs in Chrysococcyx mayreflect the following: (1) selection to reduce adult body massto facilitate entry through small domed nest holes to lay, and(2) less need for a large egg, because longer incubation periodsin dome-nesting hosts allow the young cuckoo more time to growbefore it need eject host eggs.     

Rapid increase in cuckoo egg matching in a recently parasitized reed warbler population

Parasitic cuckoos lay eggs that mimic those of their hosts, and such close phenotypic matching may arise from coevolutionary interactions between parasite and host. However, cuckoos may also explicitly choose hosts in a way that increases degree of matching between eggs of cuckoos and parasites, with female preference for specific host phenotypes increasing the degree of matching. We tested for temporal change in degree of matching between eggs of the parasitic European cuckoo (Cuculus canorus) and its reed warbler (Acrocephalus scirpaceus) host during 24 consecutive years in a recently parasitized reed warbler population. Cuckoo-host egg matching in an ultraviolet-brownness component yielding most of the chromatic variance of eggs improved during the study period. Improved matching was not due to changes in cuckoo egg phenotype. Cuckoo eggs matched host eggs for ultraviolet-brownness within nests irrespective of duration of sympatry. Ultraviolet-brownness of cuckoo eggs was similar to that of reed warbler eggs at parasitized nests, but differed from that of reed warbler eggs at unparasitized nests. These findings provide tentative support for the cuckoo preference hypothesis suggesting that cuckoo-host egg matching could partially be due to cuckoo females selecting host nests based on the appearance of their eggs.     

Modeling the cuckoo’s brood parasitic behavior in the presence of egg polymorphism

The common cuckoo Cuculus canorus is a brood parasite that utilizes many host species. These have evolved defense against parasitism to reject cuckoo eggs that look unlike their own and some cuckoos have evolved egg mimicry to counter this defense. Egg phenotype indeed plays a key role for both the cuckoo and its hosts to successfully reproduce. It has been argued that cuckoos should parasitize host nests where egg phenotype matches because this makes parasitism more successful. Details of the cuckoo’s parasitic behavior, however, largely remains unknown if they really parasitize hosts depending on “egg matching”. In this paper, we model a time sequence of parasitic events in which a cuckoo finds host nests and decides to parasitize them or not in the presence of egg polymorphism. We evaluate which strategy is optimal: (1) opportunistic parasitism where cuckoos parasitize hosts irrespective of the phenotype, or (2) non-opportunistic parasitism where cuckoos parasitize hosts where egg phenotype matches. The analysis showed that either of the two strategies can be optimal. Factors not considered in the model, e.g., ecological and evolutionary changes both in the cuckoo and the host side, are discussed to explain apparent contrasts observed in some cuckoo–host interactions.     

Predicting the responses of native birds to transoceanic invasions by avian brood parasites

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 naïve accepter and coevolved rejecter songbirds in the Northern Hemisphere.     

Outcomes of brood parasite-host interactions mediated by egg matching: common cuckoos Cuculus canorus versus Fringilla finches

Background

Antagonistic species often interact via matching of phenotypes, and interactions between brood parasitic common cuckoos (Cuculus canorus) and their hosts constitute classic examples. The outcome of a parasitic event is often determined by the match between host and cuckoo eggs, giving rise to potentially strong associations between fitness and egg phenotype. Yet, empirical efforts aiming to document and understand the resulting evolutionary outcomes are in short supply.

Methods/Principal Findings

We used avian color space models to analyze patterns of egg color variation within and between the cuckoo and two closely related hosts, the nomadic brambling (Fringilla montifringilla) and the site fidelic chaffinch (F. coelebs). We found that there is pronounced opportunity for disruptive selection on brambling egg coloration. The corresponding cuckoo host race has evolved egg colors that maximize fitness in both sympatric and allopatric brambling populations. By contrast, the chaffinch has a more bimodal egg color distribution consistent with the evolutionary direction predicted for the brambling. Whereas the brambling and its cuckoo host race show little geographical variation in their egg color distributions, the chaffinch''s distribution becomes increasingly dissimilar to the brambling''s distribution towards the core area of the brambling cuckoo host race.

Conclusion

High rates of brambling gene flow is likely to cool down coevolutionary hot spots by cancelling out the selection imposed by a patchily distributed cuckoo host race, thereby promoting a matching equilibrium. By contrast, the site fidelic chaffinch is more likely to respond to selection from adapting cuckoos, resulting in a markedly more bimodal egg color distribution. The geographic variation in the chaffinch''s egg color distribution could reflect a historical gradient in parasitism pressure. Finally, marked cuckoo egg polymorphisms are unlikely to evolve in these systems unless the hosts evolve even more exquisite egg recognition capabilities than currently possessed.     

The comparative breeding behaviour of two sympatric cuckoos, Horsfield's Bronze-Cuckoo Chrysococcyx basalts and the Shining Bronze-Cuckoo C. lucidus, in Western Australia: a new model for the evolution of egg morphology and host specificity in avian brood parasites

The breeding behaviour of two similarly sized sympatric cuckoos, Horsfield's Bronze-Cuckoo Chrysococcyx basalts and the Shining Bronze-Cuckoo C. lucidus, was studied over four breeding seasons at Gooseberry Hill, Western Australia. Both cuckoos usually began laying in late August; Shining Bronze-Cuckoos laid for up to 13 weeks and Horsfield's Bronze-Cuckoos for up to 15 weeks. Four host species were parasitized and major hosts were parasitized throughout most of their laying periods. The frequency of parasitism varied between hosts and between years, but Splendid Fairy-wrens Malurus splendens and Yellow-rumped Thornbills Acanthiza chrysorrhoa (major hosts) were always parasitized more heavily than Western Thornbills A. inornata and Scarlet Robins Petroica multicolor. Western Thornbills were parasitized by both cuckoos. Horsfield's and Shining Bronze-Cuckoos laid monomorphic eggs; those of Horsfield's Bronze-Cuckoos were highly mimetic whereas those of Shining Bronze-Cuckoos were non-mimetic and dark in colour. Both cuckoos laid one egg per host nest, deposited eggs directly into the nest, laid very quickly in the early morning, removed at least one host egg at laying, laid eggs small for the size of the birds, hatched after 12 days and evicted nest companions shortly after hatching. Laying was well synchronized with the start of incubation by hosts. Field observations and experiments with egg models indicated that neither of the major hosts, nor the secondary host in common, discriminate against foreign eggs. The nestling period for Horsfield's Bronze-Cuckoo was 17 days, and for the Shining Bronze-Cuckoo 20 days. There was a corresponding difference in nestling growth rate between the cuckoo species. About 50% of cuckoo eggs produced fledglings. Reproductive success for both cuckoos was highest in nests of the secondary host in common, the Western Thornbill. Young cuckoos reached independence 5–6 weeks after hatching. The adaptive significance of competition between cuckoos as a selective agent for cuckoo egg morphology and host specificity is discussed.     

Coevolution between Himalayan cuckoos and 2 sympatric Pycnonotidae hosts

Selection due to cuckoo parasitism is responsible for the evolution of anti-parasitism defenses in hosts. Different host species breeding sympatrically with a single parasitic cuckoo may evolve different strategies to reduce the risk of counter cuckoo parasitism, resulting in different interactions between cuckoos and hosts in areas of sympatry. Here, we studied the coevolutionary interactions between Himalayan cuckoos Cuculus saturatus and 2 sympatric and closely related potential hosts belonging to the family Pycnonotidae, the brown-breasted bulbul Pycnonotus xanthorrhous and the collared finchbill Spizixos semitorques. We investigated parasitism rates and nest-site selection (nest height, nest cover, human disturbance, perch height, forest distance, and degree of concealment) related to parasitism risk, nest defense against a cuckoo dummy, and egg rejection against cuckoo model eggs. Bulbuls used specific nest sites that were further away from forests than those of finchbills, and they behaved more aggressively toward cuckoos than finchbills. In contrast, bulbuls possessed moderate egg rejection ability, whereas the finchbill rejected 100% of cuckoo model eggs. We suggest that selection of a nest site away from forests by the bulbul explains the absence of parasitism by Himalayan cuckoos. We suggest that these interspecific differences in nest-site selection and nest defense indicate alternative responses to selection due to cuckoos.     

Egg Rejection and Brain Size among Potential Hosts of the Common Cuckoo

Interspecific brood parasitism by the common cuckoo (Cuculus canorus) lowers host fitness, and has selected for discrimination and rejection of parasitic eggs in their commonly parasitized hosts. Cognitive demands needed to discriminate and reject cuckoo eggs may have led to augmentation of relative brain size among passerine hosts parasitized by cuckoos. This hypothesis predicts for across species positive relationships of brain size with rejection rate, host suitability and parasitism level. Here we test these predictions while controlling for phylogenetic, ecological and developmental factors known to affect brain size and egg rejection in a comparative study using the cuckoo and their hosts in Europe as a model system. Contrary to expected the rate of rejection of non‐mimetic cuckoo eggs covaried negatively with relative brain size across bird species. Either suitability as cuckoo host, which reflects long‐time duration of exposure to cuckoo parasitism, and level of parasitism, did not relate to brain size. Our results do not support the hypothesis that cuckoo parasitism was a main direct force affecting brain size variation across passerine hosts.     

Brood parasites lay eggs matching the appearance of host clutches

Interspecific brood parasitism represents a prime example of the coevolutionary arms race where each party has evolved strategies in response to the other. Here, we investigated whether common cuckoos (Cuculus canorus) actively select nests within a host population to match the egg appearance of a particular host clutch. To achieve this goal, we quantified the degree of egg matching using the avian vision modelling approach. Randomization tests revealed that cuckoo eggs in naturally parasitized nests showed lower chromatic contrast to host eggs than those assigned randomly to other nests with egg-laying date similar to naturally parasitized clutches. Moreover, egg matching in terms of chromaticity was better in naturally parasitized nests than it would be in the nests of the nearest active non-parasitized neighbour. However, there was no indication of matching in achromatic spectral characteristics whatsoever. Thus, our results clearly indicate that cuckoos select certain host nests to increase matching of their own eggs with host clutches, but only in chromatic characteristics. Our results suggest that the ability of cuckoos to actively choose host nests based on the eggshell appearance imposes a strong selection pressure on host egg recognition.     

Continuous Variation Rather than Specialization in the Egg Phenotypes of Cuckoos (Cuculus canorus) Parasitizing Two Sympatric Reed Warbler Species

The evolution of brood parasitism has long attracted considerable attention among behavioural ecologists, especially in the common cuckoo system. Common cuckoos (Cuculus canorus) are obligatory brood parasites, laying eggs in nests of passerines and specializing on specific host species. Specialized races of cuckoos are genetically distinct. Often in a given area, cuckoos encounter multiple hosts showing substantial variation in egg morphology. Exploiting different hosts should lead to egg-phenotype specialization in cuckoos to match egg phenotypes of the hosts. Here we test this assumption using a wild population of two sympatrically occurring host species: the great reed warbler (Acrocephalus arundinaceus) and reed warbler (A. scirpaceus). Using colour spectrophotometry, egg shell dynamometry and egg size measurements, we studied egg morphologies of cuckoos parasitizing these two hosts. In spite of observing clear differences between host egg phenotypes, we found no clear differences in cuckoo egg morphologies. Interestingly, although chromatically cuckoo eggs were more similar to reed warbler eggs, after taking into account achromatic differences, cuckoo eggs seemed to be equally similar to both host species. We hypothesize that such pattern may represent an initial stage of an averaging strategy of cuckoos, that – instead of specializing for specific hosts or exploiting only one host – adapt to multiple hosts.     

The common cuckoo Cuculus canorus is not locally adapted to its reed warbler Acrocephalus scirpaceus host

The obligate avian brood parasitic common cuckoo Cuculus canorus comprises different strains of females that specialize on particular host species by laying eggs of a constant type that often mimics those of the host. Whether cuckoos are locally adapted for mimicking populations of the hosts on which they are specialized has never been investigated. In this study, we first explored the possibility of local adaptation in cuckoo egg mimicry over a geographical mosaic of selection exerted by one of its main European hosts, the reed warbler Acrocephalus scirpaceus. Secondly, we investigated whether cuckoos inhabiting reed warbler populations with a broad number of alternative suitable hosts at hand were less locally adapted. Cuckoo eggs showed different degrees of mimicry to different reed warbler populations. However, cuckoo eggs did not match the egg phenotypes of their local host population better than eggs of other host populations, indicating that cuckoos were not locally adapted for mimicry on reed warblers. Interestingly, cuckoos exploiting reed warblers in populations with a relatively larger number of co-occurring cuckoo gentes showed lower than average levels of local adaptation in egg volume. Our results suggest that cuckoo local adaptation might be prevented when different cuckoo populations exploit more or fewer different host species, with gene flow or frequent host switches breaking down local adaptation where many host races co-occur.     

Hosts of avian brood parasites have evolved egg signatures with elevated information content

Hosts of brood-parasitic birds must distinguish their own eggs from parasitic mimics, or pay the cost of mistakenly raising a foreign chick. Egg discrimination is easier when different host females of the same species each lay visually distinctive eggs (egg ‘signatures’), which helps to foil mimicry by parasites. Here, we ask whether brood parasitism is associated with lower levels of correlation between different egg traits in hosts, making individual host signatures more distinctive and informative. We used entropy as an index of the potential information content encoded by nine aspects of colour, pattern and luminance of eggs of different species in two African bird families (Cisticolidae parasitized by cuckoo finches Anomalospiza imberbis, and Ploceidae by diederik cuckoos Chrysococcyx caprius). Parasitized species showed consistently higher entropy in egg traits than did related, unparasitized species. Decomposing entropy into two variation components revealed that this was mainly driven by parasitized species having lower levels of correlation between different egg traits, rather than higher overall levels of variation in each individual egg trait. This suggests that irrespective of the constraints that might operate on individual egg traits, hosts can further improve their defensive ‘signatures'' by arranging suites of egg traits into unpredictable combinations.     

Cryptic gentes revealed in pallid cuckoos Cuculus pallidus using reflectance spectrophotometry

Many cuckoo species lay eggs that match those of their hosts, which can significantly reduce rejection of their eggs by the host species. However, egg mimicry is problematic for generalist cuckoos that parasitize several host species with different egg types. Some generalist cuckoos have overcome this problem by evolving several host-specific races (gentes), each with its own, host-specific egg type. It is unknown how generalist cuckoos lacking gentes are able to avoid egg rejection by hosts. Here we use reflectance spectrophotometry (300-700 nm) on museum egg collections to test for host-specific egg types in an Australian generalist cuckoo reported to have a single egg type. We show that the colour of pallid cuckoo (Cuculus pallidus) eggs differed between four host species, and that their eggs closely mimicked the eggs of the host they parasitized. These results reveal that pallid cuckoos have host-specific egg types that have not been detected by human observation, and indicate that gentes could be more common than previously realized.     

Host‐specific parasitism in the Central American striped cuckoo,Tapera naevia

Coevolutionary theories of brood parasite strategy and host defense have been informed by research on egg mimicry and host recognition. However, there is no information on the strategies of New World parasitic cuckoos and their hosts. The striped cuckoo Tapera naevia is a New World cuckoo that uses multiple host species and maintains an egg color polymorphism. To investigate if color‐matching influenced rejection behavior in hosts, I conducted an egg rejection experiment on a host that lays blue‐green eggs, the rufous‐and‐white wren Thryophilus rufalbus and a host that lays white eggs, the plain wren Cantorchilus modestus. I used spectrophotometric analysis of egg color to determine the degree of egg color‐matching. I found that at the field site the striped cuckoo lays highly mimetic eggs for the rufous‐and‐white wren, in both color and brightness. The rufous‐and‐white wren was more likely to accept mimetic artificial eggs than non‐mimetic eggs. The plain wren exhibited low rejection rates for both mimetic and non‐mimetic artificial eggs. The evidence from this study indicates that the striped cuckoo lays eggs that are closely color‐matched to those of its preferred host, the rufous‐and‐white wren, and that this mimicry improves acceptance.     

Do common cuckoos (Cuculus canorus) possess an optimal laying behaviour to match their own egg phenotype to that of their Oriental reed warbler (Acrocephalus orientalis) hosts?

Optimality theory suggests that parasitic cuckoos should evolve an optimal laying behaviour aiming to positively select host nests in which the eggs match the phenotype of their own eggs, thus minimizing the rejection risk from hosts and, in turn, maximizing the cuckoos' fitness. We tested this hypothesis by investigating cuckoo‐egg matching between parasitized and nonparasitized nests in a common cuckoo (Cuculus canorus) host, the Oriental reed warbler (Acrocephalus orientalis), by use of Vorobyev–Osorio and Nature‐Pattern‐Match models to quantify the matching of egg colour and pattern from avian vision, respectively. The results of our study indicated that cuckoo‐egg matching in parasitized nests was no better than that in nonparasitized nests, and thus we found no support for the optimal laying hypothesis in cuckoos. The mixed conclusions from all previous studies, including the present study, may be explained by (1) the parallel coevolution in different cuckoo–host systems; (2) the inappropriate methodology; and (3) the deficiency of the assumption itself. We suggest that a better methodology should be developed to solve the puzzle of whether cuckoos choose to lay eggs matching those of the host.     

How to evade a coevolving brood parasite: egg discrimination versus egg variability as host defences

Arms races between avian brood parasites and their hosts often result in parasitic mimicry of host eggs, to evade rejection. Once egg mimicry has evolved, host defences could escalate in two ways: (i) hosts could improve their level of egg discrimination; and (ii) negative frequency-dependent selection could generate increased variation in egg appearance (polymorphism) among individuals. Proficiency in one defence might reduce selection on the other, while a combination of the two should enable successful rejection of parasitic eggs. We compared three highly variable host species of the Afrotropical cuckoo finch Anomalospiza imberbis, using egg rejection experiments and modelling of avian colour and pattern vision. We show that each differed in their level of polymorphism, in the visual cues they used to reject foreign eggs, and in their degree of discrimination. The most polymorphic host had the crudest discrimination, whereas the least polymorphic was most discriminating. The third species, not currently parasitized, was intermediate for both defences. A model simulating parasitic laying and host rejection behaviour based on the field experiments showed that the two host strategies result in approximately the same fitness advantage to hosts. Thus, neither strategy is superior, but rather they reflect alternative potential evolutionary trajectories.     

Attractive blue‐green egg coloration and cuckoo−host coevolution

Recent evidence suggests that blue‐green coloration of bird eggshells may be related to female and/or egg phenotypic quality, and that such colour may affect parental effort and therefore the nutritional environment of developing nestlings. Here we suggest that these relationships and the signal function of eggshell coloration would affect the outcome of coevolution between avian brood parasites and their hosts in at least three different non‐exclusive evolutionary pathways. First, by laying blue‐green coloured eggs, cuckoo females may exploit possible sensory biases of their hosts, constraining the evolution of parasitic egg recognition, and thus avoid rejection. Second, because of the relatively high costs of laying blue eggs, cuckoo females may be limited in their ability to mimic costly blue‐green eggs of their hosts because cuckoo females lay many more eggs than their hosts. Furthermore, costs associated with foreign egg recognition errors would be relatively higher for hosts laying blue eggs. Third, cuckoos may use coloration of host eggs for selecting individuals or specific hosts of appropriate phenotypic quality (i.e. parental abilities). We here explored some predictions emerging from the above scenarios and found partial support for two of them by studying egg coloration of European cuckoos (Cuculus canorus) and that of their 25 main hosts, as well as parasitism and rejection rate of hosts. Cuckoo hosts parasitized with more blue, green, and ultraviolet cuckoo eggs, or those laying more blue‐green eggs, were more prone to accept experimental parasitism with artificial cuckoo eggs. In addition, coloration of cuckoo eggs is more variable when parasitizing hosts laying bluer‐greener eggs, even after controlling for the effect of host egg coloration (i.e. degree of egg matching). Globally, our results are consistent with the proposed hypothesis that host egg traits that are related to phenotypic quality of hosts, such as egg coloration, may have important implications for the coevolutionary interaction between hosts and brood parasites. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 154–168.     

Choosing suitable hosts: common cuckoos Cuculus canorus parasitize great reed warblers Acrocephalus arundinaceus of high quality

We investigated the hypothesis that the common cuckoo Cuculus canorus selects host pairs of good phenotypic quality. As there is some evidence that cuckoos may select hosts within a population non-randomly based on external cues reflecting their foster abilities, we predicted that great reed warbler Acrocephalus arundinaceus pairs parasitized by the cuckoo would exhibit higher quality than unparasitized ones. To test this assumption, we evaluated two different parameters indicating host quality: body condition and characteristics of host eggs. We found that parasitized females showed significantly better body condition than unparasitized ones, and the model showed that the probability of being parasitized by the cuckoos increased with increasing body condition. Moreover, the likelihood of being parasitized by a cuckoo within the great reed warbler population increased with decreasing colour variability within clutches: parasitized females allocated costly blue pigments to eggshells more equally compared with unparasitized ones. Our study revealed that cuckoos parasitize great reed warbler females of higher quality, as reflected in host body condition and egg colour characteristics. In highly mimetic systems, cuckoos may choose to parasitize hosts with eggs displaying low intraclutch variation, both because this leads to reduced rejection and because these hosts are of high quality.