The number of syllables in Chernobyl cuckoo calls reliably indicate habitat,soil and radiation levels

Cuckoos Cuculus canorus produce calls that consist of a repeated but variable number of syllables that has given name to the species and the family. Here we tested the hypothesis that cuckoo calls are reliable indicators of environmental and individual quality by determining the number of repeated ‘cuckoo’ syllables in calls in relation to habitat and soil, ionizing radiation, presence of a female cuckoo and local density of male cuckoos at Chernobyl, Ukraine. Males were consistent in their production of syllables, producing more syllables per call when a female or another male arrived. This increase in the number of syllables was larger in males that already produced many syllables in the absence of conspecifics, implying that males of superior quality were still able to increase the number of syllables. Males produced more syllables per call in habitats with black soil and in forests. Furthermore, they produced fewer and more aberrant syllables in radioactively contaminated areas of Chernobyl providing evidence of an effect of environmental perturbation on the number of syllables. These findings are consistent with the hypothesis that the number of syllables is a condition-dependent signal used for attracting mates, repelling competitors, providing information about local environmental conditions for other cuckoos, but also for humans and thus can be used by humans to infer habitat quality.     

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.     

Experimentally Constrained Virulence is Costly for Common Cuckoo Chicks

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.     

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.     

Alarm call‐based discrimination between common cuckoo and Eurasian sparrowhawk in a Chinese population of great tits

Morphological resemblance of the common cuckoo Cuculus canorus to the Eurasian sparrowhawk Accipiter nisus has been regarded as an example of predator mimicry. Common hosts could distinguish parasites as the result of coevolution, while rare hosts or non‐hosts may mistake cuckoos for hawks because rare hosts or non‐hosts behave similarly when faced with these two species. Birds usually produce alarm calls in addition to showing behavioral responses when in danger. However, previous studies of identification by rare hosts or non‐hosts of sparrowhawks usually lacked experimental evidence of alarm calls. Great tits Parus major, a rare cuckoo host, perform similar behaviors and usually produce alarm calls in response to sparrowhawks and common cuckoos. Here, we tested whether great tits could distinguish common cuckoo from sparrowhawk based on analysis of their alarm calls and the effects of playback of alarm calls on conspecific behavior. Previous studies showed that great tits have a complex communication system that conveys information about predators, and they could perform different kinds of response behavior to different alarm calls. If great tits have not made the ability to distinguish between common cuckoo and sparrowhawk, then their acoustic responses to these two species and their response behaviors in playback experiments should be similar. Specimens of a common cuckoo (parasite), a sparrowhawk (predator) and an Oriental turtle dove Streptopelia orientalis (harmless control) were used to elicit and subsequently record the response behavior and alarm calls of great tits. There was no significant difference in behavioral response among great tits when exposed to the dummy of cuckoo, sparrowhawk and dove. In contrast, they differed significantly in alarm calls. Great tits produced more notes per call that contained increasing D‐type and decreasing I‐type notes when responding to sparrowhawk as compared to cuckoo or dove. In playback experiments, we found that great tits responded more strongly to great tit hawk than to great tit cuckoo or great tit dove alarm calls. Our study suggests that great tits are able to distinguish sparrowhawks from common cuckoos and convey relevant information in alarm calls by adjusting the number and combinations of notes of a single call type.     

Western Yellow-Billed Cuckoo Nest-Site Selection and Success in Restored and Natural Riparian Forests

The western distinct population segment of yellow-billed cuckoo (Coccyzus americanus; western cuckoo) has been extirpated from most of its former breeding range in the United States because of widespread loss and degradation of riparian cottonwood (Populus spp.)-willow (Salix spp.) forests. Restoration and management of breeding habitat is important to the recovery of this federally threatened species, and identification of high-quality breeding habitat can help improve the success of recovery. In 2005, the Lower Colorado River Multi-Species Conservation Program, a long-term, multi-agency effort, was initiated to maintain and create wildlife habitat within the historical floodplain of the lower Colorado River (LCR) for federally endangered and threatened species, including western cuckoos. We conducted an empirical, multi-scale field investigation from 2008–2012 to identify habitat characteristics selected by nesting western cuckoos along the LCR. Multiple logistic regression models revealed that western cuckoos selected nest sites characterized by increased densities of small, native, early successional trees measuring 8–23 cm diameter at breast height, and lower diurnal temperature compared to available habitat in restoration and natural forests. Nesting cuckoos selected sites with increased percent canopy closure, which was also important for nest success in restoration sites along the LCR. Our results show habitat components selected by nesting western cuckoos in restoration and natural riparian forests and can help guide the creation, enhancement, and management of riparian forests with habitat conditions necessary to promote nesting of western cuckoos. © 2021 The Wildlife Society.     

大杜鹃对家燕的巢寄生

了解杜鹃(Cuculus spp.)对不同宿主鸟类的巢寄生,是研究杜鹃与其宿主之间协同进化的重要基础资料。大杜鹃(Cuculus canorus)和家燕(Hirundo rustica)分布遍及全国,且为同域分布,但两者之间的寄生现象尚未有过系统调查。2012年和2014年4~8月,对繁殖于吉林市昌邑区桦皮厂镇(34°58′44.18″N,126°13′26.83″E,海拔184 m)和海南岛的家燕种群进行调查,结果表明,吉林市昌邑区桦皮厂镇家燕种群的寄生率为2.4%(1/42),而在海南岛所调查的1 719个家燕巢未发现杜鹃寄生现象。同时在网络上搜集家燕巢寄生的报道案例,共记录到13巢家燕被大杜鹃寄生繁殖,均发生在北方的家燕种群。     

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.     

Imperfect mimicry of host begging calls by a brood parasitic cuckoo: a cue for nestling rejection by hosts?

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

Spatial variation in egg polymorphism among cuckoo hosts across 4 continents

Although egg color polymorphism has evolved as an effective defensive adaptation to brood parasitism, spatial variations in egg color polymorphism remain poorly characterized. Here, we investigated egg polymorphism in 647 host species (68 families and 231 genera) parasitized by 41 species of Old Word cuckoos (1 family and 11 genera) across Asia, Europe, Africa, and Australia. The diversity of parasitic cuckoos differs among continents, reflecting the continent-specific intensities of parasitic selection pressure on hosts. Therefore, host egg polymorphism is expected to evolve more frequently on continents with higher cuckoo diversity. We identified egg polymorphism in 24.1% of all host species and 47.6% of all host families. The common cuckoo Cuculus canorus utilized 184 hosts (28.4% of all host species). Hosts of the common cuckoo and of Chrysococcyx species were more likely to have polymorphic eggs than hosts parasitized by other cuckoos. Both the number of host species and the host families targeted by the cuckoo species were positively correlated with the frequency of host egg polymorphism. Most host species and most hosts exhibiting egg color polymorphism were located in Asia and Africa. Host egg polymorphism was observed less frequently in Australia and Europe. Our results also suggested that egg polymorphism tends to occur more frequently in hosts that are utilized by several cuckoo species or by generalist cuckoo species. We suggest that selection pressure on hosts from a given continent increases proportionally to the number of cuckoo species, and that this selection pressure may, in turn, favor the evolution of host egg polymorphism.     

Does coevolution promote species richness in parasitic cuckoos?

Why some lineages have diversified into larger numbers of species than others is a fundamental but still relatively poorly understood aspect of the evolutionary process. Coevolution has been recognized as a potentially important engine of speciation, but has rarely been tested in a comparative framework. We use a comparative approach based on a complete phylogeny of all living cuckoos to test whether parasite–host coevolution is associated with patterns of cuckoo species richness. There are no clear differences between parental and parasitic cuckoos in the number of species per genus. However, a cladogenesis test shows that brood parasitism is associated with both significantly higher speciation and extinction rates. Furthermore, subspecies diversification rate estimates were over twice as high in parasitic cuckoos as in parental cuckoos. Among parasitic cuckoos, there is marked variation in the severity of the detrimental effects on host fitness; chicks of some cuckoo species are raised alongside the young of the host and others are more virulent, with the cuckoo chick ejecting or killing the eggs/young of the host. We show that cuckoos with a more virulent parasitic strategy have more recognized subspecies. In addition, cuckoo species with more recognized subspecies have more hosts. These results hold after controlling for confounding geographical effects such as range size and isolation in archipelagos. Although the power of our analyses is limited by the fact that brood parasitism evolved independently only three times in cuckoos, our results suggest that coevolutionary arms races with hosts have contributed to higher speciation and extinction rates in parasitic cuckoos.     

Reproductive biology of the European Cuckoo Cuculus canorus: early insights, persistent errors and the acquisition of knowledge

The brood parasitic habits of the European Cuckoo Cuculus canorus have excited wonder, disbelief and speculation since the fourth century BC. Accurate knowledge of cuckoo biology, however, accumulated only slowly and mostly since 1700. The aim of this study is to review six main topics: (1) the placement of cuckoo eggs in host nests; (2) cuckoo ‘clutch’ size; (3) cuckoo egg characteristics, mimicry and rejection; (4) choice of hosts; (5) eviction of eggs and chicks; and (6) the reasons why cuckoos are brood parasites and are incapable of rearing their own young. Early errors in reporting cuckoo biology were often a consequence of poor or incomplete observations leading to erroneous interpretations. Many of the early observers were egg collectors who focussed almost exclusively on the egg-laying period, thus ignoring cuckoo chick biology. Major landmarks in cuckoo studies included the facts that: (1) cuckoo eggs often resembled those of their hosts (1760s) and that this mimicry was adaptive (1850s); (2) hosts sometimes evicted cuckoo eggs (1770s); (3) female cuckoos laid individually distinctive eggs and that specific cuckoo gentes may exist (1850s); and (4) although well recognised that cuckoo chicks were reared alone, prior to Jenner’s work in the 1780s female cuckoo parents were thought to either eat or evict the host eggs or young. Jenner’s results was more readily accepted in Britain than in Germany. Between 1700 and 1859, cuckoo brood parasitism was difficult to reconcile with the prevalent conceptual framework of physico-theology (later known as the argument from design). Thereafter, Darwin’s idea of natural selection provided a superior conceptual framework, which in conjunction with experimental testing of specific hypotheses has continued to advance our understanding of brood parasitism. Our knowledge of cuckoo biology is far from complete, however, and we predict that continuing research often incorporating new technologies will refine and extend our understanding of the cuckoo’s extraordinary biology.     

Climate change and coevolution in the cuckoo–reed warbler system

The evolution of traits in hosts may be influenced by their parasites and vice versa and a coevolutionary arms race often develops between the two. As part of such an arms race, the common cuckoo mimics the eggs of its hosts to avoid egg rejection. Traits related to this arms race may also be influenced by climatic conditions, such as temperature, affecting, for example, food availability and, thus, female condition and egg size (therefore may reflect Bergmann’s rule or the resource rule). The potential interaction between coevolution and climate has rarely been studied. We investigated whether egg and body size of cuckoos and reed warblers from Britain and Denmark had undergone change between 1868 and 1956, and whether such changes were correlated with climatic factors. Cuckoo egg size decreased during the studied period while warbler egg size remained stable. Hence, cuckoo and warbler eggs have become more similar in size over time. Cuckoo egg volume decreased with increasing annual precipitation, but annual precipitation decreased over time. Warbler egg volume increased with spring temperatures (which could not reflect Bergmann’s rule, but may support the resource rule). Hence, it seems that the measured climatic indices did not affect cuckoo egg size but may in part affect warbler egg size. Therefore, the decrease in cuckoo egg size may be the result of the coevolutionary arms race. Body and egg sizes in the cuckoos were negatively correlated whereas warbler body and egg sizes were uncorrelated, suggesting that selection probably acted on egg size directly and not via selection on body size. Taken together, these findings may indicate that climate change, the coevolutionary arms race, or both, affected egg sizes. It is suggested that drawing conclusions regarding the arms race without taking into account other selective pressures (e.g., climate) may confound conclusions regarding parasite-host systems.     

Cuckoo females preferentially use specific habitats when searching for host nests

Egg mimicry is an important adaptation of common cuckoos, Cuculus canorus, against rejection of eggs by their respective hosts. A precondition for the maintenance of egg mimicry is that female cuckoos find hosts with a matching egg type. Experimental evidence indicated that habitat imprinting may be important for host selection. We tested whether the spacing and laying patterns of female cuckoos in the field are compatible with the supposed habitat-imprinting mechanism. We observed 16 females, with the help of radiotelemetry; of seven females, we observed directly 26 egg layings and 27 nest visits without laying. As expected if females were imprinted on different vegetation types, (1) the distribution of vegetation types differed between female home ranges, (2) female habitat use differed from average habitat availability within the egg-laying area (habitat preference), (3) females visited nests and deposited their eggs in the habitat they preferred, and (4) females laid their eggs consistently in a particular habitat type, irrespective of the host species. These results indicate that cuckoo females show habitat preference when searching for suitable host nests. Hence our data are compatible with the habitat-imprinting hypothesis, but owing to the habitat specificity of hosts the data cannot disprove a potential role of host specificity in cuckoo females.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

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.     

Nestling cuckoos,Cuculus canorus,exploit hosts with begging calls that mimic a brood

Nestling cuckoos, Cuculus canorus, eject host eggs or young from the nest and are then raised alone by the hosts. Using reed warblers, Acrocephalus scirpaceus, as hosts, we investigated how the single cuckoo chick can command the same provisioning rate as a whole brood of host young. Large size alone is not sufficient to stimulate adequate provisioning because single blackbird, Turdus merula, or song thrush, T. philomelos, chicks of the same mass as a cuckoo were fed at a lower rate. Our experiments show that the key stimulus is the cuckoo chick''s rapid begging call (''si, si, si, si ...''), which sounds remarkably like a whole brood of host chicks, and which it matched in calling rate. When single blackbird or song thrush chicks were accompanied by loudspeakers that broadcast either cuckoo begging calls or calls of a brood of reed warblers, the hosts increased their provisioning rate to that for a cuckoo chick. We suggest that the cuckoo needs vocal trickery to stimulate adequate care to compensate for the fact that it presents a visual stimulus of just one gape.     

Habitat-dependent call divergence in the common cuckoo: is it a potential signal for assortative mating?

The common cuckoo (Cuculus canorus) is an obligate brood parasite that mimics the eggs of its hosts. The host-specific egg pattern is thought to be inherited matrilinearly, creating female-only host-specific races. Males are thought not to be adapted to their host and they maintain the species by mating arbitrarily with respect to host specialization of females. However, recent results suggest that male cuckoos may also show host-specific adaptations and these may require assortative mating with respect to host. The calls males produce on the breeding grounds could provide a potential mechanism for assortative mating. We tested whether male cuckoo calls differ more between nearby populations that parasitize different hosts than between distant populations that parasitize the same host. We recorded the calls of geographically distant pairs of populations in Hungary, with each pair consisting of a forest population and a nearby reed bed population. Each habitat is characterized by one main host species for the common cuckoo. Our results show that calls of distant cuckoo populations from the same habitat type are more similar to each other than they are to those of nearby populations from a different habitat. These results suggest that cuckoo calls differ sufficiently to allow recognition of habitat-specific individuals.     

A comparative study of host selection in the European cuckoo Cuculus canorus

Certain kinds of hosts are commonly regarded as being more suitable than other for rearing European cuckoos (Cuculus canorus) – insectivores that lay small eggs and have open, shallow nests – although empirical tests of cuckoo host selection are lacking. We analysed host use by the European cuckoo in 72 British passerines that are potential hosts and for which there was information available on life-history variables and variables related to cuckoo-host coevolution, such as rate of parasitism, rejection rate of non-mimetic model eggs and degree of cuckoo-egg mimicry of host eggs. The relative population size of the host species affected parasitism rate most strongly, followed by relatively short duration of the nestling period, and the kind of nest, with cuckoos selecting open-nesting hosts. However, the effect of the nestling period could be related to host body size and the kind of nest used, because hole-nesting species normally have longer nestling periods than open-nesters. We re-analysed the data excluding hole nesters and corvid species (species with larger body mass), but the results remained identical. The European cuckoo may benefit from selecting hosts with short nestling periods because such hosts provide food for their nestlings at a very high rate. When only those species known as cuckoo hosts were analysed, the variable that best accounted for the parasitism rate was duration of the breeding season. Therefore, availability of potential hosts in both time and space is important for cuckoos in selecting hosts. Received: 16 July 1998 / Accepted: 27 October 1998     

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.     

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.