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The evolutionary equilibrium hypothesis was proposed to explain variation in egg rejection rates among individual hosts (intra- and interspecific) of avian brood parasites. Hosts may sometimes mistakenly reject own eggs when they are not parasitized (i.e. make recognition errors). Such errors would incur fitness costs and could counter the evolution of host defences driven by costs of parasitism (i.e. creating equilibrium between acceptors and rejecters within particular host populations). In the present study, we report the disappearance of host eggs from nonparasitized nests in populations of seven actual and potential hosts of the common cuckoo Cuculus canorus. Based on these data, we calculate the magnitude of the balancing parasitism rate provided that all eggs lost are a result of recognition errors. Importantly, because eggs are known to disappear from nests for reasons other than erroneous host rejection, our data represent the maximum estimates of such costs. Nonetheless, the disappearance of eggs was a rare event and therefore incurred low costs compared to the high costs of parasitism. Hence, costs as a result of recognition errors are probably of minor importance with respect to opposing selective pressure for the evolution of egg rejection in these hosts. We cannot exclude the possibility that low or intermediate egg rejection rates in some host populations may be caused by spatiotemporal variation in the occurrence of parasitism and gene flow, creating a variable influence of opposing costs as a result of recognition errors and the costs of parasitism. [ABSTRACT FROM AUTHOR]

Sex allocation theory and empirical evidence both suggest that natural selection should favour maternal control of offspring sex ratio in relation to their ability to invest in the offspring. Generalist parasites constitute a particularly interesting group to test this theory as different females commonly utilize different host species showing large variation in provisioning ability. The common cuckoo (Cuculus canorus) is a generalist brood parasite that lays its eggs in the nest of many different passerine birds, but each female tends to specialize on one particular host species giving rise to highly specialized host races. The different host species show large variation in their ability to invest in the parasitic offspring, presenting an opportunity for female cuckoos to bias offspring sex ratio in relation to host species quality. Here, we investigate host-race specific sex allocation controlling for maternal identity in the common cuckoo. We found no evidence of any significant relationship between host race and sex ratio in one sympatric population harbouring three different host races, or in a total of five geographically separated populations. There was also no significant association between host quality, as determined by species-specific female host body mass, and cuckoo sex ratio. Finally, we found no significant relationship between individual cuckoo maternal quality, as determined by her egg volume, and sex ratio within each host race. We conclude that the generalist brood-parasitic common cuckoo show no significant sex-ratio bias in relation to host race and discuss this finding in light of gene flow and host adaptations. [ABSTRACT FROM AUTHOR]

There is at present considerable variation in the level of antiparasite defences among different host species of avian brood parasites, but in many potential hosts some individuals reject poorly matching parasite eggs. Here we present unique absence of egg discrimination behaviour backed up by a lack of egg recognition abilities in a suitable common cuckoo Cuculus canorus host, the skylark Alauda arvensis. Skylarks did not show any clear rejection response to experimentally added highly non-mimetic foreign eggs in any behavioural context, even before they had started laying or when the whole clutch was exchanged with foreign eggs. This absence of antiparasite defence can be explained by the breeding habitat of larks consisting of largely treeless open landscapes where cuckoos have little access to the nests, thereby eroding the possibility of coevolutionary interactions. Our results are strikingly consistent with the spatial habitat structure hypothesis proposed to explain the occurrence and extent of avian host-parasite co-adaptation. [ABSTRACT FROM AUTHOR]

Using multiple logistic regression analysis, we investigated the influence of nest site characteristics, laying date and nest size in marsh warblers Acrocephalus palustris on the risk of parasitism by common cuckoos Cuculus canorus. Marsh warblers breed in more diverse and dense herbaceous vegetation than other cuckoo hosts investigated in comparable studies. The “perch proximity” hypothesis was supported as parasitized nests were situated closer to trees than non-parasitized ones. Furthermore, demonstrated for the first time in a cuckoo host, tree height was an important predictor of parasitism, with higher trees increasing the parasitism odds ratio. The “nest exposure” hypothesis was also supported since parasitized nests had a shorter stand of vegetation in the close vicinity than non-parasitized nests. However, visibility of the nest from the nearest potential cuckoo perch (cuckoo view) was not selected by the model, probably because most nests were well concealed. Laying date, height of nest above ground and the distance from the nest to the nearest edge of the vegetation were not important predictors of parasitism. Though smaller nests tended to be parasitized more frequently than larger ones, nest size only approached significance, making its importance unclear. [ABSTRACT FROM AUTHOR]

Eggs of several brood parasites have thicker and stronger shells than expected for their size. The present study evaluated the puncture resistance hypothesis for the occurrence of thick-shelled eggs in common cuckoos Cuculus canorus by investigating costs of cuckoo egg ejection in four Acrocephalus warblers—the great reed warbler A. arundinaceus, reed warbler A. scirpaceus, marsh warbler A. palustris and sedge warbler A. schoenobaenus. The three latter species all suffered ejection costs, while ejection was not costly in the larger great reed warbler. The occurrence of ejection costs was negatively related to host bill size. In the marsh warbler, we compared ejection costs in naturally parasitized nests and two experimental treatments, in which broods were parasitized artificially with great reed warbler and conspecific eggs. Hosts damaged their own eggs significantly more often when ejecting the thick-shelled cuckoo eggs than when ejecting the similarly sized but thinner-shelled great reed warbler eggs, providing some support for the puncture resistance hypothesis. Ejection of conspecific eggs did not involve any costs. Furthermore, contrary to predictions derived from the laying damage hypothesis, there was no evidence that egg damage was associated with cuckoo egg laying. Hosts damaging their own eggs during ejection were more likely to subsequently desert their clutches than those that did not. The frequency of clutches smeared with the contents of the ejected egg were positively related to the hypothesized difficulty of foreign egg puncturing. Potential advantages of thicker shells in common cuckoo eggs are discussed. [ABSTRACT FROM AUTHOR]

In the coevolutionary "arms race" between Common Cuckoos (Cuculus canorus) and their hosts, several adaptations and counter-adaptations have evolved. Here, we investigated natural parasitism and host sensitivity to egg rejection in Marsh Warblers (Acrocephalus palustris) in Bulgaria. The level of Common Cuckoo parasitism was high (28%), and average mimicry of Common Cuckoo eggs was good. Experimental parasitism with four egg-types that showed various degrees of mimicry of the host eggs revealed a generally high rejection rate of foreign eggs (37.5-100%). In addition, naturally laid Common Cuckoo eggs were rejected at a moderate rate (50%). The Marsh Warbler's ability to reject foreign eggs was strongly dependent on the degree of mimicry of the parasite egg but apparently not on differences in size between host and foreign eggs. Furthermore, intraclutch variation in host egg appearance was not related to the probability of egg rejection. The Marsh Warbler's highly developed egg-recognition ability and the good mimicry of Common Cuckoo eggs suggests that this poorly known host-parasite arms race has reached an advanced stage. The present study provides new insight into variables that are important for egg rejection in a heavily parasitized host population. [ABSTRACT FROM AUTHOR]