Why do cuckoos lay strong-shelled eggs? Tests of the puncture resistance hypothesis
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- Institutt for biologi 
Coevolution occurs when two or more interacting species have a reciprocal effect on each other. Avian obligate brood parasites and their hosts represent a classical example of coevolution and studies of such interactions have led to a better understanding of evolution in general. Brood parasitic birds such as cuckoos, cowbirds and honeyguides all lay eggs of unusual structural strength for their size. In this thesis, I test one of the hypotheses proposed to explain the evolutionary significance of this trait. The “Puncture resistance hypothesis” postulates that strong eggshells have evolved to resist attempts by the hosts to destroy the foreign egg and increase probability of acceptance. Most hosts of brood parasites are small passerines which cannot grasp the parasitic egg in their bills to selectively remove it from the nest. Such hosts have to first puncture the foreign egg and then grasp it in the opening to carry it away from the nest. A central assumption of the puncture resistance hypothesis is the existence of rejection costs – hosts destroy some of their own eggs as a side effect of the process of rejection. A scenario of strong eggshells acting as a constraint to rejection have been regarded theoretically possible in “non-evicting” brood parasites like cowbirds, which are less virulent in that hosts in many cases also can raise some own offspring. In “evicting” parasites such as most of the cuckoos Cuculidae, however, parasitic chicks destroy all host progeny, therefore increased eggshell strength has been considered unlikely to influence host rejection decisions. However, given the costs of rejection and the role of phenotypic plasticity/motivation, I hypothesized that strong eggshells may function to increase the probability of acceptance also in small host species of “evicting” brood parasites. More specifically, I predicted that the increased eggshell strength of cuckoo eggs may interact with mimicry in leading to a low motivation of hosts to actually complete rejection even though the parasitic egg is actually recognized. By using observational data, experiments and video recordings, I tested the puncture resistance hypothesis for the first time in the common cuckoo Cuculus canorus and some of its regular small Sylviid hosts. My results showed that the hard eggshell of the cuckoo egg indeed generates egg damage as a byproduct of the rejection process (rejection costs) in a considerable proportion of rejections in several of these host species. Cuckoo egg mimicry had a slight but non-significant effect in increasing further rejection costs beyond the effect of eggshell strength. However, I found no evidence for the validity of the puncture resistance hypothesis in marsh warblers Acrocephalus palustris, the study species in which most of my detailed experiments were done. Marsh warblers were generally able to puncture the strong-shelled cuckoo egg and all individuals that showed the first signs of egg recognition (pecking), subsequently rejected the cuckoo egg. Cuckoo eggshell strength did not influence rejection probability, alone or through an interaction with the mimicry. Nevertheless, selection for strong eggshells in the cuckoo may stem from coevolutionary interactions with other host species, less capable of successfully puncture ejecting the cuckoo egg. According to this expectation, I found that the olivaceous warblers Hippolais pallida, a slightly smaller puncture ejector species, recognized all non-mimetic cuckoo eggs added to their nests by selectively pecking them but accepted a considerable proportion of them. One such acceptance took place even after the host had destroyed one own egg. Therefore, the puncture resistance hypothesis is not irrelevant even in evicting brood parasites and their hosts and further investigations might identify more host species in which ‘forced’ acceptances of strong-shelled cuckoo eggs exist. Quantification of the effort marsh warblers actually needed to puncture the cuckoo egg revealed that most of the individuals were able to puncture eject the cuckoo egg much more efficiently than they actually did. Such seemingly suboptimal behaviour can represent either an adaptive strategy to diminish the risk of own egg damage or it may just be attributable to inexperience of most of the host individuals at puncture ejection.