Does the cuckoo benefit from laying unusually strong eggs?

Brood-parasitic birds such as cuckoos and cowbirds lay eggs with unusual by strong shells. Three main hypotheses have been proposed to explain the adaptive significance of this trait. The present study focused on the puncture resistance hypothesis and tested its critical prediction that increased eggshell strength enhances the probability that the parasitic egg is accepted by the host. To address the problem, we experimentally parasitized marsh warbler, Acrocephalus palustris, nests with three types of real eggs having similar size but differing in eggshell strength and/or mimicry: (1) great reed warbler, Acrocephalus arundinaceus, eggs painted to be nonmimetic; (2) common cuckoo, Cuculus canorus, eggs painted in the same way; and (3) unmanipulated cuckoo eggs. When we controlled for mimicry, ejection of strong-shelled (cuckoo) eggs was considerably more costly than ejection of weak-shelled (great reed warbler) eggs. However, nonmimetic cuckoo eggs were not more likely to be accepted than nonmimetic great reed warbler eggs, suggesting no effect of eggshell strength alone on rejection decisions. Mimetic cuckoo eggs were accepted more often than the eggs painted to be nonmimetic suggesting that mimicry primarily determines the probability of rejection. Thus, we found no support for the puncture resistance hypothesis in marsh warblers, which is a host with well-developed defence mechanisms against cuckoo parasitism.