How to hatch from the Common Cuckoo ( Cuculus canorus ) egg: implications of strong eggshells for the hatching muscle ( musculus complexus )

One of the most obvious adaptations to the brood parasitic mode of reproduction is the formation of eggs with unusually strong shells, which apparently reduce chances of egg breakage during laying and puncture ejection attempts of parasitic eggs by the hosts. We tested a hypothesis that strong eggshells of the Common Cuckoo, Cuculus canorus , may have also led to a stronger hatching muscle, musculus complexus . First, the Cuckoo hatching muscle had a higher density of fibers than that of the similarly sized Great Reed Warbler Acrocephalus arundinaceus chicks; and second, the cross-sectional area of fibers of the hatching muscle was smaller in the Cuckoo than in the Great Reed Warbler. We propose that the increased density of muscle fibers in the Cuckoo facilitates hatching out of structurally strong eggshells because chicks possessing this trait should be able to exert greater pressure on the shell during the hatching process. Our results are consistent with the hypothesis that the higher density of fibers in the musculus complexus represents another adaptation facilitating hatching from unusually strong parasitic eggs that has presumably evolved during coevolution involving the Cuckoo and its hosts.