How strong are eggs of the common cuckoo Cuculus canorus

The common cuckoo Cuculus canorus is an obligate brood parasite that lays its eggs in the nests of small passerines. It has long been hypothesized that cuckoo eggs should be structurally stronger than host eggs or those of non-parasitic cuckoos to reduce chances of breakage during laying, to prevent accidental damage during incubation and/or to hinder their rejection through puncture ejection by the host. Therefore, we analysed selected characteristics of a sample of freshly laid eggs of the common cuckoo with two of its major hosts, the reed warbler Acrocephalus scirpaceus and great reed warbler Acrocephalus arundinaceus, and a sample of species with known puncture resistant eggs. We found that in puncture resistance tests cuckoo eggs tolerated on average 231 g. The cuckoo eggs were 3.3 and 2.5 times stronger than those of the reed warbler and great reed warbler, respectively. Greater shell thickness can explain only 17% of the total extra strength of the cuckoo eggs (125.97 g). When we controlled for the confounding effects of egg size (using a sample of eggs of normal strength from bird species of varying size), the common cuckoo eggs were 2.2 times stronger than expected for their size. Our results are consistent with the hypothesis that cuckoo eggs are structurally stronger and this trait probably represents an adaptation for a brood parasitic life style.