IS GREATER EGGSHELL DENSITY AN ALTERNATIVE MECHANISM BY WHICH PARASITIC CUCKOOS INCREASE THE STRENGTH OF THEIR EGGS

Brood parasites lay unusually strong eggs, presumably to prevent punct ure ejection of the eggs by hosts. However, it has been suggested that eggs of some parasitic cuckoos have normal strength. This suggestion was based on the eggshell thickness and shape of these eggs. Here, we propose that there may be other structural adaptations (such as increa sed eggshell density) that make the cuckoo eggs unusually strong. In t his study, we compared the eggshell density of parasitic cuckoos to th e eggshell density of two control groups, non-parasitic cuckoos and a sample of non-passerine species. The comparisons to both control group s demonstrated that the parasitic cuckoos have eggshells of significan tly higher density than would be expected for their size. Our results support the hypothesis that the higher eggshell density is an alternat ive mechanism by which some cuckoos increase the strength of their egg s.