Are cowbird eggs unusually strong from the inside?

Abstract

The shells of Brown-headed Cowbird (Molothrus ater) eggs are unusually strong from the outside, presumably to reduce chances of puncture ejection by small hosts. To evaluate the potential effects of 'outside' eggshell strength on inside strength, I measured the force required to puncture the inside of eggshells. When the confounding effects of variation among species in egg size were controlled, cowbird eggshells tolerated 1.7 times greater pressure than eggs of nine control species from several families. Within-species analyses of cowbirds and two nonparasitic blackbirds indicated that most of the variation in the inside strength of eggshells is related to their thickness. Analyses among species demonstrated that the greater inside strength of cowbird eggs could be explained mostly by their unusually thick eggshells. However, because cowbird eggshells were stronger than expected for their thickness, other eggshell characteristics also must play a role in inside strength. These results suggest that hatching from cowbird eggs is more difficult than would be expected for eggs of their size.