A Comparative Study of Eggshell Thickness in Cowbirds and Other Passerines

Abstract

We directly measured the eggshell thickness of the three brood parasitic Molothrus cowbirds, 17 other icterids, and 13 additional passerines. By correcting these shell thickness measurements for variation attributable to interspecific differences in egg volume, we show that the Molothrus cowbirds lay eggs with shells that are 30% thicker than expected for their size. Our samples of nonparasitic icterids and of other passerines do not differ significantly in shell thickness values corrected for differences in egg volume. We evaluate two hypotheses for the evolution of unusually thick-shelled eggs in cowbirds. The first, an old idea proposed for parasitic cuckoos, is that thick shells resist damage to the parasite's egg at laying. From indirect tests we could find little or no evidence that the thin-shelled eggs of the hosts of cowbirds were damaged by impact when cowbird eggs were laid in their nests; thus, we tentatively conclude that resistance to laying damage has not been critical in favoring the evolution of thick shells in cowbird eggs. As an alternative hypothesis we propose that thick shells have evolved in cowbird eggs to resist puncture ejections by hosts that are too small to grasp whole cowbird eggs for ejection. We show that a puncture specialist, the Marsh Wren (Cistothorus palustris), has greater difficulty puncturing cowbird eggs than the thinner-shelled eggs of various other passerines. Implications of our hypothesis concerning resistance to puncture ejections are discussed.