Phylogenetic analysis of life-history adaptationsin parasitic cowbirds

Parasitic cowbirds lay eggs in the nests of other species and dupe them into caring for their young. Unlike other brood parasites,cowbirds have not developed egg mimicry or bizarre chick morphology. However, most of them parasitize a large number ofhosts. Several features of cowbirds have been proposed as more general adaptations to brood parasitism. In this study, we useda recent molecular phylogeny as a historical framework to test the possible adaptations of the parasitic cowbird, including eggsize, eggshell thickness and energy content of the eggs, length of the incubation period, and growth pattern of cowbird nestlings.We used a recently developed extension of independent contrasts to test whether the five cowbird species deviate from generalallometric equations. We generated prediction intervals for a nonparasite that evolved in the place of the cowbirds. By usingthese prediction intervals, we found that parasitic cowbirds had not reduced weight or energy content of their eggs, nor theirincubation period over evolutionary time. Cowbird chicks and those of nonparasitic relatives had similar growth pattern. Theonly characteristic that separated parasitic cowbirds from their nonparasitic relatives was an increase in eggshell thickness. Allthese findings were robust and resisted the use of three models of character evolution. The fact that most traits exhibited bycowbirds were inherited from a nonparasitic ancestor does not rule out that they are advantageous for parasitism. Futureresearch should focus on such traits of cowbird relatives and on how these traits preadapted a particular lineage to becomeparasites.

Myriam E. Mermoz and Juan Francisco Ornelas, Behav Ecol 15:109-119 (2004)