Phylogenetic analysis of life-history adaptations in parasitic cowbirds

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Abstract
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 of hosts. Several features of cowbirds have been proposed as more general adaptations to brood parasitism. In this study, we used a recent molecular phylogeny as a historical framework to test the possible adaptations of the parasitic cowbird, including egg size, 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 general allometric equations. We generated prediction intervals for a nonparasite that evolved in the place of the cowbirds. By using these prediction intervals, we found that parasitic cowbirds had not reduced weight or energy content of their eggs, nor their incubation period over evolutionary time. Cowbird chicks and those of nonparasitic relatives had similar growth pattern. The only characteristic that separated parasitic cowbirds from their nonparasitic relatives was an increase in eggshell thickness. All these findings were robust and resisted the use of three models of character evolution. The fact that most traits exhibited by cowbirds were inherited from a nonparasitic ancestor does not rule out that they are advantageous for parasitism. Future research should focus on such traits of cowbird relatives and on how these traits preadapted a particular lineage to become parasites.

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