Life history theory predicts evolutionary trade-offs between investing in immune defense and other traits. We investigated whether reduced longevity was associated with increased investment in immunity in an avian brood parasite, the brown-headed cowbird (Molothrus ater). Previously we had found that the brown-headed cowbird was unusually resistant to infection with West Nile virus and other pathogenic arboviruses when compared with three closely related species in the same Family, Icteridae, the New World blackbirds. In this study, we hypothesized that the cowbird’s more effective immune responses may be associated with a trade-off in somatic maintenance that results in a shorter lifespan. We measured lifespan using the North American bird banding database and compared the lifespan of the cowbird with the lifespan of red-winged blackbird (Agelaius phoeniceus) and Brewer’s blackbird (Euphagus cyanocephalus), two related species whose immune defenses were previously found less effective against West Nile virus. The cowbird lifespan was significantly shorter than those of both the red-winged blackbird and Brewer’s blackbird. We also found that both male and female cowbirds had a shorter lifespan than the males and females in the two non-parasitic blackbird species. These data suggest that the brown-headed cowbird is a good study species for examining the trade-offs between immunity and longevity.
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