Immune function in an avian brood parasite and its nonparasitic relative

Organisms that breed multiple times must trade off resources between current and future reproduction. In many species, sexual selection can lead to reduced levels of immune function in males because they invest heavily in current reproduction at the expense of self-maintenance. Much less is known about whether the same trend is seen in species such as the brood-parasitic brown-headed cowbird Molothrus ater (hereafter "cowbird"), in which females invest heavily in current reproduction. We examined two measures of immune function (bactericidal capacity of the plasma and the phytohemagglutinin swelling response) and baseline levels of corticosterone in both sexes of the cowbird and its nonparasitic relative the red-winged blackbird Agelaius phoeniceus (hereafter "redwing") during the breeding and subsequent nonbreeding seasons. We found that female cowbirds exhibited significantly lower levels of both measures of immune function than did male cowbirds and female redwings during the breeding season but had comparable levels during the nonbreeding season. Female redwings, in contrast, exhibited higher or comparable levels of immune function when compared with male redwings during the breeding season. In conjunction with published accounts documenting significantly higher rates of mortality for female cowbirds compared with male cowbirds and the fact that female cowbirds produce very high numbers of eggs (25-65) in a single breeding season, our results suggest that female cowbirds invest heavily in current reproduction at the expense of self-maintenance.