Effects of Cowbird parasitism on parental provisioning and nestling food acquisition and growth

Brood parasitism by Brown-headed Cowbirds (Molothrus ater) is known to affect the fitness of many hosts by causing a reduction in the number of chicks that fledge from parasitized nests. However, little is known about less immediate effects on host fitness. We studied nestling growth and food acquisition and parental provisioning in parasitized and unparasitized nests of the Indigo Bunting (Passerina cyanea). Indigo Bunting nestlings in parasitized nests exhibited reduced rates of mass gain, but not tarsus growth, relative to bunting chicks in unparasitized nests. Bunting nestlings in parasitized nests received less food than did buntings in unparasitized nests. Buntings in parasitized nests spent more time begging than did those in unparasitized nests, but energy expended in this behavior may not have detracted greatly from the amount of energy available for growth. Adults at parasitized nests exhibited a higher provisioning rate than those at unparasitized nests. Increased provisioning by adult buntings at parasitized nests did not come at the expense of time spent brooding nestlings, but increased provisioning has the potential to affect the survival and future reproductive success of host adults. Because cowbird parasitism appears to impose substantial costs on Indigo Bunting nestlings and adults, concern over the conservation implications of parasitism should not be limited to species that suffer total reproductive failure when parasitized.