Physiological responses of host parents to rearing an avian brood parasite: An experimental study

Raising an obligate avian brood parasite is costly for host parents because it redirects valuable parental resources from one's own offspring to genetically unrelated young. The costs of raising a brood parasite may be mediated by physiological stressors for foster parents if it requires greater or biased parental effort compared to raising non-parasitized broods. For example, upregulating glucocorticoid hormones or reducing immune responses may mediate a trade-off between resource allocation to a current brood versus future reproductive opportunities, but published data on parasitized hosts' physiology are scarce. Here we used an experimental approach to test if host parents respond to the presence of brood parasitic young through dynamic changes in their own physiology. We conducted our experiments with parasitic brown-headed cowbirds (Molothrus ater) fostered into nests of host prothonotary warblers (Protonotaria citrea). Relative to parents caring for non-parasitized control broods, parasitism increased baseline corticosterone levels and reduced body mass in adult male, but not in female, hosts. Immune responses to a novel antigen were depressed in both parents of parasitized broods compared to parents of non-parasitized broods. Additionally, we found that immune function increased along the breeding season regardless of treatment. These experiments also confirmed prior observational data that parasitized adult males have reduced return rates to breeding sites in years subsequent to raising cowbirds. The findings demonstrate diverse physiological effects of parasitism on the foster parents in our particular host-brood parasite system, yet we found no evidence of individual trade-offs between endocrine and immune responses of adult hosts.