Immune response covaries with corticosterone plasma levels under experimentally stressful conditions in nestling barn swallows (Hirundo rustica)

Traits related to fitness are often pleiotropically linked or otherwise constrained in their expression. Organisms therefore trade between fitness components such as number and viability of their offspring. The physiological mechanisms mediating such trade-offs, however, have been poorly investigated. We manipulated brood size and satiation of nestling barn swallows, Hirundo rustica, to simulate the effect of two kinds of natural stresses, i.e., long-term intense competition in a large brood and acute food deprivation, and we measured their effect on body condition, T cell-mediated immune response, and corticosterone, the main hormone mediating the adrenocortical stress-response. Brood enlargement increased corticosterone levels compared with those for brood reduction, and brood enlargement depressed immune response, body mass, and condition. Corticosterone levels markedly increased after food deprivation. Immune response negatively covaried with corticosterone levels measured after long-term stress. Hence, living in a crowded nest and with food deprivation elicited a stress response mediated by corticosterone, and depressed an important component of offspring fitness such as T cell-mediated immunity. The negative covariation between circulating corticosterone and immunity suggests that the trade-off between offspring number and quality is mediated by variation in plasma levels of corticosterone, which has immunosuppressive effects.