Despite their higher metabolic rates and lifetime energy expenditures, birds generally outlive similar-sized mammals even in the wild, often reaching maturity and aging considerably more slowly. Wild populations of many bird species have been monitored for years using banding-and-recapture methods, allowing field ornithologists to document age-related declines in survival and reproductive success. Although elderly birds rarely reach advanced stages of senescence in nature, many show other signs of physiological deterioration. In this Perspective, we review recent reports of aging-related changes in the immune response of two small European songbirds, the barn swallow and the collared flycatcher. Researchers in both studies challenged birds' humoral immune response by administering antigen to free-ranging adults during the breeding season. Older barn swallows--particularly breeding females--showed lower antibody responses (both primary and secondary) to vaccination with Newcastle disease virus, an avian pathogen. In flycatchers, older females raised lower antibody titers than younger breeders did in response to an injection of sheep red blood cells, a nonpathogenic antigen, and produced offspring with lower average body masses. Although the relevance of such measures of "immunosenescence" to actual fitness, reproductive success, and mortality is still unclear, studies of wild vertebrate populations may ultimately provide an important link between laboratory research and our understanding of the natural history and evolution of basic mechanisms of aging.
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