Cell-mediated immunosenescence in birds.

The phytohaemagglutinin (PHA) skin test response, used to assess cell-mediated immunity, is known to vary with many social and energetic factors, but the effects of age have received little attention. We found that the PHA response of immature birds was lower than those of the youngest breeding adults and were decreased in adults. Whenever possible, age should be included as a covariate when the PHA skin test is used to assess immunocompetence in ecological immunology. The rate of decline in PHA response differed between species and was inversely correlated with survival. The decrease in the PHA response averaged 57% over an average 80% of the maximum life span, but the absolute rate varied with species lifespan such that the short-lived species showed a greater loss per year than the long-lived species. This link between declining immune function and survival may reflect differences in resource partitioning between species, and suggests that selection may act on investment in immune function to influence maximum life span.