Intrinsic aging-related mortality in birds

Actuarial senescence in captive populations of 28 species of bird was quantified by estimating the parameters of Weibull models fitted to survival curves constructed from data obtained from toes. Samples of natural and captive populations were compared using phylogenetically independent contrasts, which revealed that extrinsic mortality rates in captive populations are, on average, less than 30% of those of natural populations but that the component of mortality related to aging does not differ significantly between natural and captive birds. This result supports the hypothesis that aging-related mortality is associated with intrinsic causes of death that kill independently of the external environment. A logical implication of this result is that birds in natural populations maintain a high level of physical fitness into old age and do not become more vulnerable to extrinsic mortality factors with increasing age. Additional comparisons showed that the rate of aging in this sample of birds is correlated with body mass, but not with embryonic or postnatal growth rate. These analyses suggest that studies of aging in captive populations can provide powerful tools to help us understand senescence in natural populations.