Detecting and measuring senescence in wild birds: experience with long-lived seabirds

This paper points out and discusses several practical and methodological pr oblems that arise in attempts to detect and measure senescent declines in s urvival or breeding performance of wild animals, with specific emphasis on long-lived seabirds. Birds have no anatomical markers of age, so studies of age-related biology require marking individuals at the time of hatching an d following them throughout their lives. Seabirds live longer than the work ing lifespan of biologists, and longer than the turnover times of study tec hniques or theories of senescence. Seabirds are exposed to changing environ mental and demographic conditions and cannot be assumed to be in demographi c equilibrium. Sample sizes of the oldest age-classes are always small, req uiring either marking very large numbers of birds at hatching or continuing studies of old birds over many years. Incomplete sampling requires the use of mark-recapture models that have only been developed in the last 20 year s. Mortality selection resulting from demographic heterogeneity (selective survival of high-quality individuals) can offset or confound the effects of senescent changes within individuals. Many of these problems are amenable to solution and will be probably solved within a few years. In the meantime , this paper recommends that reviewers should be cautious about accepting p ublished reports of senescent declines in natural populations. (C) 2001 Els evier Science Inc. All rights reserved.