ACTUARIAL SENESCENCE AND DEMOGRAPHIC HETEROGENEITY IN THE FLORIDA SCRUB JAY

Analysis of mortality from a 25-yr sample of 380 Florida Scrub Jays, A phelocoma c. coerulescens, shows that actuarial senescence (increase i n mortality with age) occurs. This refutes the notion that adult morta lity is independent of age in birds, and has important implications fo r evolutionary dynamics. We point to two major factors that may act to mask underlying patterns of actuarial senescence: (1) Selection throu gh time (demographic heterogeneity): because selection inevitably weed s out lower quality individuals first, overall mortality may appear to be constant or even decreasing, despite an increasing force of mortal ity acting on birds of higher quality. (2) Conflicting processes: one source of mortality may act to decrease mortality over time (e.g., gro up size effects), while another acts to increase it (e.g., degenerativ e senescence). Age-specific data, measures of intrinsic quality that a re independent of mortality, and exposure of sources of heterogeneity and of conflicting processes allow us to demonstrate a clear pattern o f actuarial senescence in an unmanipulated, natural population of long -lived birds. The slow rate of increase in mortality is consistent wit h the hypothesis that mortality increases more slowly with age in bird s than in mammals.