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.