N. Nur et Wj. Sydeman, Survival, breeding probability and reproductive success in relation to population dynamics of Brandt's cormorants Phalacrocorax penicillatus, BIRD STUDY, 46, 1999, pp. 92-103
The size of the breeding population of Brandt's Cormorants Phalacrocorax pe
nicillatus on Southeast Farallon Island, off the coast of central Californi
a, LISA, shows large annual fluctuations and has declined by 50% since the
1970s. We investigated patterns of variation in adult survival, breeding pr
obability, resighting probability, juvenile survival and reproductive succe
ss, to determine the extent to which variation in demographic processes ref
lected variation in environmental conditions (e.g. food availability) versu
s density-dependence, using observations of breeders and non-breeders from
1976 to 1995. Resighting and breeding probabilities varied significantly am
ong years, and both were positively correlated with an annual index of prey
availability the abundance of juvenile rockfish. Adult survival between ye
ars was significantly correlated with changes in juvenile rockfish abundanc
e and differed between sexes (0.77, males; 0.71, females). Female, but not
male, survival appeared age-specific: older females showed reduced survival
, especially during El Nino years. Intermittent breeding appeared common in
this population: 54% of males and 49% of females estimated to be alive in
a given year were not observed breeding in that year. Reproductive success
varied in relation to the juvenile rockfish index but not in relation to po
pulation size. The return rate of juveniles, an inner of survival during th
e first three years of life, was negatively related to sea surface temperat
ure, both in the year of hatching and in the third year of life, when indiv
iduals first return to the colony to breed. We conclude that this populatio
n is strongly susceptible to environmental fluctuation. All parameters of s
urvival and reproduction deteriorate under poor environmental conditions, w
ith no evidence of buffering, and there was no evidence of density-dependen
ce. Thus extrinsic, not intrinsic, forces appear most important in explaini
ng recent population fluctuations.