Age at First Return to the Breeding Colony and Juvenile Survival of Sooty Shearwaters

Abstract

Using mark-recapture, we estimated the age at first return and survival of juveniles of the Sooty Shearwater (Puffinus griseus), a medium-sized burrow-breeding procellariiform, at two New Zealand breeding colonies, Taiaroa Head and The Snares. Sooty Shearwaters were first observed at the colony between 2 and 10 years after being banded as chicks, with a mean of 5.7 years at Taiaroa Head and 5.4 years at The Snares. After accounting for imperfect detectability with a mark-recapture model, we estimate the mean age at first return as 4.8 years. Extrapolation from observed delays between arrival and breeding of the Short-tailed Shearwater (P. tenuirostris) suggests that the mean age at first breeding was 7.7 years. Annualized estimates of survival from 0 to 2 years were 0.54 at Taiaroa Head and 0.41 at The Snares. Overall estimates of the generation time and maximum rate of population growth were 13.1 years and 7.7%, respectively. Many published estimates of age at first return and age at first breeding are biased because they implicitly assume perfect detectability of individuals. Such an assumption in turn leads to bias in estimation of generation time and maximum rate of population growth. A generic population model for the Procellariiformes, in which the parameters are scaled against generation time, could prove to be a useful tool in guiding management of bycatch and sustainable harvest.