Comparison of different approaches to the study of local recruitment of breeders

Although the recruitment of breeders is as important to population dynamics as mortality, it has received far less attention from statisticians workin g at modelling capture histories of marked animals. However, two main appro aches to studying local recruitment have become available in the last three years. Both deal with birds marked as chicks and then possibly resighted o r recaptured only as breeders (the same approaches may be applied more gene rally whenever breeders and non-breeders can be distinguished in a populati on by working with the sole observations 4 breeding animals). This situatio n is natural to species of colonial birds sampled on the breeding site wher e the young remain away From the breeding site previous to their first bree ding attempt. The first method published (Clobert el al., 1994) contrasts, in the context of a survival analysis, the capture rates of known breeders and the capture rates of animals tip to their first observed breeding attem pt. Young animals as a group are less likely to be 'captured' if some have not yet started to reproduce. It is then possible to estimate age-specific breeding probabilities, i.e. the proportions of breeding animals in each ag e-class. The second approach (Pradel, 1996, Pradel et al., 1997) addresses recruitment directly. Capture histories are read backwards starting from th e last observed breeding event and asking the question: when in the past di d an animal start breeding? In this way, one can estimate the probability t hat a breeding animal is a first-time breeder as a function of age, individ ual characteristics or environmental conditions. A third approach based on the history of individual animals is presented here for the first time. It involves estimating the probability that an as yet inexperienced animal sta rts to breed and can be implemented as a particular case of a two-state mod el with a non-observable state (Lebreton et al., 1999, this volume). Under general assumptions, there are simple relationships between the different p arameters that measure accession to reproduction but not all seem equally f lexible nor relevant to the same questions.