Using Winter Juvenile/Adult Ratios as Indices of Recruitment in Population Models

Abstract

Abstract There is an increasing interest in using winter juvenile counts as indices of recruitment in species that show delayed plumage maturation, especially for species, such as dispersed breeders, for which it is difficult to obtain good productivity estimates. To date, however, the needed mathematical work to assist in interpreting these winter juvenile/adult ratios has not been conducted. A matrix-based population model is presented that is modified to allow the fecundity component to be measured in mid- to late winter. This model is simplified to a set of equations that relate adult survival, winter juvenile/adult ratios and population growth rates, allowing an assessment of population trend with only one survival rate and age ratio data. These models have the advantage of not requiring that age of first breeding be well known. As an example, age ratios and survival rates of Harlequin Duck (Histrionicus histrionicus) populations in British Columbia and Maine are presented. Models for both population...