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1 December 2008 Using Winter Juvenile/Adult Ratios as Indices of Recruitment in Population Models
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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 populations suggest recruitment of young is insufficient to compensate for adult mortality, which is contrary to observed trends. Although some further methodological work is needed, such as better estimates of true adult survival and a further understanding of how to obtain unbiased estimates of juvenile/adult ratios, these models may prove to be a useful tool to assess population trends when detailed demographic data are not available.

Gregory J. Robertson "Using Winter Juvenile/Adult Ratios as Indices of Recruitment in Population Models," Waterbirds 31(sp2), 152-158, (1 December 2008).
Published: 1 December 2008

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