Winter Age Ratios and the Assessment of Recruitment of Harlequin Ducks

Recruitment is the process by which young birds are added to the breeding population. The two most commonly used methods to estimate recruitment in ducks are pair/brood counts, and fall age ratios based on wings returned by hunters. Direct counts on the wintering area to determine the proportion of young males to adult males (age ratios), can be used for species with delayed plumage maturation. This is useful for species that are difficult to study on the breeding grounds and are seldom hunted, such as Harlequin Ducks (Histrionicus histrionicus). Such an approach is only valid if age classes are equally sampled. Between 1994 and 1999, the proportion of male Harlequin Ducks that were immatures in the Strait of Georgia, British Columbia, was estimated at 0.068. The proportions differed across years. Little difference occurred through the winter months until spring, when there was a significant increase in proportions in March. If these age ratios are accurate, then recruitment would not be compensating for annual adult mortality and the population could have been declining during the period of our study. However, if immature males are not distributed evenly among the population, then our assessment may underestimate or overestimate proportions. Delayed breeding means that immature birds experience several additional seasons of mortality prior to breeding, and thus the actual recruitment rate into the breeding population would be lower than that presented here.