Winter age ratios and the assessment of recruitment of Harlequin Ducks

Recruitment in the process by which young birds are added to tile breeding population. The two most commonly used methods to estimate recruitment in d ucks are pair/brood counts, and fall age ratios based on wings returned by hunters. Direct counts on the wintering al-ea to determine the proportion o f young males to adult males (age ratios), can be used for species with del ayed plumage maturation. This is useful for species that are difficult to s tudy on tile breeding grounds and are seldom hunted, such as Harlequin Duck s (Histrionicus histrionicus). Such an approach is only valid if age classe s are equally sampled. Between 1994 and 1999, the proportion of male Harleq uin Ducks that were immatures in thr Strait of Georgia, British Columbia, M as estimated at 0.068. The proportions differed across years. Little differ ence occurred through the winter months until spring, when there was a sign ificant 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. Ho wever, if immature males are not distributed evenly among the population, t hen our assessment may underestimate ol overestimate proportions. Delayed b reeding means that immature birds experience several additional seasons of mortality prior to breeding, and thus the actual recruitment I ate into the breeding population would be lower than tl-lat presented here.