Age and sex distributions of wintering Surf Scoters: Implications for the use of age ratios as an index of recruitment

We assessed age- and sex-specific distribution patterns of Surf Scoters (Melanitta perspicillata) wintering in southern coastal British Columbia, Canada, and evaluated potential biases associated with the use of male age ratios as an index of their recruitment. For surveys conducted during 2000 through 2002, annual variations in male age ratios were evident, with estimates ranging from 0.07 +/- 0.02 to 0. 13 +/- 0.03 (SE; first-year males:total males). Flock composition patterns indicated first-year males did not distribute independently, but tended to associate with other first-year males. With respect to habitat, male age-class proportions did not vary among shoreline substrate types, but higher proportions of first-year males were found in sites with low exposure to wind and waves (<50 km fetch). To determine the efficacy of male age ratios for indexing recruitment, we used a power analysis, which incorporated overdispersion in age-class segregation and determined the sample sizes required for precise estimates of the proportion of first-year male Surf Scorers. Samples of approximately 600-1000 total males were required to obtain 95% confidence limits within 5% of the estimated mean, with sampling accuracy leveling off at about 2% when 6000 or more males were aged. Recruitment among waterfowl species is typically modeled using the ratio of female recruits to breeding-age females. Based on the sex and male age-ratio estimates obtained in this study, we calculated a female age ratio of 0.23 (first-year females:adult females).