Indices of structural size and condition of American wigeon (Anas americana)

Abstract

American wigeon (Anas americana) were studied throughout their annual cycle in British Columbia, Saskatchewan, and Manitoba between 1976 and 1978. Body measurements were obtained from 145 collected and 128 livetrapped birds. Collected birds were dissected and analyzed for lipid, protein, and water content, and organs and muscles were weighed. Similar measurements were made on two each of starved adult captive males and females to determine basal levels of reserves. The skeletons were excised from 13 other wigeon and the lipid-free dry weights of bone were determined. Body length + wing length was the best predictor of skeletal weight and could be used as a structural index (SI). Body weight varied greatly over the year and so is a poor indicator of structural size. The lipid and protein reserves were calculated and the total reserve weight was used as the condition index (CI) for each bird. Body weight/SI was a good predictor of CI but better estimates were obtained with plucked, eviscerated carcass weights. The best predictors of lipid reserves and protein reserves were weights of abdominal fat + skin fat and carcass weight/SI, respectively.