BODY-COMPOSITION OF WINTERING CANVASBACKS IN LOUISIANA - DOMINANCE AND SURVIVAL IMPLICATIONS

I studied effects of sex, age, and month on body mass and composition of Canvasbacks (Aythya valisineria) at Catahoula Lake (CL) and the Mis sissippi River Delta (MRD), Louisiana, in winter 1987-1988. Size-adjus ted body mass and fat varied by age, month, and site. Fat levels in im matures were greater at MRD than at CL, but changed similarly at both sites. Adults at MRD were also fatter than those at CL in early winter , especially in December, but fat levels of all birds increased from e arly to late winter and were equivalent by late winter. Body protein a djusted for structural size varied by age, month, and site, and sex, m onth, and site. Protein levels by site were higher in adults and males than in immatures and females, but sex- and age-related differences a veraged < 11 g (5%). Changes in size-adjusted leg muscle protein (inde x of feeding activity) and body fat were positively associated. This r elation (my measure of feeding efficiency) was not affected by sex or age of Canvasbacks, implying that all birds at CL or MRD fed at simila r efficiencies by month. At both sites, females were fatter than males ; thus, although males were larger and potentially dominant to females , it appears that females were not disadvantaged in their acquisition of nutrients during the year of study. Body mass of CL and MRD birds i n winter 1987-1988 was greater than that of Canvasbacks wintering else where. If overwinter and annual survival of Canvasbacks are related to their relative body mass during winter, then survival probabilities o f CL and MRD birds may be high compared to other wintering populations .