VARIATION IN BODY-MASS OF WILD CANVASBACK AND REDHEAD DUCKLINGS

We assessed variation in body mass of ducklings in single- and mixed-s pecies broods of wild Canvasbacks (Aythya valisineria) and Redheads (A ythya americana) 20-50 days old. Body mass of canvasback ducklings was not affected by year and season (early vs. late hatch date) despite c hanges in water conditions. Mean body mass of male and female Canvasba cks did not differ in Class IIA but did differ in older age classes. W ithin-brood differences in body mass tended to be higher in Class IIA ducklings (6-7% of mean body mass for Canvasbacks, 9-11% in Redheads) and generally declined to 4-6% in Class IIC and older ducklings. Some within-brood differences were as high as 20-30% of mean body mass. Tes ts to assess sources of within-brood variation (age, sex, and season) in body mass for Canvasbacks were inconclusive. Variation within brood s was generally less than that among broods for both Canvasbacks and R edheads. The lack of differences in duckling body mass between single- and mixed-species broods for any age class, sex, or species suggests that mass was not affected by interspecific brood parasitism.