The postbreeding ecology of the redhead duck (Aythya americana) on Long Island Bay, Lake Winnipegosis, Manitoba.

Postbreeding ecology of the redhead duck (Aythya americana) was studied on Lake Winnipegosis from 1974-1977.Daily energy expenditure, including energy for activities and molt, showed little seasonal variation. Expenditures for molt were reduced by temporally staggering molt in different feather tracts. Redheads conserved energy through behavioral adaptations and by compensating higher demands with reduced expenditure elsewhere.Plant material comprised 92 and 98% of the dry aggregate weight of food consumed by males and females. Drought in 1977 influenced distribution of redheads through its' effect on submergent vegetation.Body molt of pennaceous feathers in tracts II, III, and IV exhibited a bimodal pattern indicative of basic and alternate feather generations in males. Tracts I and VI molted irregularly, and tract V molted once coinciding with the single down molt. Prebasic molt was advanced in most tracts with the earlier termination of breeding during drought, whereas the prealternate molt was unaffected.Little annual variation in timing of wing molt in males occurred, suggesting that this molt was under photoperiodic control. Upper-wing coverts were molted gradually to provide protection to the exposed wing.Digestive organ size in males fluctuated seasonally in response to diet quality, quantity, and innately with predictable events. Changes in the gastro-intestinal tract at the beginning of molt were related to enhanced protein uptake.The concept of a labile protein reserve was applicable to seasonal variability in the size of proteinaceous tissues. Greater food intake correlated with higher tissue weights, whereas lower intake and increased protein demand caused a reduction in tissues. Skeletal muscles contributed most to the reserve. Reserves were drawn on throughout the flightless period to meet protein requirements.Lipids were stored in the abdominal cavity, on mesenteries of the small intestine, and in the subcutaneous depot, which was also the largest. Total fat content in males was lowest during breeding and averaged 8 to 9% of body weight at the onset of wing molt. Total fat varied inversely with body water content, and independently of fat-free carcass weight. Foraging time explained 81% of the variation in seasonal lipid levels