BARBOZA, P.S. ; JORDE, D.G.: Effects of winter diet on enrichment of 13C & 15N in ducks

Waterfowl select many foods during winter but depletion of lipid and conservation of protein may delay the effect of diet on body tissues. We predicted that retention of dietary isotopes would vary among tissues of black ducks (Anas rubripes) from January to March. Yearling ducks were raised on a common diet and housed in identical pens and ponds. Birds were switched from a maintenance diet to a ration formulated for breeding waterfowl: an animal diet of estuarine prey with C3 plants or a plant diet of C4 species. The animal diet was enriched in 15N (5.9 vs. 4.3ppt) and depleted in 13C (-22.0 vs. –13.7ppt) when compared with the plant diet. Body mass loss was similar between diets (7�11% of initial mass). Abundance of 13C in blood and muscle was similar to diets in March but tissues were more enriched in 15N than diets after 67d (10.6–9.5ppt animal; 8.2-7.4ppt plant) suggesting reutilization of N from dispensable amino acids. Dietary 15N in the essential acid threonine was low (-2.9 animal vs. –0.5ppt plant) and similar to those of blood cells at 3-11d. Intermittent fasting (4d/wk) did not affect isotopic C or N in blood cells after 8 weeks. However, females were depleted in 15N when compared with males on both diets in March. Diet groups were different in plasma 15N at 3d whereas differences in cells of blood (11d) and muscle (11 to 42d) were delayed and indicated slow turnover of cellular protein. Conversely, 13C abundance differed between groups for blood sampled at 3d and for muscle at 11d suggesting rapid turnover of energy substrates. Plasma 13C, and threonine 15N reflect acute changes in diet whereas sustained dietary shifts change isotopic levels in cells after 14-30d. Therefore, isotopic C and N in wild birds probably reflect changes with season and sex in diet and metabolism of tissues.