Nutrient-Reserve Dynamics and Diet of Breeding Female Gadwalls

We analyzed food habits, digestive organs, and nutrient reserves (lipid, protein, and mineral; the latter two corrected for structural size) of yearling and adult female Gadwalls (Anas strepera) collected in southwestern Manitoba. Gadwalls are herbivores outside the breeding season and we predicted, therefore, that they would be relatively inefficient at foraging for invertebrates. Thus, they provided a test of our hypothesis that if female ducks normally encountered protein deficiencies during egg production, they would store and then use protein reserves, but would not use lipid reserves at a greater rate than do other ducks. Predictably, we found that prelaying and laying females consumed a low proportion (50%) of animal matter relative to other Anas spp. Overall, lipid reserves declined by 0.78 g for every 1 g of lipid deposited in eggs, a rate similar to those reported for closely related species. Separate analyses suggested that yearlings began rapid follicle growth (RFG) with 15g less lipid reserve than did adults and used these reserves at a lower rate (-0.62 g vs. -0.94 g per gram lipid committed to eggs). Protein reserves declined by 0.16 g for every 1 g protein deposited in eggs, but neither mass nor rate of use of these reserves differed between yearlings and adults. In females collected late in the laying cycle (≤5 developing follicles), number of developing follicles was positively related to protein reserves (r = +0.52). Females in early RFG (1 or 2 developing follicles) had significantly more lipid (27 g) and protein (8 g) reserves than did non-breeders (no developing follicles), suggesting a threshold amount of these reserves is reached before initiation of RFG. Mineral reserves were unrelated to eggshell production. Female Gadwalls are exceptional among temperate-nesting ducks in use of protein reserves during egg production, yet are unexceptional in rate of use of lipid reserves. Thus, we conclude that data from them support the hypothesis that temperate-nesting ducks are generally lipid-, not protein-, limited. Finally, we hypothesize that among-female Variation in size of reserves, especially lipid, at the end of egg laying reflects different tactics for successfully completing incubation.