Nutrient-Reserve Dynamics and Control of Clutch Size in Northern Pintails Breeding in Alaska

We collected breeding Northern Pintails (Anas acuta) in subarctic Alaska during 1987 and 1988 to assess nutrient-reserve dynamics. Greatest body mass of both sexes occurred during the rapid-follicle-growth (RFG) period of females. Males lost lipid, but not protein reserves, between RFG and laying periods. Females lost an average of 94 g of lipid from RFG to early incubation. An average clutch contained less than half the amount of carcass lipid lost between RFG and early incubation; thus, endogenous lipid was used to meet both reproductive and maintenance energy costs. Females lost protein during laying; we estimate endogenous protein provided 21 to 62% of the protein requirement for egg production. Carcass protein reserves were negatively related to the proportion of clutch laid (P = 0.01), whereas lipid reserves were not, suggesting that protein limited clutch size. These patterns differ from those for most temperate-breeding ducks, but we are uncertain whether this results from geographic and/or phylogenetic variation.