Mallard (Anas platyrhynchos) populations breeding in temperate North America obtain a significant part of the energy and lipid requirements of reproduction at sites occupied prior to arrival on the breeding grounds. Protein for egg formation, however, is obtained principally from the diet during the nesting period. Both sexes arrive heavy and fat in North Dakota but experience substantial weight loss and lipid depletion during the nesting cycle. Weight loss is most pronounced among females and averages 25% from prelaying to late incubation. Body weights of both sexes are positively correlated with carcass lipid content. The paired male draws upon lipids early in the nesting season when an activity center is being established and defended and when females are preparing to nest. The female's lipid reserves are utilized primarily during laying and early incubation. The significance of lipid reserves diminishes as the nesting season progresses, and females do not acquire substantial lipid stores prior to renesting when the initial clutch is destroyed. The magnitude of lipid reserves carried in female carcasses is positively correlated with clutch size from mid-April to early June. Protein transfer for egg formation from flight and leg muscle and body organs can account for only a small part of the protein requirement for the clutch. By utilizing lipid reserves to meet energy requirements, the female can acquire sufficient protein from the diet to produce a large initial clutch even when foods are relatively scarce, whereas the renesting female must rely entirely upon food resources available at the breeding site for its nutrient and energy requirements.