Small birds likely face energetic challenges in temperate zone winters posed by cold weather coupled with food scarcity. These challenges are often assumed to occur, but are rarely experimentally tested. I hypothesized that the naturally occurring, lower abundances of food in temperate zone winters limit a bird's ability to acquire optimal energy and ultimately limit fitness. In this dissertation, I show that supplementation of food decreased mortality and improved traits potentially associated with future reproductive success of wild swamp sparrows (Melospiza georgiana), supporting the hypothesis that winter food abundance limits fitness. These results come from a replicated and controlled food supplementation experiment conducted over three years. First, I demonstrated that following food addition, immigration increased, leading to higher densities, and that all age/sex classes experienced higher survival and maintained larger energy reserves (Chapter I). Survival was positively related to energy reserves, indicating that food availability limits survival through a bird's ability to maintain sufficient fat. In addition to causing mortality in winter, food limitation of energy reserves may carry over to affect future reproductive success by influencing timing of preparations for breeding, including migration. In Chapter II, I show that swamp sparrows decreased fat reserves over each winter, despite unlimited food availability, indicating that they adaptively regulated fat reserves, potentially to balance starvation and depredation risks. Fat reserves of control birds tracked recent temperature and control birds lost muscle throughout the winter, indicating that they were limited by food and were unable to reach optimal fat levels on a daily basis. These results suggest that limitation of energy reserves by food availability can be influenced by temperature and predator abundance. Lastly, I demonstrated that food abundance limits the timing of molt in the wild (Chapter III), an unprecedented finding. Because molt, migration, and breeding typically do not overlap, early molt might lead to earlier migration and breeding. Therefore, we hypothesize that timing of molt is another mechanism by which winter food abundance can limit reproductive success. These results provide strong evidence that food availability can limit wintering temperate migrants in a variety of ways.