| Conserving avian populations requires understanding how they are limited by density-independent factors and regulated by density-dependent processes. To better understand the relative importance of limiting factors and regulatory processes in wintering American Redstarts (Setophaga ruticilla), I carried out two related studies. First, to determine how food availability affects space use, body composition, and migration timing, I experimentally decreased food availability in high-quality mangrove habitat. Using an insecticide, I reduced food by ∼80%, which mimicked natural losses in nearby scrub habitats. I found that food-reduced redstarts deposited fat and lost muscle compared to control birds. Subsequently, food-reduced redstarts experienced on average a one-week delay in departure on spring migration. Previous work has demonstrated that for each day delayed after the first male arrival on the breeding grounds, redstarts experience an 11% decrease in the chance of successfully reproducing. Thus, my results demonstrate experimentally, for the first time, that fluctuations in winter food-availability can lead to fitness costs for migratory birds, and that the mechanism involves a fat-muscle trade-off. Second, to understand how limiting factors and density-dependence interact to drive population dynamics, I used four years of data on redstarts wintering in Jamaican scrub and mangrove forests. In a dry and food-limiting year in scrub, I found that individuals on territories surrounded by a high density of conspecifics experienced large losses in food availability, suggesting a density-dependent depletion of resources. These losses in food were correlated with poor body condition, and individuals on high-density territories delayed departure on spring migration. In two wetter and less food-limiting years in scrub, and in all years in high-quality mangrove habitat, no effects of neighbor density were evident and density-independent factors alone appeared to determine body condition and departure date. Previous research has shown that poor body condition reduces annual survival and that delayed departure has carry-over effects into the breeding season, resulting in lower fecundity. Thus, both food limitation and neighbor density appear to drive population dynamics, but density-independent factors may override the negative effects of density when weather conditions are favorable. |
