Migrating birds make choices en route about where and when to rest and replenish energy reserves, often in areas they have never visited before and at a time when food and weather may be unpredictable. Their ability to reach their destinations in good condition and in a timely manner can influence their chances of gettting the best wintering or breeding territories and, thus, influence their survivorship or reproductive sucess. The Blackpoll Warbler, Dendroica straiata, almost doubles its body mass with fat reserves as it prepares for a trans-oceanic migration over the North Atlantic to winter in South America. This species has been found to have the lowest rate of weight loss during migration. This suggest that they use fat more efficiently and can spare skeletal muscle for use as energy reserves longer than many other birds. In contrast, while the Yellow-rumped warbler, Dendroica coronata, overlaps with the Blackpoll warbler on its breeding grounds, it is a short-distance migrant (primarily overland) and does not put on such extensive fat reserves. The focus of this study is to investigate differences in the behavior and phsiology of these two species of warblers that reflect their different energetic demands of migration. In this project, the primary emphasis is on the role of corticosterone and related metabolites in controlling food intake and fat reserves. A series of laboratory and field studies will investigate possible differences in patterns of corticosterone secreation in these two species that may reflect their different patterns of fattening and their respective need to protect skeletal muscle during migration. In laboratory studies, the PIs will investigate the possibility that corticosterone has multiple roles in regulating the seasonal changes in energy demand in birds and that these two species may have different threshold for corticosterone's effects on fattening and muscle protein utilization. By understanding how birds with different energetic demands have evolved to meet the challenges of migration under optimal conditions. we will have a greater understanding of the behavioral and physiological constraints of birds when conditions such as weather and habitat quality deteriorate. The results from this project may illustrate how environmental factors encountered during the migratory period may impact bird populations throughout the annual cycle.
