Resources and arrival of landbird migrants at northerly breeding grounds: Linking en route with breeding season events
For migratory birds, timing of arrival and energetic condition on arrival at the breeding grounds are influenced by en route events. Both arrival timing and condition are thought to influence breeding season survival and reproductive performance, thus linking the spring migratory and breeding phases in a migrant's annual cycle. This dissertation examined the behavioral ecology of intercontinental landbird migrants as they approached and arrived at high latitude breeding grounds and their reproductive performance in relation to energetic condition upon arrival. Four related questions formed the basis of this research: (1) assess the fitness consequences of habitat use by migrants stopping in forests bordering northern Lake Huron in Michigan's eastern Upper Peninsula; (2) evaluate the importance of emergent aquatic insects (Diptera: Chironomidae) as an energy source; (3) evaluate the significance of arrival fat on reproductive performance; (4) gain insight into how events during the spring migratory phase affect breeding season events. In a study examining the fitness consequences of habitat use by migrants in Michigan's eastern Upper Peninsula, I found that several species gained mass during stopover. Mass gain during migratory stopover reflects addition of lipid stores, critical for fueling migration. This finding supports earlier researchers' suggestions that areas adjacent to northern Lake Huron provide important habitat for spring migrating landbirds. Further, evidence suggests that emergent aquatic insects are an important resource for landbirds migrating through Michigan's eastern Upper Peninsula. Both arrival timing and arrival condition appeared to influence fitness in American Redstarts (Setophaga ruticilla ). Redstarts arrived with fat stores, which evidence suggests may have been important as insurance against poor weather typical of early spring. Further, both arrival timing and arrival fat appeared to influence reproductive performance. Early females initiated clutches early and produced heavier nestlings. Early males appeared to settle on higher quality territories and hatched nestlings sooner than later arrivals. Finally, both females and males arriving with fat experienced gains in reproductive performance as shown by increased clutch size, egg volume and nestling mass. The results have implications for understanding how events occurring during one phase of the annual cycle influence survival and reproductive performance in subsequent phases.