Carry-over effects and the dynamics of migratory bird populations

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Long-term declines in the abundance of migratory birds have led to renewed interest in whether migratory bird populations are most affected by events on their breeding grounds, on wintering areas or on migration. Some studies suggest that events on breeding grounds may be sufficient to explain long term declines but others show that events in wintering areas have greater impacts on migratory bird populations. Recent studies have also demonstrated that events on wintering grounds can carry over into the breeding season and influence individual reproductive success and the dynamics of populations on breeding grounds. Investigation of the importance of carry-over effects has been hampered by our inability to follow individuals throughout the annual cycle and the lack of information on the links between wintering and breeding grounds. However, recent technological advances allow examination of migratory connectivity and more detailed investigation of how events on both breeding and wintering grounds influence populations. We will use radiotelemetry and stable isotope analyses of feathers to investigate how winter and breeding grounds are linked, and evaluate the importance of carry-over effects for the population dynamics of a short-distance altitudinal migrant (the American Dipper) and a long-distance neotropical migrant (the Yellow Warbler). Specifically, we will, 1) ask how weather, food availability and density on wintering grounds influence the timing and duration of migration, the onset of breeding and annual productivity of migratory dippers, 2) evaluate how variation in climatic conditions and winter habitat use influence the arrival times, condition and reproductive success of yellow warblers breeding in western Canada (Revelstoke, BC and Inuvik, NWT), and 3) use population models to assess the importance of carry over effects for migratory bird populations. This research will evaluate the importance of carry-over effects and contribute to our understanding of how climate change and habitat loss on wintering grounds will influence populations of migratory birds.