There is growing evidence of carry-over effects in migratory birds. Aerial insectivores are declining across North America; therefore, to determine the cause of these declines, we must have a holistic view of their annual cycle. I use geolocators to map the annual movements of tree swallows breeding in Nova Scotia, Canada, determine if reproduction has carry-over effects on migration, and examine the effects of geolocators. Geolocators revealed that tree swallows began migration in July and had an extended stopover in the northeastern United States. They wintered in Florida or Cuba, returning from spring migration in late April. This study revealed that later breeding swallows began migration later than earlier breeding swallows, but all birds arrived on the wintering grounds around the same time, due to differences in stopover length. No short-term effects of geolocators were found, although the sample size was small, and thus these results must be interpreted cautiously.

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Movements of long-distance migratory birds are frequently well timed and routed in order to maximise individual fitness. However, individual timing of events or environmental conditions experienced have a potential to carry over into subsequent annual cycle stages and may have delayed fitness consequences. Therefore, knowledge of these seasonal interactions is crucial to identify key periods and regions that limit survival beyond the breeding period and thus to fully understand population dynamics of migratory species. Despite current evidence for seasonal interactions, there is no detailed knowledge of the complex of relationships within the full annual cycle, differences between sexes and populations or impact of environmental conditions during moult in one species. To unveil this system of seasonal interactions within the annual cycle, we directly tracked 103 great reed warblers (Acrocephalus arundinaceus) using light-level geolocators from five breeding sites across the breeding range to obtain information on timing and positions of annual cycle events. We then combined this information with various remotely sensed habitat condition metrics and stable isotopic composition of feathers from the non-breeding grounds to identify and quantify carry-over effects within the full annual cycle. Our...