Bone collagen and muscle dC in relation to the timing of the migration of Garden Warblers Sylvia borin during return migration from Africa

In migratory birds, seasonal factors interacting at different stages of the annual cycle can influence individual life histories. These have been well documented for Nearctic-Neotropical birds. Yet, seasonal interactions between the wintering, migration, and breeding periods have not been demonstrated for Palearctic-African passerine migrants. We tested whether variation in longand shortterm dietary choice of the Garden Warbler Sylvia borin can influence events during the subsequent spring migration from the African winter quarters to the Palearctic breeding grounds. Using bone collagen and muscle carbon stable isotope (dC) analysis, we assessed the relationships between dietary history, body condition, and migration timing in the GardenWarbler during a return migration from Africa. We predicted that Garden Warblers arriving early will have significantly different muscle and collagen dC relative to those arriving later. Whereas muscle dC (referring to events in the immediate past) was not related to body condition, we found a strong association between body condition and collagen dC signatures (representing the integration of long-term events). Collagen and muscle dC indicate that birds passing through later originated from moister or cooler geographic areas. The observed variation in isotope signatures might relate to differences in habitat and geographic/population origin, or in dietary intake.