Does hatch date set the clock? Timing of post‐fledging movements for families of a colonially breeding, long‐distance migratory songbird

Factors that influence the development of migration timing in juvenile songbirds have implications for the ability of individuals to respond positively to rapid environmental changes. We investigated the impacts of nest timing on the post‐breeding movement timing of juveniles and adults of a migratory songbird. We tested whether first egg date and environmental factors predicted the initiation of post‐breeding stages: fledge date for juveniles, and colony departure date for both adults and juveniles. At breeding colonies of purple martin Progne subis in southern Ontario, Canada, we monitored nests to determine the date of nest initiation (‘first egg date') and deployed 122 coded radio‐frequency tracking tags on young and adults to determine the timing of post‐breeding stages. We found that first egg date, the number of nestmates, and age of parents were the main predictors of fledge date. Of these three factors, only first egg date carried through post‐breeding to influence colony departure date for juvenile birds, but this relationship weakened between first egg date and departure date. Nestmates tended to fledge together (range 0–4 days) but exhibited greater variation in colony departure timing (range 0–11 days). Further, while first egg influenced departure date, increasing variation between fledging and departure led to some birds departing breeding colonies at a younger age, suggesting influence of local environmental factors (e.g. social or photoperiod change) in departure decisions. The timing of adult colony departure date was independent of first egg dates. In sum, our results suggest a role for first egg dates in setting timing of post‐breeding movements, but with variation introduced between fledge and departure dates. Experimental manipulations of photoperiod in a wild setting are needed and future research should investigate whether the timing of movement driven by nesting timing, holds across the rest of migration or even the lifetime of bird.