A range-wide domino effect and resetting of the annual cycle in a migratory songbird

Latitudinal differences in timing of breeding are well documented but how such differences carry over to influence timing of events in the annual cycle of migratory birds is not well understood. We examined geographic variation in timing of events throughout the year using light-level geolocator tracking data from 133 migratory tree swallows (Tachycineta bicolor) originating from 12 North American breeding populations. A swallow’s breeding latitude influenced timing of breeding, which then carried over to affect breeding ground departure. This resulted in subsequent effects on the arrival and departure schedules at fall stopover locations and timing of arrival at non-breeding locations. This “domino effect” between timing events was no longer apparent by the time individuals departed for spring migration. Our range-wide analysis demonstrates the lasting impact breeding latitude can have on migration schedules but also highlights how such timing relationships can reset when individuals r...