Autumn migratory timing and pace are driven by breeding season carryover effects

Abstract

Carryover effects occur when an individual's experience during one stage of the annual cycle influences their behaviour in subsequent stages. This includes stages of the annual cycle, such as migration, when behaviour, such as departure timing, is also affected by circannual and circadian cues. In birds, the initiation of autumn migratory movements is often related to the timing of the cessation of the breeding season. However, less is known as to whether breeding season carryover effects also affect the pace of migration. To determine whether there are carryover effects from the breeding season to migration in barn swallows, Hirundo rustica, we monitored nests and used the Motus Wildlife Tracking System to determine migratory departure dates and the pace of migration. Barn swallows that finished breeding later in the summer initiated migration later and migrated faster through North America than swallows that finished breeding earlier in the summer. However, daily flight speeds were unrelated to the timing of the breeding season, suggesting that late-departing individuals increased their overall pace by reducing stopovers. Our results suggest that although barn swallows do not establish a nonbreeding territory, there are constraints during migration that require late-departing individuals to compensate for carryover effects from the breeding season. Late departure is not necessarily disadvantageous if individuals are able to ‘catch-up’ to early-departing individuals, but more work is needed to assess whether late departure and the faster pace of migration affect the subsequent survival of these individuals.