The Relationship between Latitude and the Timing of Spring Migration of North American Landbirds

Under laboratory conditions, two mechanisms have been identified that control the onset of migration. One mechanism inflexibly controls the initiation of migration, while the other requires external input from a changing environment. It has been suggested that the evolution of these mechanisms has an ecological basis related to the wintering latitude of the species. To establish whether the timing of migration was consistent with this ecological explanation, we studied the long-term mean timing of spring migration, and within-year and among-year variance in timing, in 27 free-living Nearctic migrant species. Species that wintered in higher, temperate latitudes migrated significantly earlier than tropical-wintering species. Tropical-wintering species showed significantly less within-year and among-year variation in the timing of migration, suggesting that the mechanism regulating their migration is primarily endogenous, with little influence of external stimuli. In contrast, breeding latitude showed no relationship to the mean date of migration, or the within-year or among-year variation in timing. These results are consistent with the notion that conditions that indicate time-of-year are unreliable or change imperceptibly (photoperiod) in the tropics, making an endogenous circannual clock valuable for controlling the onset and progression of migration. Temperate-wintering species were more likely to show interspecific correlations in timing than tropical species, further suggesting that external cues modulate spring migration in temperate-wintering species. Our results indicate that the migration regulation system employed might be ultimately determined by wintering latitude and associated environmental cues.