Investigating the plasticity of migration timing to en-route environmental variability in a long-distance migratory songbird

Background/Question/Methods

Long-distance avian migrants are particularly threatened by a rapidly changing environment which induces a growing mismatch between critical environmental conditions and the breeding season, forcing them to greatly rely on their phenotypic plasticity in migration timing to remain synchronised. One such neotropical migrant, the purple martin (Progne subis) is a colonial aerial insectivore that exhibits a high intraspecific variation in migration strategies. How this plasticity responds to en-route environmental factors and whether migration timing will remain synchronised with advancing springs is largely unknown. The objective of this study is to investigate how weather variability (e.g. temperature, precipitation, wind) and change in daylength experienced along purple martin spring migration routes influences: (1) spring migration timing, (2) migration pace, and (3) the duration of stopovers, in distinct purple martin populations using precise individual migration tracks. Direct tracking (2007-2019) using light-level geolocators deployed across the eastern breeding range provided full migration route and timing data. These tracks were aligned with daily weather data using the Environmental Data Automated Track Annotation System on Movebank (movebank.org) and used to determine daylength.

Results/Conclusions

Our full data set includes 314 migration tracks from 5 latitudinally distributed regions (Northern n = 75, mid-interior n = 100, mid-coastal = 57, low-interior n = 18, low-coastal n = 16, and Southern n = 48). Overall, migrating purple martins traveled at a rate of 360 ± 245 km/day and had stopover durations ranging from 2 to 46 days. We found a latitudinal pattern in weather encountered by birds en-route, where northern breeders encountered colder and rainier conditions with longer daylengths than southern breeders. Preliminary results (using GLMMs) indicate that as stopovers duration increases (estimate = -8.43, CI: -9.71, -7.15) migration rate decreases. Similarly, stopover duration increased with warmer temperature (estimate = 1.0, CI: 0.5, 1.4), increased precipitation (estimate = 6.9, CI: 2.2, 11.6), and less favorable winds (estimate = -1.6, CI: -2.4, -0.8). These results suggest that favorable temperature, opposing winds, and precipitation may slow spring migration pace and increase stopover duration, possibly because birds could take advantage of warmer weather for foraging while more rain and unfavorable wind impedes migration. Understanding the environmental factors influencing migration timing will allow us to better predict how long-distance migrants respond to our changing environment and better direct future conservation efforts.