Individual timing consistency across purple martin (Progne subis) migrations
Migration timing in long-distance migratory birds plays an essential role in individual survival and fitness. Migration schedules determine when individual birds depart and arrive seasonally between their overwintering and breeding sites. Bird migration timing may be largely driven by internal routines with some plasticity to environmental conditions, but individual timing across migrations has been little explored. To investigate the consistency of individual timing across migrations, I examined the individual order of migration timing in purple martins (Progne subis), a neotropical migratory songbird that travels between breeding sites throughout eastern North America and winter sites in Brazil. Migration timing data were collected for 295 different individual purple martins spanning over nine years by using light-level geolocators deployed during the breeding season at sites across the range and collected at the same sites the following year. I used a linear mixed-effect model (LMM) to examine the influence of the rank order of departure dates in one season on the rank order of four subsequent migration events while controlling for the effects of breeding latitude, sex, and age. Overall, I found that the individual rank order of migration timing in purple martins was conserved across migrations. Rank order timing was consistent between fall departure date from the breeding site and spring arrival dates in the following year (0.28 0.03, 95% CI 0.21-0.34), as well as the finer scale across fall migration departure and arrival dates (0.33 0.05, 95% CI 0.23-0.42), over the wintering period (0.39 0.04, 95% CI 0.30-0.48), and across spring migration (0.03 0.001, 95% CI 0.026-0.033). These results demonstrate that purple martin exhibit consistency in individual migration timing throughout the annual cycle. Migration distance also played a significant role, as the consistency of rank order timing lessened with distance traveled. Understanding how individual birds time migrations and if individuals are consistent between events can provide insight into how shifts in the environment with climate change could lead to a mismatch if migratory birds are unable to adapt. Future studies should examine if purple martins are able to adjust their migration timing and how long these changes persist in response to environmental alterations.
Purple martin, Progne subis, Migration, Migratory birds