Direct-tracking technologies ( Stutchbury et al., 2009) provide the means to examine complete annual migration tracks, but studies where multiple migrations by the same individual are monitored are still rare ( Both et al., 2016). These studies report broad variation ( r = 0.04–0.51), both within and among migratory species ( Potti, 1998 Brown and Brown, 2000 Moller, 2001 Ninni et al., 2004 Cooper et al., 2009 Studds and Marra, 2011). For songbird migration, most previous studies have focused on the use of observational data to determine the individual repeatability (r) of spring migration departure and arrival dates. Longitudinal data are ideal for research on these themes because they provide the opportunity to investigate phenotypic variation within individuals in response to varying environmental conditions across years ( Charmantier and Gienapp, 2014). Steep population declines among migratory species ( Both et al., 2010), lends urgency to determining whether constraints on adaptive timing are a contributing factor.
To what extent phenotypic plasticity and/or micro-evolution are the mechanisms responsible for population-level advances in the spring migration timing of some landbirds has been hotly debated ( Knudsen et al., 2011 Charmantier and Gienapp, 2014). Phenotypic plasticity in animal migration timing could provide the means for rapid acclimation to environmental change, as compared to adaptive responses through genetic change ( Charmantier and Gienapp, 2014). However, whether phenotypic plasticity will be sufficient to keep up with the pace of climate change remains to be determined. Further, large, within-individual variability in migration dates (0–24 days) suggest that advances in spring arrival dates with climate change that have been reported for multiple songbird species (including purple martins) could potentially be explained by intra-individual flexibility in migration timing. In this first, repeat-tracking study of a diurnal migratory songbird, the high within-individual variability in timing that we report may reflect the greater influence of environmental and social cues on migratory timing, as compared to the migration of more solitary, nocturnally migrating songbirds. Fall migration timing was more variable at the individual level (depart range = 0–19 days gulf crossing range = 1–15 days arrive range = 0–24 days) and less repeatable, with fall crossing of the Tropic of Cancer being the least repeatable ( r = 0.0001). Spring departure and arrival dates were fairly repeatable across years (depart r = 0.39 arrive r = 0.32). In contrast to previous studies of songbirds, we found broad, within-individual variability between years in the timing of spring departure (0–20 days), spring crossing of the Gulf of Mexico (0–20 days), and breeding site arrival (0–18 days). Using the largest, repeat-tracking data set available to date for a songbird ( n = 33, purple martin Progne subis), we investigated individual variability in migration timing across 7,000–14,000 km migrations between North American breeding sites and South American overwintering sites. Longitudinal data for individuals sampled across migration are ideal for investigating phenotypic plasticity in migratory timing programs, but remain exceptionally rare.
It is unclear whether individual long-distance migratory songbirds can flexibly adjust their timing to varying inter-annual conditions. Migratory animals may be particularly at-risk due to global climate change, as they must match their timing with asynchronous changes in suitable conditions across broad, spatiotemporal scales. 2Purple Martin Conservation Association, Erie, PA, United States.
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