Individual Variability in Migration Timing Can Explain Long-Term, Population-Level Advances in a Songbird

Abstract

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. It is unclear whether individual long-distance migratory songbirds can flexibly adjust their timing to varying inter-annual conditions. Longitudinal data for individuals sampled across migration are ideal for investigating phenotypic plasticity in migratory timing programs, but remain exceptionally rare. 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. 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). Spring departure and arrival dates were fairly repeatable across years (depart r = 0.39; arrive r = 0.32). 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). 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. 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. However, whether phenotypic plasticity will be sufficient to keep up with the pace of climate change remains to be determined.

New indices for assessing changes in seasons and in timing characteristics of air temperature

“…Increases in global average temperatures have been welldocumented in the literature, but other characteristics of temperature, like regional and seasonal variability or timing, have been less well studied (Allen and Sheridan 2016;Rogers 2013;Trenberth 1983). Among the different thermal characteristics of climate change, timing is an important issue for many bio-climatological processes, as, e.g., earlier onset of spring/summer or delayed onset of autumn and shortening of winter can have impacts on flowering dates (Sperling et al 2019;Woznicki et al 2019), migratory species (Fraser et al 2019;Helm et al 2019;Schmaljohann 2019), and human adaptation to extreme temperature environments. In this work, we developed a new configuration of the seasons in order to quantify changes in timing in thermal climate characteristics.…”

Timing to temperature: Egg‐laying dates respond to temperature and are under stronger selection at northern latitudes

“…Additional constraints on lay dates could result from asynchronous changes across life history stages (Visser and Gienapp 2019). While purple martins exhibit high within‐individual variation in migration timing (Fraser et al ), it is not known if there are latitudinal patterns in individual timing that may constrain spring arrival dates. Our finding of stronger selection at more northern latitudes may be an early warning that these populations are more vulnerable, if they are already at or near the minimum time needed between spring arrival date and lay dates.…”

Endogenous Programs and Flexibility in Bird Migration

“…Variation within the population can reveal interesting characteristics, where interactions of the inherited migration phenotype with different environmental factors can lead to the evolution of diverse migration patterns. Phenotypic plasticity may for example lead to advancement of migration timing in response to environmental conditions, in particular in flock-migrating birds (Fraser et al, 2019). Populations may furthermore comprise migratory and resident fractions (i.e., partial migration), or migration may differ between sex and age classes (i.e., differential migration) (Terrill and Able, 1988;Newton, 2008).…”