Using citizen science efforts to examine spatiotemporal variation in avian migration phenology

Background/Question/Methods

In response to large-scale warming, many organisms have shifted their distributions and altered the timing of seasonal life events such as flowering, growing, hatching, breeding and migrating.  Understanding how the strength and magnitude of such responses varies across species and with ecological context is critical for being able to predict the consequences of ongoing and future climate change. Migration poses a particularly unique phenological challenge in that organisms must time their arrival with environmental conditions at distant locations. Individuals arriving too early may face adverse conditions and limited resources, while individuals arriving too late may face disadvantages in establishing breeding territories or finding high quality mates. We utilized data from eBird, an online checklist program where amateur birders can submit their bird observations for science, to examine spatiotemporal variation in spring arrival date for 18 common migratory species of eastern North America over the past 9 years. We examined the extent to which arrival date was related to spatiotemporal variation in minimum spring temperature, and investigated which species and geographical regions exhibited the strongest phenological responses.

Results/Conclusions

All species exhibited the expected latitudinal gradient in arrival date, but species differed in the speed with which they advanced northward. Red-eyed vireo (Vireo olivaceus) and common yellowthroat (Geothlypis trichas) are among the species that advance most slowly, while house wren (Troglodytes aedon) and barn swallow (Hirundo rustica) cover the same latitudinal distance in 50-67% of the time. Over the past decade, there has been a tendency for all species to arrive earlier (grand mean 0.16 days earlier per year, t = -3.17, p = 0.002). However, mean arrival dates vary even more strongly with minimum spring temperature across all species, locations, and years (grand mean 0.76 days earlier per °C, t = -7.53, p = 4.1e-13). Species that advanced northward more slowly during migration tended to have stronger phenological responses to minimum spring temperature. Phenological responses to temperature change were not uniform across a species range, but were greatest in the southeastern US. This study represents the most spatially comprehensive study of migration phenology of birds in North America, and demonstrates the taxonomically and geographically heterogeneous responses to climate change.