The role of atmospheric conditions in the seasonal dynamics of North American migration flyways

Abstract

Aim Avian migration strategies balance the costs and benefits of annual movements between breeding and wintering grounds. If similar constraints affect a large numbers of species, geographical concentrations of migration routes, or migration flyways, may result. Here we provide the first population‐level empirical evaluation of the structure and seasonal dynamics of migration flyways for North American terrestrial birds and their association with atmospheric conditions. Location Contiguous USA. Methods We modelled weekly probability of occurrence for 93 migratory species using spatio‐temporal exploratory models and eBird occurrence data for the combined period 2004 to 2011. We used hierarchical cluster analysis to identify species with shared migration routes based on normalized spatio‐temporal representations of autumn migration. We summarized atmospheric conditions within flyways using nocturnal wind velocity and bearing estimated at three isobaric levels (725, 825 and 925 mbar) for the combined period 2008 to 2011. Results We identified three migration flyways: an eastern and western flyway whose paths shifted westwards in the spring, and a central flyway whose core boundaries overlapped with the eastern flyway and whose width was more constricted in the autumn. The seasonal shift of the eastern flyway created potentially longer migration journeys in the spring, but this longer route coincides with a low‐level jet stream that may enhance migration speeds. Atmospheric conditions appeared to have a more limited role in the seasonal dynamics of the western flyway. Main conclusions Migration routes for terrestrial species in North America can be organized into three broadly defined migration flyways: a geographically distinct flyway located west of the 103rd meridian and two interrelated flyways located east of the 103rd meridian. Seasonal shifts in flyway locations reflect the influence of looped migration strategies that for the eastern flyway can be explained by the trade‐off between minimizing total migration distance while maintaining an association with favourable atmospheric conditions.

Convergence of broad-scale migration strategies in terrestrial birds

“…We expect spatial convergence to be most pronounced for transoceanic migrants and characterized, based on previous findings [23,24], by clockwise looped migration trajectories. When contrasted with other Western Hemisphere migrants, we anticipate that populationlevel migration speeds for transoceanic migrants will be faster owing to the use of non-stop flights over longer distances.…”

“…Seasonal variation in atmospheric conditions [19,24] and seasonal changes in ecological productivity [25] have been identified as likely drivers for these strategies. For species that conduct transoceanic migration within the Western Hemisphere, a migration flyway has been identified within the eastern portion of North America [24]. Migration strategies within this eastern flyway are characterized by clockwise looped migration trajectories where potentially longer spring migration routes occur west of autumn migration routes [23,24].…”

Spring phenology of ecological productivity contributes to the use of looped migration strategies by birds

“…Using a macroecological perspective, we examined how the weekly distributions of multiple species identified in the eastern and western North American migration flyways [2] intersect rspb.royalsocietypublishing.org Proc. R. Soc.…”

“…In previous work [2], we clustered 93 species into three autumn migration flyways in North America, classified as western (n ¼ 31), central (n ¼ 17) and eastern (n ¼ 45). From these species, we selected for analysis 26 from the western flyway, 6 from the central flyway and 25 from the eastern flyway (see the electronic supplementary material, table S1).…”

“…An interesting feature shared by the eastern and western flyways is that birds tend to migrate northward in the spring west of their southward trajectory in the autumn, generating a looped migration trajectory [3]. Looped migration strategies in the eastern and western flyways result in potentially longer migration journeys in the spring for species in both flyways [2].…”