Response of Migrating Birds to Hurricane Sandy

Science Center Objects

The Challaenge: Each autumn, millions of landbirds migrate southward from breeding areas in the U.S. and Canada. Because these migrations occur during hurricane season, the impacts of hurricanes on migrating birds and their en route resting and foraging sites (stopover sites) are a real concern. Hurricane Sandy occurred late in the landbird migration season. By the time the storm made landfall in late October 2012, most birds that migrate to tropical wintering areas had already passed through the Mid-Atlantic region. However, the storm likely impacted temperate migrants that winter in Mid-Atlantic coastal habitats or were en route to wintering areas in the southern U.S. We applied a variety of techniques and data to assess possible impacts of Hurricane Sandy on the movement of nocturnally migrating landbirds: weather surveillance radar data, field surveys and eBird citizen science data, and dynamic simulation of migratory flights using an individual-based model (IBM) together with high-resolution modeled wind data.

The Science: Analysis of radar reflectivity data from coastal Virginia to Massachusetts revealed that many migrants avoided direct contact with the hurricane by departing in moderately supportive (northerly) winds as the storm approached. For a week following landfall, areas close to the hurricane’s path showed reduced migratory activity compared to previous years. In contrast, there was increased activity to the north and south. Post-storm densities of birds leaving stopover sites increased in developed areas and decreased in ag fields and wetlands, indicating some shifts in the habitats and locations used by migrants.

Distinct peaks in migratory departures around sunset, earlier than is typical among landbird migrants, were apparent in the radar data; field surveys confirmed these as dominated by American Robins. Analysis of departure reflectivity from October 27, as the hurricane approached, indicated that these migrants likely became grounded by rain in coastal Delaware and Virginia where they remained for a week. Simulated flights using the IBM supported this scenario, and more generally of temperate migrants being able to exploit the unusually supportive winds in late fall 2012. Unseasonably late eBird records of neotropical migrants in the northeastern U.S. and Atlantic Canada suggest that some birds were flung northwards with the storm system. IBM simulations of nocturnal flights to the Caribbean support this conjecture, and indicated that the strong circulation spanning the Atlantic seaboard continued to displace individuals for up to a week following Sandy’s landfall. Migrants could avoid drifting to sea or to the northeast by selecting nights with moderate wind profit and partially compensating for drift on departure.

The Future: While hurricanes pose a threat to individual landbird migrants in North America, the general risk-averse behavior of nocturnal migrants mitigates both the mortality of extreme weather events and conversely the potential benefit of exploiting strong supportive wind flow. However, the response of neotropical migrants to hurricanes warrants further study, including effects of hurricane damage to stopover and wintering habitats, particularly in the tropics where hurricanes typically are more severe and more frequent.

Collaborators: Jeffrey Buler, Matt Boone, and James McLaren, University of Delaware