Migration Ecology of a Declining Songbird, the Rusty Blackbird (Euphagus carolinus)
James R. Wright, MS
Advisor: Christopher M. Tonra
Each spring and fall, hundreds of species of North American birds migrate thousands of kilometers between breeding and wintering grounds. Most of the migratory period is not spent in flight, but rather at stopover sites where birds rest and refuel. This is a vulnerable time for migrants, since they are often in an unfamiliar landscape, balancing the necessities of rapidly refueling with increased risk of predation and competition for resources with other migrants. Many migrant songbirds are in steep decline, and understanding the movement behavior and habitat use of these birds on stopover is crucial to developing effective conservation action.
One of the most rapidly declining songbirds in North America is the Rusty Blackbird (Euphagus carolinus), which has experienced population declines of more than 85% over the last 50 years. Some possible causes of this decline have recently been identified on the breeding and wintering grounds, but very little is known about their behavior and habitat requirements during migration. We attempted to fill this knowledge gap by studying the species during spring and fall migration at a high-traffic stopover site in northern Ohio, on the southwest shore of Lake Erie. We utilized an automated telemetry array in the western Lake Erie basin and across Ontario to track landscape-scale movements of radio-tagged individuals during and after stopover events. Specifically, we wanted to know how long individuals spent at the study site, since stopover duration is one way to determine how reliant a population is on particular stopover sites.
We found that stopover duration of Rusty Blackbirds at our site was unusually long for a songbird, at an average of ~25 days during both fall and spring migrations. During the spring, nearly all captured birds (98%) were molting body feathers, which may partly explain the long stopover in that season. Furthermore, many individuals made landscape-scale (10-35km) movements during their stopover event. Taken together, these behaviors describe a migration strategy that closely resembles shorebird migration strategies, where birds congregate to forage for several weeks at a few key staging areas. This indicates that high-quality stopover habitat may be critically important to Rusty Blackbird populations. We also found that departure decisions were largely determined by favorable tailwind conditions, and birds made nocturnal migratory flights that exceeded 400km. This is the first time nocturnal migration has been described in this species, which could expose a large portion of the population to risk of collision with offshore wind turbines being installed in Lake Erie.
In addition to our study of movement behavior, we investigated Rusty Blackbird fine-scale habitat preferences on stopover. We tracked radio-tagged individuals by hand to locate foraging flocks, and compared habitat characteristics at foraging locations versus random locations. Rusty Blackbirds selected microhabitat with shallow water and leaf litter, often avoiding grass, forbs, and herbaceous shrub cover. Outside of forest patches, they preferred areas closer to habitat edges and with some canopy cover, as well as areas with a mixture of different habitat types. Our description of Rusty Blackbird stopover habitat use and behavior will help inform conservation and management practices across the species’ migratory range.