Radar data proves Rhode Island's importance to migrating birds

            Rhode Island is a small state, but it plays an outsized role in providing food and habitat for migrating birds traveling through the region each fall.

That’s the preliminary result of the first year of data collected for the Rhode Island Bird Migration Atlas, which combines satellite data of mass movements of birds during migration with field observations collected along 10 transects scattered throughout the state.

“We’ve confirmed what we’ve always suspected – Rhode Island is a very important stopover site for migrants. We’ve just never been able to put numbers to it before,” said Charles

Clarkson, the ornithologist who leads the project on behalf of the University of Rhode Island and the Rhode Island Department of Environmental Management.

Clarkson said that the bulk of the birds migrating north along the Atlantic flyway in the spring travel to the west of Rhode Island, whereas most of those traveling south each autumn do so by crossing through the state.

“It seems like there is a funneling effect, with large numbers of birds heading south and avoiding going over open ocean,” said Clarkson. “They’re not heading toward the beaches in eastern Massachusetts but are instead hooking west, passing through Rhode Island and along coastal Connecticut and New York before turning south along the New Jersey coast. They all seem to funnel through our state on a southwest trajectory.”

Among the other findings noted so far is the discovery that Rhode Island has what Clarkson called a very protracted migration.

“We have just one year of data, but it seems that one of our busiest migratory nights was the last night of data collection,” Clarkson said. “We had huge pulses of birds moving through the state early and much later in the migratory season, suggestive of a much more protracted migratory season in Rhode Island than you might expect.”

No other state in the country has attempted to map and quantify the migratory movements of birds. Clarkson said that Rhode Island’s small size makes the project more feasible than in larger states.

Clarkson spent three days in March at the AeroEcology Laboratory at the University of Delaware, which uses weather radar to analyze migratory bird movements.

He said that weather radar “shoots a laser beam” into the sky, which bounces off an object and creates an image. The beam can also determine how quickly objects are moving in the sky, which enables scientists to differentiate between, say, wind-borne leaves and birds.

“You can use these two metrics to get an idea of when, how quickly and how many birds are moving through an area,” he said. “When we looked at the data for Rhode Island, we saw that Rhode Island is used very heavily during fall migration relative to neighboring areas. Rhode Island seems like a hotbed of fall migratory movement by birds.”

            The radar data was combined with data collected by two researchers regularly walking 10 transects during the migratory season. They recorded every bird they saw and heard, as well as data about insect abundance as a measure of food availability. This on-the-ground data provided what Clarkson referred to as “bird use days,” or the number of days during the migration period that birds used each site.

            “That gives us an idea of how long birds hang out at these spots,” he said. “And the insect counts give us an idea of what fuel source is available to them as they hang out. It also gives us an idea of turnover. If we see 50 robins one day and two days later we see 50 robins and two days later we see no robins, that gives us an idea of how long that flock hung out and refueled before they picked up and moved on.”

            While the data has not yet identified particular species that use Rhode Island habitats more than others, Clarkson said the state appears to be an especially important stopover location for songbirds that breed north of the state and winter in the Tropics, including blackpoll, magnolia and black-throated blue warblers.

            “We had plenty of the standard stuff, too – flocks of hundreds of robins, huge flocks of grackles moving through,” he said. “And almost always in those monospecific flocks you’d see Cooper’s hawks following them everywhere they went. Inevitably we’d be able to predict when we’d see birds of prey based on those monospecific flocks moving through.”

            The migration atlas project will continue collecting data for two more years, increasing the number of transects studied from 10 last year to 50 this year.

This article was first published on EcoRI.org on April 5, 2017.