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Migrating birds caught over Great Lakes at dawn return to shore

Blackburnian Warbler migrates through the Great Lakes region.
Blackburnian Warbler at Swan Lake, Markham, Ontario, May 12, 2012, Alexis Hayes.

Birds prefer to migrate at night — so much so that if day breaks while they’re over water, they’ll turn back toward the nearest shore rather than pressing on. That’s the key finding of a new study just published in The Auk: Ornithological Advances.

A team led by Kevin Archibald, of the University of Delaware, used three weather-surveillance radars to observe birds heading north across Lake Michigan and Lake Erie during four spring migrations.

Birds caught over water as dawn approached increased their elevation and often turned back, leading to an increase in density of birds along the near shore. The density of birds taking off from the southern shores of the Great Lakes on subsequent spring evenings was 48 percent higher than on the northern shores.

The researchers speculate that birds increase their altitude at dawn in order to make it easier to see how much farther they have to fly. If they decide it’s too far, they return, to try again the next night, leading to higher concentrations of migrants on near shores.

When birds migrate south in the fall, the pileups happen on the north side of the lakes, rather than the south.

“Our study justifies the high value of shoreline habitats for conservation of migratory bird populations in the Great Lakes region,” says co-author Jeffrey J. Buler, of the University of Delaware.

“It also emphasizes that the extent of stopover use in shoreline habitats is context-dependent. We hope professionals charged with managing stopover habitats recognize that shoreline areas can receive high migrant use by virtue of the proximity to a lake and how many migrants are aloft at dawn from day to day, rather than [just] by the presence of abundant food sources in these habitats.”

The data used in the study was collected in four spring migration seasons from 2010 to 2013 from NEXRAD stations in Cleveland, Ohio; Grand Rapids, Michigan; and Green Bay, Wisconsin. Cleveland was the only station that did not observe birds increasing their elevation at dawn, possibly because Lake Erie is narrow enough for them to see across without an increase in altitude.

The Auk: Ornithological Advances is a peer-reviewed, international journal that began in 1884 as the official publication of the American Ornithologists’ Union. The AOU merged with the Cooper Ornithological Society in 2016, becoming the American Ornithological Society.

Read the paper

Kevin M. Archibald, Jeffrey J. Buler, Jaclyn A. Smolinsky, and Robert J. Smith (2017) Migrating Birds Reorient toward Land at Dawn over the Great Lakes, USA. The Auk: Ornithological Advances, Volume 134. DOI: 10.1642/AUK-16-123.1

Bird populations steady in western Great Lakes forests (August 2016).

 

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