Kirtland's Warbler starts its annual movements by flying west in the spring and east in the fall.. — Kirtland’s Warbler starts its annual movements by flying west in the spring and east in the fall. Photo by Nathan W. Cooper.

A new study has confirmed not only that the endangered Kirtland’s Warbler winters almost exclusively in the Bahamian Archipelago but also that it makes a loop migration and multiple stopovers.

Researchers Nathan W. Cooper, Michael T. Hallworth, and Peter P. Marra, of the Smithsonian Migratory Bird Center, deployed tiny solar geolocators on warblers in Michigan in 2012 and 2014. Data recovered from 27 devices showed that two-thirds of the birds spent the winter in the central Bahamas (Eleuthera, Cat Island, the Exumas, Long Island, Rum Cay, and San Salvador), and that almost every one started its spring migration by flying west to Florida.

The average trip north took 15.7 days. Most migrants stopped over three times: first in Florida, after making landfall; then in northern Florida, southeastern Georgia, or southwestern South Carolina, before crossing the Appalachians; and finally in Ohio and areas around the western end of Lake Erie.

The fall migration was slower, lasting 18.1 days on average. Almost all of the warblers first flew east from Michigan to southern Ontario, northwestern Pennsylvania, and western New York. Then they migrated to the coast of North and South Carolina, where they rested before continuing to the Bahamas.

Four warblers that wintered in the eastern Bahamas or the Turks and Caicos made an additional stopover, in the western Bahamas (Grand Bahama, the Abacos, New Providence, and Andros).

“By documenting the general location of stopover sites, we have begun the process of understanding how events during stopover might shape individual performance and population dynamics,” write the scientists. “Unfortunately, our tracking data are too coarse to identify specific stopover sites and determine habitat use.”

To fill in gaps, biologists are planning an eBird migration blitz, a citizen-science effort similar to the one that helped locate wintering sites of Rusty Blackbird.

A version of this article appeared in the January-February 2017 issue of BirdWatching.

Endangered Kirtland’s Warblers in Wisconsin have most successful season ever (Dec. 2015).

Kirtland’s Warblers found on Bahamian island for the first time in 46 years (Jan. 2014).

Where to look for rare overwintering Eurasian Wigeon.

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