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Connecticut warbler down to single nesting site in Wisconsin

The little yellow bird's numbers are crashing in Minnesota as well.

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A Connecticut warbler in northern Wisconsin. Biologists in the state are part of an international effort to find out why the yellow warblers are declining, down to just one known nesting site in Wisconsin. Contributed / Ryan Brady

A little yellow bird once seen in jack pine and spruce forests across northern Wisconsin is down to a single patch of nesting habitat in the state, and bird experts aren’t certain the birds can hang on there.

The Connecticut warbler has been declining for a half-century, down 60% nationally, declining nearly 9% annually in northern Minnesota and down 80% in Wisconsin.

But when Wisconsin biologists went back to look for them this year, they found none at 60 sites where the birds had nested before.

The species is now known to breed in only one small area of upland jack pine in Northwestern Wisconsin, the Department of Natural Resources announced Tuesday.

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A nest of Connecticut warbler hatchlings in a northern Minnesota spruce bog. The birds, which have been dramatically declining in Minnesota and Wisconsin, nest on the ground and use moss to make their nests. Contributed / Brett Howland

Ryan Brady, an Ashland-based DNR conservation biologist, said “habitat loss, development, climate change and other threats throughout their annual life cycle make it harder for these birds to survive and reproduce.”

The Connecticut warbler is listed as a “Species of Greatest Conservation Need“ in Wisconsin and a “Species of Continental Concern’’ nationally, although those titles don’t give the bird any formal protections. State officials discovered the more recent, more rapid decline as part of a follow-up investigation after the statewide Wisconsin Breeding Bird Atlas effort conducted from 2015-2019.

The DNR is working with landowners, foresters and other resource managers to protect and enhance the warbler’s habitat of upland jack pine forests in eastern Douglas and western Bayfield counties.

“In short, we need to create more suitable breeding habitat,’’ Brady told the News Tribune. “On sandy soils, that means older/mature jack pine with open mid-canopy and nice ground layer of blueberry or similar low shrubs. On heavier soils, we need to promote healthy black spruce-tamarack bogs and encourage conifer retention or regrowth in adjacent uplands. Private landowners, especially those with larger acreages in the northwest corner of the state, can help.”

Connecticut warbler range
Connecticut warblers nest in a relatively small area of southern Canada, far northern Minnesota and in some isolated areas of northern Wisconsin and northern Michigan. They spend their winters in South America. Contributed / borealbirds.org

But the little birds may be facing issues on their migration routes and where they spend winter, too. So the DNR and the nonprofit Natural Resources Foundation of Wisconsin are working with international partners to develop conservation plans for critical migratory stopover sites and overwintering habitat in Central and South America. Money raised through the foundation’s Great Wisconsin Birdathon supports that effort.

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The poorly named warbler in fact doesn’t breed in Connecticut at all, only passing through as an uncommon migrant. The species is sparsely distributed throughout its breeding range with the bulk of its population now nesting in parts of Ontario and northern Minnesota.

Minnesota decline steep

In Minnesota, the bird has been declining at an average rate of 8.5% since 1995. Once considered simply hard to find due to its remote nesting sites in bogs, it’s now becoming outright rare. Alexis Grinde, a bird researcher for the University of Minnesota Duluth’s Natural Resources Research Institute, said that the Connecticut warbler is now declining faster than any other bird studied in Minnesota’s Chippewa and Superior national forests.

“It is very startling to see population trends drop this quickly,’’ Grinde told the News Tribune.

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This kind of prime nesting habitat for Connecticut warblers — semi-open spruce and tamarack bogs — is declining in Minnesota, possibly one reason the bird's numbers are crashing here. Contributed / Alexis Grinde

Grinde said that, while the warbler uses jack pine in Wisconsin, it favors spruce and tamarack almost exclusively in Minnesota. That habitat is declining, in part due to the eastern larch beetle infestation that’s killing tamarack trees. Climate change also appears to be impacting spruce bogs, Grinde noted. (The bird, which nests on the ground, needs sphagnum moss to build its nests.)

“Unfortunately, we are seeing several bird species that breed in lowland conifer forests with significant declining trends,’’ Grinde said.

The Connecticut warbler is a small gray-hooded bird with a bold white eye ring, yellow chest and belly and olive back. Younger birds and females have more muted colors than males. During summer nesting season, they live in jack pine and black spruce forests primarily in northern Wisconsin, In Minnesota, they are known to nest across the northern tier of counties, especially from St. Louis County north and west to Lake of the Woods County.

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For more information on the Connecticut warbler, go to allaboutbirds.org/guide/Connecticut_Warbler/id .

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