The Yellow-rumped Warbler is the workhorse of Michigan's warbler migration — the one that pours through in enormous numbers each April and again from September into November, when nearly every other warbler has already gone. Birders affectionately call it the "butter-butt" for the bright yellow patch above the tail, which flashes as the bird flits through branches. Spring males are handsome slate-blue with yellow crown, sides, and rump; by fall everyone fades to streaky brown, keeping only that diagnostic yellow rump.
What sets this warbler apart is its stomach. Alone among North American warblers, it can digest the waxy coating of bayberry and juniper berries, and that trick lets it winter far north of other insect-eaters. A handful survive Michigan winters along the Great Lakes shorelines where fruit hangs on, though most continue to the southern states.
Insects dominate in the breeding season, hawked from the air or gleaned from foliage. In migration and winter it switches heavily to fruit — bayberry, juniper, poison ivy, and viburnum — a rare digestive ability that fuels its unusually northern winter range.
The nickname comes from the bright yellow patch on the rump, visible in every plumage and season. It is the quickest way to identify the bird as it moves through foliage.
Late April and early May in spring, and a long window from mid-September through late October in fall, when they are often the most numerous songbird in the woods.
A few do, especially near Lake Michigan and Lake Erie where bayberry and juniper berries persist. Their ability to digest waxy fruit lets them survive cold snaps that push other warblers south.
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