The warbler species in the genus Vermivora are small birds (4 to 41/2 inches long) with sharp, thin, pointed bills. The bill is a great tool for capturing small insects, spiders and other invertebrates.

Vermivora means worm-eater, hardly an appropriate name for these birds unless one interprets worm to mean caterpillar.

Two species in the genus nest regularly in Maine. Nashville warblers occur throughout the state in the summer, nesting in second-growth or scrubby woodlands. The margins of the many bogs we have in Maine provide great habitat for these birds. Tennessee warblers are much more common in the northern half of Maine. These birds are residents of spruce-fir forests. Tennessee warblers depend on spruce budworms to feed themselves and their nestlings.

Blue-winged warblers have nested sporadically in extreme southern Maine. In the past few years, this species has been seen north of Portland during the breeding season. This spring, blue-winged warblers seem more widespread than ever. We may well be seeing the vanguard of a wave of this species expanding their breeding range north.

Blue-winged warblers are gorgeous birds. Males are yellow underneath except for white undertail coverts. The head is yellow as well, with a thick black line through the eye. The back is greenish-yellow and the wings are blue-gray with two prominent white wing bars. Females are similar, with the yellow on the head restricted to the forehead and with less prominent eyelines and wingbars than the males.

Alexander Wilson, a Scottish immigrant, covered the species in his “American Ornithology,” the first publication on the birds of North America. The species was called the blue-winged yellow warbler at that time and had been confused with the pine warbler until Wilson figured out two species were involved.

Blue-winged warblers prefer overgrown old field and brushy habitats for nesting. They occur in patchy fashion from Missouri, Iowa and Minnesota eastward, from New Jersey to New Hampshire on the Eastern Seaboard.

Before Europeans colonized the eastern United States, blue-winged warblers migrated to Arkansas, Tennessee and Kentucky to nest. As forests were cleared throughout the East, the blue-winged warblers expanded their range because favorable habitat was being created. The species had reached northern Ohio by 1900.

Blue-wings were quite rare in Massachusetts in 1924, but now are found nesting widely across that state.

With the reversion of agricultural fields to forests and destruction of habitat for buildings, the amount of shrublands is decreasing, particularly in the northeastern United States. As a result, blue-winged warbler numbers are down in some parts of their range.

The incursion of blue-winged warblers into Maine may simply be a continuation of the range expansion that has been going on for more than 200 years. Warming of the climate and destruction of nesting habitat in southern New England may also be contributing to this apparent increase in blue-winged warblers in our state.

Blue-winged warbler males have a distinctive, buzzy song consisting of two parts. It is often described as “beee-buzzz.” You can hear a recording at www.allaboutbirds.org.

A closely related species to the blue-wing is the golden-winged warbler. Golden-wings occur rarely as migrants in Maine. The two species overlap over much of their range. Golden-winged warblers have a yellow cap, a black throat, a broad, black mask on the sides of the face, and yellow wing bars. Their song consists of four buzzy parts: zee zaa-zaa-zaa.

Blue-winged warblers and golden-winged warblers occasionally interbreed to yield fertile hybrids. Some early ornithologists collected some of these hybrids and described them as new species. It took some genetics sleuth work to determine the true hybrid nature of these birds.

The most commonly encountered hybrid is called Brewster’s warbler, the product of a first-generation cross between a blue-wing and a golden-wing. The Brewster’s warbler has the white underparts of a golden-winged warbler, and often yellow wing bars, but the black eyeline of the blue-winged warbler. A Brewster’s warbler was recently found by Marie Jordan and friends in Saco.

Now it gets more complicated. A Brewster’s warbler may breed with either a golden-winged warbler or a blue-winged warbler. A geneticist calls such a mating a backcross. The offspring of the backcrosses fall along a continuum of golden-wing and blue-wing traits. One variant, the Lawrence’s warbler (also thought originally to be a new species) has the yellow underparts of a blue-winged warbler and the head pattern (bold black mask and black throat) of a golden-winged warbler.

The genetics of this system were worked out by Kenn Parkes in 1951. You can download a copy of his paper (with paintings of different hybrids) at my website: web.colby.edu/mainebirds/.

 

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at:

[email protected]