Brewster's Warbler

Musings About Habitat               25 May 2015

Brewster's Warbler singing in prickly ash

The Brewster's Warbler is a hybrid, the outcome of a mating between a Blue-winged Warbler and a Golden-winged Warbler.  The two species are very recently diverged, sharing some similarity in basic song and nearly identical Type II aggressive song. Though it is clear a number of events have caused them to be two distinct species, hybridization still occurs when ranges overlap.  While Brewster's Warbler hybrids lack the black throat of the Golden-winged Warbler, each one seems to be unique. Some are pale yellow with a white face.  Others are largely gray and pale, like a Golden-winged Warbler, but with splashes of lemon yellow on the breast.  Some are not easily distinguished from the Blue-winged Warbler except for a pale gray cheek.   Check the face, the back, the breast, the wing patterns, and it becomes clear that there are many plumage possibilities at play!

Brewster's Warbler in young burr oak

 Both nest in younger, brushier habitats, though ideal habitat for each species is really quite different, the differences being felt better than described.   To me, Blue-winged Warbler habitat is found near young farm forests, mowed walking trails bordered by dogwood and prickly ash, the thickets within a stone's throw of a bluebird house trail, maybe even older upland forest adjacent to a wide and marshy clearing with a stream, dead trees, and a margin of sun-dappled grape.  In Blue-winged Warbler habitat, the red fox slips by just out of view, cows bellow in the distance, and elements of the wild are woven amid neatened rows of agriculture and forgotten back field edges. A lazy "tzeeeeeee bzzzzzz" rings out while Field Sparrows, Gray Catbirds, Common Yellowthroats, and Eastern Towhees join the song chorus.

Golden-winged Warbler habitat is a little more stoic, a picture of "up north" wild.  To me, Golden-winged Warbler habitat is found in a grove of young aspen at the edge of flooded wiregrass sedge, a broad finger of alder running far out into a wild and timeless wetland criss-crossed by deer trails.  It may be, perhaps, a wide clearcut patch of county forest stumps, brush and young chokecherry adjacent to a deep and dark forest of oak, birch, maple, and white pine,  Wolves have marked the sand road with their droppings here, and bears keep cool in the shade of older trees just down the slope.  Scarlet Tanagers, Nashville Warblers, Sedge Wrens, and Chestnut-sided Warblers provide the background ambiance to the invigorated "beee-bzzzz-bzzzz-bzzzz-bzzzz". 

A problem emerges in my habitat generalization, of course.  I have found Blue-winged Warblers in equally wild land.   But I also have found a common player where ranges overlap--European Honeysuckle.  My realization is just tantalizing enough as a hypothesis to maybe find its way into research.  As I write this, I find myself looking into the research of the Golden-winged Warbler Working Group to see if they have had a common thought... Could a European exotic provide ecological stimulus to influence hybridization?

Brewster's Warbler in young white oak

Brewster's Warbler lurking behind the honeysuckle flowers

Foraging in the honeysuckle

A wonderfully wet May, all around life is flourishing.  Time moves along relentlessly, and for just one month each year, Wisconsin shares nearly the same climatological data as a Costa Rican rain forest. The birds are here, back from the tropics, joining our resident birds, and they are busy!

Belted Kingfisher and beaver pond

Tree Swallow and overcast sky

All images were made with a Canon 7D, Canon 300mm f4L IS lens, and a lot of drizzle, fog, and cloud. A beautiful, tropical day along the St. Croix River!  Spend a little time afield with the Brewster's Warbler here at my YouTube channel!  https://www.youtube.com/watch?v=2xcsjcRIjaU