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Blackburnian Warblers

by Mike Scully

This article first appeared in the May 2006 issue of the San Antonio Audubon Society newsletter.

The experiences of our youth remain in memory more vividly than any other, or so it has been for me. Back in June of 1971, I attended a Boy Scout camp in the Adirondack Mountains of New York. The summer was young and the forest rang with song. I recall the Chipping Sparrow nest by our tent. Bobolinks and goldfinches gargled and bounced respectively over the meadows where bluebirds called. Swainson’s Thrushes and Black-throated Green Warblers sang from the forest, Rusty Blackbirds creaked in the swamp, and kingbirds fussed over the lake. I recall, too, the wheezy songs of Black-throated Blue Warblers, and how drab their mates were. It was as if the illustrations from the pages of my bird guide had sprung to life.Blackburnian Warblers

One evening I went walking along a winding lakeside path when I turned a corner and there, at eye level, was the most beautiful bird I have ever seen: a little warbler, beautiful as warblers generally are, but this one with a vivid flame-orange throat and breast that glows in memory still. Such was my introduction to the Blackburnian Warbler (Dendroica fusca), the smallest and perhaps the most beautiful of the “spruce warblers.”

The origin of the name “Blackburnian” will be obscure to most. In their day, though, the sibling team of Ashton and Anna Blackburn were noted British naturalists. In the years prior to and during our Revolution, Austin traveled in the American Colonies, collecting specimens and sending them to Anna at her estate in Lancashire, England. No less than seventeen type specimens of our native birds were later described from their collection, so perhaps we can forebear the Blackburns their warbler.

Students of ecology may recall the classic study by MacArthur (1958) wherein the foraging habits of five warblers that breed in our northern coniferous forests were compared. The study demonstrated resource partitioning, wherein the canopy was roughly divided up between species, each exhibiting different foraging preferences. Along with the Cape May Warbler, the Blackburnian foraged in the top-most branches of the canopy. Between these two, Cape Mays more frequently went after aerial prey and foraged at the very end of branches, while Blackburnians traversed the high foliage in a more methodical fashion.

Cape Mays and Blackburnians also winter in very different locales, the Cape May winters primarily in the West Indies whereas the Blackburnian is among those few warblers which commonly travel clear to South America. Wintering Blackburnians were recently reported by some of our traveling local birders as far south as Ecuador.

After their long migration, male Blackburnians typically arrive on the breeding grounds about a week before the females, singing their high-pitched songs from prominent branches at the very top of the canopy. Mature mixed or coniferous woodlands are the preferred habitat and the nest is typically placed in the fork of a conifer branch high above the ground. Indeed, to this species goes the dubious honor of having the highest nest (85 feet) ever recorded as being parasitized by a cowbird. Four eggs are usual, the male may feed the incubating female, both sexes then feeding the young. A few days after fledging, the brood will split, part of the brood following each parent. Adults trailed by begging young will often leave their breeding territories to travel with mixed-species feeding flocks.

The Blackburnian Warbler does not appear to be well adapted to exploiting the periodic massive outbreaks of the Spruce Budworm caterpillar that occur in the north woods. Some studies have indicated that breeding populations may actually fall in outbreak areas, perhaps due to competition with the Cape May Warbler, which along with the Bay-breasted is known to actively seek out and exploit these areas, resulting in large broods and high population densities.

Blackburnian Warblers are more tolerant of mixed woodlands than are those two budworm specialists, breeding as far south as the Smoky Mountains, often in association with hemlocks. Neither do Blackburnian populations appear to rise and fall as much according to the fortunes of the budworm. Indeed, thus far the numbers of this warbler appear remarkably stable, despite ongoing habitat destruction on the wintering grounds and widespread losses of our hemlocks to the introduced wooly adelgid insect pest.


 

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