Aug. 27, 2012

A Warbler Rises from the Ashes

by Kara Rogers

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On a thin, sap-speckled branch of a jack pine, a male Kirtland’s warbler perches, its yellow chest puffed out and gleaming brilliantly under the sun. He poses proudly for a group of bird-watchers, whose eagerness to document this moment digitally sends their 400mm Canons and Nikons whirling in a shower of shutter clicks.
The sense of isolation that unfolds over the field of dry, brown grass, low shrubs, and stubby pines makes the excitement seem excessive at first. But this is the jack pine barrens of Michigan’s northern Lower Peninsula, the place where Kirtland's warblers (Dendroica kirtlandii) -- one of North America's rarest songbird species --  come to breed. Kirtland's warblers can be seen in the barrens only from late May until August or September, when they depart for their wintering grounds, more than 1,200 miles away in The Bahamas and nearby islands.
Down to just 167 singing males in 1987, the Kirtland's warbler population has rebounded, with 1,828 males counted in 2011. The species has risen, almost literally, like a phoenix from the ashes. The Kirtland's warbler builds its nest on top of sandy, well-drained soils and depends on shrubs to provide cover and food -- characteristics that in the barrens are made possible by fire. This association led to the warbler's nickname, the “bird of fire.”
The best breeding forests for Kirtland's warblers emerge about six years after a burn, when low shrubs, such as sweetfern, sand cherry, and blueberry, grow densely beneath and near young jack pines. By this time, the pines are at least five feet in height, and stands are separated by sprawling grassy openings.
Jack pine forests historically burned at regular intervals as a result of natural and human-associated factors. Native Americans, for instance, may have set fires intentionally in the barrens. But with the expansion of European settlement in the 19th and 20th centuries, fire suppression became the norm, and ecological succession suddenly was allowed to proceed in the barrens. As the forests grew up and the grasslands closed, the warblers' breeding habitat disappeared. And the warbler, too, began to disappear.
The Kirtland's warbler's survival was also endangered by the presence of brown-headed cowbirds (Molothrus ater). As their name suggests, cowbirds associate with cattle, and they presumably followed cattle, introduced by humans, into the Great Lakes region. Cowbirds, however, never build nests of their own. Instead, they lay their eggs in the nests of small birds like Kirtland's warblers and cause a reduction in host chick survival.
Fortunately, intense fire management and efforts to control cowbird nest parasitism in the jack pine barrens have paid off. There appear to be more Kirtland's warblers alive today than at any other time since the first specimen was collected in 1851 by Charles Pease. The species has extended its summer range, too, with nests found in places such as Wisconsin and north of the Lower Peninsula, into Ontario, Canada.
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About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.

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