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Two warbler species find the West isn't big enough for both of them

A songbird species known as the Townsend's warbler, which lives in the damp Douglas fir forests of Western North America, has been steadily displacing its more timid sister species, the hermit warbler, for thousands of years.

New research suggests that substantially higher levels of androgens specifically the hormones testosterone and dihydrotestosterone have given male Townsend's warblers the upper hand in competition for territory and mates. The Townsend's greater aggressiveness, fueled by the higher androgen levels, has pushed hermit warblers out of their former range and into territory, often less desirable, that is farther south and west.

But Douglas fir forests only extend so far, and current trends would mean the hermit warbler probably will become extinct in another 5,000 years or so, said Luke Butler, a University of Washington doctoral student in biology.

"The hermits have slowly been pushed out of Alaska and British Columbia, and now they are being pushed out of Washington," Butler said. "They are running out of places to go."

A Townsend's warbler is a small songbird with yellow and black streaking on the sides and a dark crown, throat and upper breast. A hermit warbler has a yellow head and a dark chin and throat, but its dark feathers are much less dominant and its appearance is less striking.

The hermit warbler is now mostly confined to coastal forests in Washington, Oregon and northern California. There are three known places two in Washington and one in Oregon where the two species overlap. In these "hybrid zones," instead of just two types of plumage, observers typically see a notable array of feathers.

"Many of these are presumably the result of hybrid matings between the 'tough-guy' Townsend's and the females of the 'wimpy' hermit species," Butler said.

The hybrid birds appear to carry androgen levels similar to Townsend's warblers. That corresponds with previous research, which f
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Contact: Vince Stricherz
vinces@u.washington.edu
206-543-2580
University of Washington
24-Aug-2004


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