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

08/23/04

Hormones prove to be weapon of choice



Photo Credit: Peter Gibert
A male Townsend's warbler attacks a decoy of the same species while being observed by researchers.

Full size image available here

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 found that in places once inhabited by hermits and now populated only by Townsend's large parts of Washington, British Columbia and Alaska the existing birds carry genetic evidence that hermits once interbred with Townsend's.

The research by Butler and Noah Owen-Ashley, who recently completed UW doctoral work in biology, is being published online Aug. 25 in Biology Letters, a journal of The Royal Society, the United Kingdom's national academy of science. The work was funded by the National Science Foundation, the Society of Integrative and Comparative Biology and the Garrett Eddy Ornithological Endowment at the Burke Museum of Natural History and Culture at the UW.

In the spring of 2001 and 2002, Owen-Ashley and Butler worked at one site where the species come together, in the Washington Cascades south of Mount Rainier. They used a fine net to capture individual Townsend's, hermit and hybrid warblers. All the captures came between 7 a.m. and 1 p.m. to minimize the effect on their measurements of hormone level fluctuations throughout the day. They lured the birds with taped warbler song and decoys, and males were captured before females were present in the area or before the birds were socially mated. The researchers also worked in two other areas of Washington state, one with only Townsend's warblers and the other with only hermits.

Once the birds were captured, the researchers drew blood from veins in the wings, then placed it on ice until they could use a centrifuge to separate the plasma, done within six hours. The plasma was stored at minus-20 degrees Celsius to preserve the hormones until it could be returned to the UW for analysis.

The scientists found substantially higher androgen levels in both the Townsend's warblers and the hybrid species than in the hermit, which could explain why the hermit has consistently, over thousands of years, lost its territory to the Townsend's. It also could explain why previous studies have shown that hermit males become agitated only when other hermit males are encroaching, while Townsend's males react strongly toward other Townsend's and to hermits. Hybrids tend to exhibit behavior closer to Townsend's.

But the competition isn't just for territory. When the two species are in the same locale, the Townsend's also seem to have the upper hand in attracting females even hermit females. That is particularly true when the quality of habitat declines, leaving the female with a choice on which her survival could depend.

"In low-quality habitat, Townsend's males are more able to attract females than hermits are," Butler said. "And that research is done in the hybrid zone, where there are both types."

Source: Eurekalert & others

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