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Out on the Limb

Hormone-Fueled Songbird Update

Thursday, September 09, 2004

Luke Butler has been kind enough to respond to my questions about the Townsend and Hermit warblers. Please see my post dated 8-27-04 for background info.

Luke says:
"My co-author, Noah Owen-Ashley, and I have been studying androgen levels in these species since 2000, and in Townsend's warblers since 2001. The two researchers do not know if the Townsend levels were naturally selected to be higher in the past, or if hermit levels were selected to be lower.

Dr. Sievert Rohwer, an advisor to Luke and Noah, has this hypothosis:
"During times when glaciers covered much of North America, Townsend's were sequestered into tiny breeding areas at the tops of mountains which we not covered by ice. Because there was so little space, there was extreme competition among Townsend's males for breeding territories. In that situation, the most aggressive males would be selectively favored. Then, when the glaciers receded, Townsend's expanded their range into hermit range and began to slowly replace them. Because hermits were living near the Pacific Coast and did not face similarly extreme competition for territories, hermit males had not been selected to be aggressive. Thus, hermit males lost (and continue to lose) fights with Towsend's males."

I stated in my previous post my concern that the females were "victims" of the more aggressive Townsend males during mating. In response, Luke said "What females do is complicated." I don't imaging Luke will get much disagreement there! It appears that the females are often more interested in picking the right spot than the right mate.

"Males return to the breeding grounds 2 weeks before females from wintering grounds in the south. In those two weeks males set up territories and start defending them from neighbors and other, later-arriving males. When females arrive on the scene they probably "shop" for territories based somewhat on territory quality related to reproductive success (e.g., good spot for nesting, abundant food, etc.), and somewhat on aspects of male quality. We know that in low-quality territories females tend to choose to settle on the territories of Townsend's males (assuming they're around).

Another complicating factor is that there is evidence that some females choose a territory/mate, and settle on that territory. Then a new male displaces the original male through aggressive behavior. In some cases, females stay on the territory and mate with the new male."
posted by Deb, 2:44 AM

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