Wednesday, September 15, 2010

Yellow-rumped (Myrtle) Warbler

How different Yellow-rumped Warblers are now compared to their glorious spring plumage.  To see the contrast, look at my 1 May 2010 blog entry.

I have titled today's entry as Yellow-rumped (Myrtle) Warbler because recently ornithologists have merged western and eastern populations of this warbler into one species.  Western birds used to be called Audubon's Warblers; eastern ones Myrtles.  Spring birds are easy to tell apart.  Audubon's Warblers have bright yellow throats. Myrtles are white-throated. Bird banders still report them as either one form or the other, but most birders just call them all Yellow-rumped Warblers.

Fall Myrtle Warblers, like this one, have brown auriculars (ear covering feathers) that are more or less surrounded by at least a thin pale edge. Audubon Warblers lack pale-edged auriculars, giving them a more hooded appearance.  We should be alert to Audubon Warblers in Minnesota, where a handful of records exist. I occasionally observed them in eastern South Dakota, and they breed in the Black Hills.  

Because most birders now call all of these birds Yellow-rumped Warblers, we may rue the day these forms were lumped into one species. Their range overlap is limited to a 200 km band of passes in the Rocky Mountains along the border of British Columbia and Alberta.  The genetic differences between the two types are subtle, but distinct.  Hybrids between the two may suffer higher mortality than do either parental type (Hunt and  Flaspohler 1998).  A betting person might put his/her money on our seeing the two forms again split into discrete species. If they are made into separate species, we will have to guess which form today's birders observed.

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