Let's document Myrtle x Audubon's Warblers in Colorado

[Ted Floyd]

Hey, all. Earlier this Friday evening, May 4, I saw a couple of Myrtle x Audubon's Warblers at the University of Colorado South Campus near Tantra Road, Boulder County. Which got me to thinking about something...

Away from the breeding grounds, Myrtle and Audubon's Warblers co-occur across a huge swath of western North America, easily a million square miles. Yet The Sibley Guide informs us that hybrids are rarely detected away from the breeding grounds. I have no reason to doubt the overall accuracy of that statement, but I also note that, at least here in eastern Boulder County, hybrids (or intergrades, if you prefer) are legion.

Here's the deal. Even though Myrtles and Audubon's spread out across a vast region when they're not breeding, that's not necessarily the case for their hybrids. By analogy, consider the enormous winter range of the Dark-eyed Junco vs. the quite limited range in winter of White-winged Junco. (White-winged Junco in winter occurs chiefly in the ponderosa pine belt of the Colorado Front Range.) I wonder if Myrtle x Audubon's hybrids are similarly range-restricted. For what it's worth, I never knowingly saw a Myrtle x Audubon's hybrid in all my years of looking at Myrtle and Audubon's warblers in New Mexico (1992-1994) and Nevada (1999-2002).

It would be cool if we could say with some precision where Myrtle x Audubon's warblers go when they're off the breeding grounds. Definitely here in eastern Boulder County! But how much more extensively?

Here's a photo of one of the hybrids I saw this evening:

What to do? Easy! Put it all on eBird.

Ted Floyd

Lafayette, Boulder County

[Cody Porter]

Hi all,

I'd like to echo what Ted said. We certainly see a lot of intergrades here in Laramie, which has also got me thinking about the overwintering and migratory behavior of Yellow-rumpeds a lot. I've spent a lot of time this spring trying to get an accurate representation of numbers of Myrtles, Audubon's, and intergrades on my eBird checklists, which has been a really fun experience (hey, you need to find something to do on the high plains when crossbills are sparse and you're not getting Zone-tailed Hawks and Summer Tanagers).

One of the interesting things about the hybrid zone between Myrtle and Audubon's is that there is apparently no assortative mating (in other words, anyone will mate with anyone, regardless of phenotype). We certainly see the result of this (i.e., intergrades of all flavors) in our area. However, there appears to be moderately strong selection against the hybrids, with hybrid fitness being about 18% lower than either Myrtle or Audubon's. No one knows why this is. A very likely explanation is that there is something going on during migration and/or over winter. Maybe the hybrids take an inferior migratory route in the spring and fall? Maybe they overwinter in an inferior location? Getting a better handle on the frequency of hybrids, and even what hybrid phenotypes we are seeing, could tremendously help solve this riddle.

Good birding,
Cody Porter
Laramie, WY