Date: 5/5/18 7:28 am
From: Cody Porter <empidonaxdvg...>
Subject: [cobirds] Re: Let's document Myrtle x Audubon's Warblers in Colorado
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

On Friday, May 4, 2018 at 8:13:39 PM UTC-6, Ted Floyd wrote:
> 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

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