04172021 FEATHER western butterbutt

The male Audubon’s warbler, the western

butterbutt

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By Bill Chaisson

When a fellow birdwatcher mentioned to me on Thursday that he had seen some “butterbutts,” I confess I drew a blank. In the last year I have started to check the New Hampshire Bird Forum on a semi-regular basis and have noticed the various nicknames for common bird species, but I have never used them myself. For example, folks are now seeing “woodies” and “hoodies,” i.e., wood ducks and hooded mergansers. A couple of weeks ago, someone spotted MODOs (mourning doves) in their yard. In addition to birders’ pet names, back in the day hunters gave nicknames to various shorebirds and waterfowl. American widgeons were “baldpates,” greater scaups were “green-heads” or “blue-bills,” woodcocks were “timberdoodles,” and sanderlings were “bull peeps.”

My colleague relented when he saw my confusion and began referring to the birds in question as “yellow-rumped warblers,” which still gave me pause, because when I started bird watching as a kid, they were called “myrtle warblers.” Being rather fixed in my ways, I still think of them by that name. He said he also hoped to see some palm warblers soon, as they are another early-season migrant.

I think of the bulk of the warbler species as following “bud break” northward, which is to say, we’ll see them some time in May. Most warblers are diminutive Neotropical birds that spend most of the year in the Caribbean and Latin America and only come up here to breed. The Parulidae is a species-rich family confined to the New World, and they seem to have diversified, at least initially, in a classically allopatric fashion. In this mode, at least one population at the edge of a species’ range will become isolated in a peripheral territory and begin breeding only with other members of that population. Any mutation that arises in that group will be preserved among its members and not shared with other populations of that species. As mutations accumulate, that peripheral population becomes a new species, reproductively isolated from the populations from which the original group sprang.

The parulids evolved by seeking new resources during the breeding season and returning to the “motherland” for the balance of the year. But not all of them go all the way “home.” The range maps in field guides tell you the story. Page through the warbler section and most of them will have an orange breeding range, a yellow migration range … and that’s it.

But when I did a Christmas Bird Count on Martha’s Vineyard a few years ago, I saw yellow-rumped warblers during the winter for the first time. Many of them spend the colder months as far north as the southern states, but a plucky few camp out along the east coast as far north as, well, New Hampshire. The palm warbler, for which my colleague was also on the look-out, is another warbler species that winters north to the southeastern United States. Aside from black-throated-greens, black-and-whites, and ovenbirds, a small number of which tough out winter in south Florida, most other warblers brazenly ignore U.S. immigration policies.

Another exceptional taxon is the pine warbler. Some populations of this parulid are permanent residents of the southeastern United States, and the rest migrate north as far as southeastern Canada to breed. Unique among members of this family, pine warblers are year-round North Americans.

The yellow-rumped (Setophaga coronata) is among the more common and widespread warblers. It is also among those North American species, like the northern flicker and northern oriole, that at times has been divided by the American Ornithological Society into eastern and western species. At age 10 in 1971 I learned to refer to the eastern bird as the myrtle warbler or Dendroica coronata and the western populations as Audubon’s warbler or D. auduboni. Two years later they were merged into one species because a zone of widespread hybridization had been recognized through the Midwest.

As is usual, the females are more similar looking: both have breasts and sides vertically streaked with black over white, two white wing bars on gray-brown wings, thin vertical black streaking on gray-brown backs, gray-brown tails with flashes of white on the outer feathers, and of course their “butter butts,” the square patches of yellow above the base of the tails plus yellow patches on the sides. The males are a higher contrast, blue-gray version of all of the above, and they also have yellow patches on top of their heads (hence coronata or “crowned”).

The most easily seen difference between the Audubon’s and myrtle is the yellow throat of the former versus the white throat of the latter. Male Audubon’s wing bars merge into a broader white patch and their black chest streaks merge into a solid smear. Male myrtles have a defined black mask around the eyes.

But in some parts of the Midwest, many birds have a mixture of characters. The prevailing hypothesis is that the species was separated into two breeding populations by the repeated advance of continental ice sheets during the last 2 million years, a phenomenon known as vicariance. Now, during the longest interglacial of the Pleistocene (14,000 years and counting), they may be merging into a single species again.

The former Dendroica coronata and D. auduboni became D. coronata in 1973, and then about 20 years ago, several of the Dendroica warblers, including coronata, were reclassified—as a result of molecular genetic studies — as Setophaga. (Ironically, in 2016 the American Ornithological Union and the Cooper Ornithological Society merged to form the American Ornithological Society.) Scientific names are supposed to be more constant than vernacular names like butterbutt, but in the case of the myrtle … I mean, yellow-rumped warbler, the target keeps moving.

This morning, as I watch the snow fall heavily and listen to the red-breasted nuthatches return to my suet feeders, I’m glad that most of the warblers are still sipping margaritas.

Bill Chaisson has been a birdwatcher from the age of 10. He is a former managing editor of the Eagle Times and now lives and works in the town of Wilmot. Contact him at wpchaisson@gmail.com.

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