Of a Feather: Varying from east to west

By BILL CHAISSON
By Bill Chaisson

I haven’t been to the western U.S. in many years, so I am excited to see bird species that are like old friends, and I am hoping to see some new birds too. We are going to Colorado. I have never done much more than drive through it before. I’m looking forward to wandering through montane woodlands looking for Townsend’s solitaire and searching rushing brooks for dippers.

One of my fascinations with western birds is seeing those species that are similar to the eastern ones. These slight differences either cause the American Ornithological Union to label them as different species, call them a subspecies, or to change their minds about species status repeatedly through the last century or so.

Three well known examples of the last type are the northern flicker, the yellow-rumped warbler, and the northern oriole. In 1964, when National Geographic published its “Song and Garden Birds of North America,” there were three species of flicker: the yellow-shafted (Colaptes auratus), the red-shafted (C. cafer), and the gilded (C. chrysoides).

By 1990, when Roger Tory Peterson put out his field guide to western birds, they had collapsed to one species (C. auratus) with the others regarded as geographic variants. The red-shafted differs from the yellow-shafted in having salmon-colored wing and tail linings, a brown crown and nape (no red crescent), gray cheeks, and a red mustache on the male instead of a black one. It is not uncommon to find populations in the Midwest with orange wing linings halfway between red and yellow and males with a red mustache on one side and a black one on the other.

Gilded flickers have the head of a red-shafted and the body of a yellow-shafted. They live in the U.S. Southwest and northwestern Mexico and nest in saguaro cactuses rather than in trees. By 2000, when David Sibley put out his field guide, the gilded flicker had been broken out as a separate species again.

A molecular study published in 2017 called the three species the auratus “superspecies” and noted that there were not clear genetic differences among them, especially when compared to Caribbean and Latin American Colaptes species. But C. chrysoides remains separate.

The sapsuckers, genus Sphyrapicus, show a similar pattern of variation across the continent, with three similar species ranging east to west: S. varius, the yellow-bellied, S. nuchalis, the red-naped, and S. ruber, the red-breasted. The cross-continent trend is toward more red and less white. Sibley notes that any hybrid combination among the three species is possible and that variation within the species sometimes makes identification difficult.

In the 1940s it was proposed that S. varius was the actual species and nuchalis and ruber were subspecies of it. More recently (1995) molecular studies have shown a fourth species, the Williamson’s sapsucker (S. thyroideus) of the western mountains, is the basal species. It is a mostly black bird with little red or white on it. The yellow-bellied split off from the Williamson between 3 and 4 million years ago. The red-naped and red-breasted are very closely related with very few genetic differences; they differentiated during the late Pleistocene, only tens of thousands of years ago.

For a detailed exploration of this complex, read Elizabeth Natola’s 2017 University of New Hampshire master’s thesis (online). She found evidence for sympatric development of ruber from nuchalis. That is, they weren’t geographically separated, but diverged via behavior. These are migratory woodpeckers; some members of the population arrive earlier each year and begin breeding with each other, resulting in differentiation within the same region.

In contrast, nuchalis and varius seem to recognize plumage to choose mates; the females with red on their heads (nuchalis) mate more often with males having more red on their heads. Varius females lack red on the head, and the males do not have red napes or a red patch behind the eye. That is, natural variation within a population led to genetic divergence.

Natola compared the above phenomena to the divergence of lazuli and indigo buntings, another east-west species complex. Woodpeckers rarely sing and so must rely on plumage alone. The lazuli and indigo buntings have similar songs, so where the species’ ranges overlap in the Southwest and Midwest, the females use plumage to differentiate among males.

Clearly related species like the Williamson’s and yellow-bellied sapsuckers differentiated before the Pleistocene Epoch, which began about 2.5 million years ago. Molecular studies have shown that successive ice ages during the Pleistocene drove populations previously continuous across the country into separate refugia, where they began to differentiate, only to be reunited during interglacial periods.

The last interglacial ended about 125,000 years ago and the ensuing glacial period ended only about 12,000 years ago. Very closely related specials like the red-naped and red-breasted sapsuckers differentiated during this interval. According to Natola, the genetic differentiation developed in the glacial refugia and is maintained imperfectly during interglacials. Hybridization occurs among closely related species when all that separates them is the timing of their return to their shared breeding grounds or how much red they have on their heads.

The yellow-rumped warbler — formerly Audubon’s in the west and the myrtle in the east — is another example of this phenomenon. So, was the northern oriole, but it is presently regarded as two species again: the Bullock’s in the west and the Baltimore in the east.

I look forward to seeing the western populations of familiar eastern species in Colorado. I have flown just as far in the opposite direction and been baffled by the birdlife of Britain. A coal tit is a long way from a chickadee, and a chaffinch is a really long way from an indigo bunting. But a flicker with a red mustache and pink wing linings is just fun.

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 [email protected].