‘Butterbutt’ warbler might really be 3 species

Myrtle warbler. (Credit: Paul VanDerWerf/Flickr)

One of North America’s most beloved songbirds—the yellow-rumped warbler—may need to be considered at least three separate species, says a new study.

This discovery, reported in The Auk, contradicts how ornithologists classified the bird for almost five decades.

The new work brings a wealth of genetic evidence to bear on the question and for the first time identifies where in the warblers’ DNA species have diverged from each other. That opens the door to discovering what’s responsible for differences in how the birds look, sound, and act, says lead author David Toews, a postdoctoral researcher at the Cornell Lab of Ornithology.

“The yellow-rumped warbler—this species that’s familiar to just about every birder in North America—has four distinct forms, and now we have quite compelling evidence that three of them are full species,” says Toews, who began the work while he was a doctoral student with study coauthor Darren Irwin at the University of British Columbia.

yellow-rumped warbler (Audubon's)
Audubon’s warbler. (Credit: iStockphoto)

It’s not the first time the 5-inch-long, half-ounce birds have prompted debate among ornithologists. For most of the last century the yellow-rumped warbler was known to birders as two species, the myrtle warbler of the East (and far north) and the Audubon’s warbler of the West. But in 1973 evidence the two species routinely hybridize in a narrow zone in western Canada led scientists to reclassify them as a single species.

Affectionately known to bird watchers as “butterbutts,” yellow-rumped warblers are abundant migrants that pass through North America each spring and fall in most of Canada and in every US state except Hawaii. Summer finds the gray-and-yellow birds in the evergreen forests of Canada, the mountainous western US, and the Northeast. They spend winters farther north than most warblers, in the southern US as well as in Mexico and Central America.

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Now, evidence from more than 37,000 regions of the birds’ DNA suggests myrtle and Audubons are separate species—and so is a third, isolated form known as Goldman’s warbler that is almost entirely restricted to Guatemala. If given full species status, its restricted range and low numbers would make it an immediate target for conservation.

A fourth form known as the black-fronted warbler lives in the mountains of northern Mexico but its species status is more debatable, the study authors report.

The wealth of data provided enough resolution for the researchers to see clear patterns of natural selection despite relatively small overall genetic differences. For instance, differences between myrtle and Audubon’s forms were clustered within only about 60 relatively short regions of DNA. These hotspots likely contain the genes responsible for making myrtle and Audubon’s warblers different, Toews says. Natural selection likely caused those genes to change while leaving the rest of the birds’ DNA relatively unaltered.

It is thought the myrtle and Audubon’s forms became separated over the last million years. Toews thinks that at some point, birds that became the myrtle form moved eastward from northwestern North America and became cut off from the other three forms by ice sheets.

That long period of isolation explains why myrtle is the most distinctive of the four forms, he says.

Although the myrtle and Audubon forms interbreed freely in a narrow zone just 80 miles across, some sort of weakness in hybrids keeps them from surviving and carrying the genetic mixing farther afield.

That weakness is the reproductive barrier that qualifies myrtle and Audubon’s as biological species, Toews says, although what it is, exactly, remains to be discovered.

Additional coauthors are from the University of California, Riverside; University of Zurich; and the Spanish National Research Council.

Source: Cornell University