All through September, I’ve been seeing warblers in the backyard: not the yellow-rumps that spend the winter, but migrants of other species stopping over during their southbound flight—yellow warblers running the gamut from bright to drab, orange-crowns, a female Townsend’s, at least one Wilson’s.
Fall warbler identification on our flyway is easier than back east, where you have to contend with a myriad of species and some really nondescript first-year plumages. East or west, Wilson’s is one of the easiest: a small bright yellow bird with a green back and, in adult males, a black yarmulke. Its name honors Alexander Wilson, a contemporary and competitor of Audubon who lacked his rival’s flair for the dramatic and talent for self-promotion.
There’s always an element of mystery when it comes to migrants: Where have they come from, and where are they headed? Some western warblers, like the Townsend’s warbler, end up somewhere in California; most, including the Wilson’s, are bound for Mexico and Central America. With both breeding and wintering habitats under pressure, it’s become important for conservationists to learn which populations winter where and what routes they take. For warblers, that’s something of a challenge.
Historically, banding (or as the British call it, “ringing”) was the best key to the travels of migratory birds. It’s still important: thanks to banding studies, we know that less than 11 percent of first-year Wilson’s warblers nest near where they were hatched, although they tend to return to their first nest site in succeeding years. The technique has its limitations, though. No matter how many songbirds you band in their summer habitat, the odds of detecting banded birds on their wintering grounds are vanishingly small.
As of the turn of the century, 140,000 Wilson’s warblers had been banded in the United States and Canada; only three of those were ever recovered in Mexico and Central America.
For larger birds, like the stars of Winged Migration, there’s now a high-tech alternative: portable radio transmitters that allow satellite tracking. A recent study of northern pintails, for instance, followed one duck from the Sacramento Valley through the Warner Valley in Oregon and the Kenai Peninsula and Yukon-Kuskokwim Delta in Alaska to her stopping point on the Kanchalan River in Siberia. Radiotelemetry has also worked with hawks and albatrosses. But a Wilson’s warbler weighs about a third of an ounce, and rigging it with a transmitter is just not practical.
To a limited extent, genetic analysis can indicate a migrant’s point of departure. Studies of both nuclear and mitochondrial DNA show a clear distinction between eastern and western populations of Wilson’s warblers. But with nuclear DNA, at least, it’s harder to discriminate between, say, a California breeder and a bird from the northern Rockies. Biologists speculate that there’s a lot of gene flow among western populations.
The best clue, it turns out, is feather chemistry. The isotopic content of a warbler’s feather can reveal how far north it was when the feather grew—and to a degree, how far uphill and away from the seacoast. An isotope, remember, is a stable form of an element like carbon or hydrogen, with a specific atomic weight.
Carbon can be either C12, with six protons and six neutrons, or C13, with an extra neutron. Add a neutron to plain hydrogen and you get deuterium. The ratio of heavy to normal carbon and hydrogen isotopes is related to latitude: the nearer the North Pole, the higher the proportions of deuterium and C13. These elements follow a path from rainfall to plants to plant-eating insects to insect-eating-warblers. When a warbler goes through its summer molt after nesting, the new set of feathers it grows contains a distinctive isotopic signature.
So all you have to do is mist-nest a warbler in Mexico or Costa Rica, snip a feather sample, and run it through your mass spectrometer, and you’ll have a rough idea of the location of its breeding grounds. The technique was apparently first used by biologists at Dartmouth with the black-throated blue warbler, an eastern species that winters in the Caribbean. Other researchers, including Sonya Clegg and Mari Kimura at San Francisco State’s Center for Tropical Research, then applied it to western birds like the McGillivray’s and Wilson’s warblers. The San Francisco State group also looked at genetic patterns for a finer-grained resolution.
As reported in a 2003 article in Molecular Ecology, the hydrogen isotopes had some interesting stories to tell about the travels of the Wilson’s warbler. The bird shows a pattern of “leapfrog migration”, with the northernmost nesters wintering farthest south—a phenomenon previously documented for a few other species, like the fox sparrow. Since coastal nesters have higher deuterium values than interior nesters, Clegg, Kimura, and their colleagues were also able to report that warblers from coastal sites like Oregon’s Siuslaw National Forest and California’s Pillar Point spend the winter in western Mexico—Baja California Sur and Sinaloa, to be exact. Colorado warblers, on the other hand, migrate to comparable latitudes in eastern Mexico.
Given that, I could conclude that the Wilson’s I saw from my back steps was headed for the neighborhood of Cabo San Lucas, or maybe Mazatlan. And I could only hope that it found semi-intact habitat when it got there. Wilson’s is less specialized in its wintering-ground preferences than some other warblers; in Costa Rica, it’s even been observed above timberline, in the chilly paramo. But it would find a cornfield, or a beach resort, less than ideal.