When the top bird experts on this continent came together to form the American Ornithologists’ Union (AOU) in the 1880s, one of their first priorities was to set up a committee to produce an official list of North American birds. Up to that time, anyone writing about birds might use their own set of names and their own definition of which ones were valid species. The committee’s first publication, the AOU Checklist, established an official classification and a standardized set of names, and it was a great help in putting ornithology on a firmer foundation.
Today the organization is called the American Ornithological Society (AOS), but it still has an active committee that maintains the standardized list of bird species. Because there’s so much research in this field, the AOS Checklist Committee now puts out a report every July, and birders wait each year to see what that report will do to our life lists. Will we gain new species from “splitting” or lose some to “lumping?” Will we see changes in some names?
The 2020 report was released online on June 30th. It shifts the sequence of birds within a few families and splits some birds in Mexico and Central America, but the following are the only changes that will affect most birders in the U.S. and Canada:
1. The Mexican Duck (Anas diazi) of Mexico and the southwestern U.S. will now be treated as a separate species from the Mallard (Anas platyrhynchos).
2. The Northwestern Crow (formerly called Corvus caurinus), of coastal regions from southern Alaska to Washington, will now be considered just a smaller variation of the widespread American Crow (Corvus brachyrhynchos).
3. The scientific names of a few hummingbirds are changing.
White-eared Hummingbird: Hylocharis leucotis becomes Basilinna leucotis
Xantus’s Hummingbird: Hylocharis xantusii becomes Basilinna xantusii
Violet-crowned Hummingbird: Amazilia violiceps becomes Leucolia violiceps
Berylline Hummingbird: Amazilia beryllina becomes Saucerottia beryllina
In terms of impact on birding, this year’s changes were minor. But the contrasting results for the Mexican Duck and Northwestern Crow—“splitting” one, “lumping” the other—are worth exploring. They give us some intriguing glimpses into history, and into the factors weighed by the experts on the checklist committee of the AOU/AOS as they make these decisions.
Mexican Duck: A Borderline Case
Everyone knows the Mallard, the most widespread duck in the northern hemisphere. The adult male, with green head and reddish chest, is readily recognized, although the subtle brown female is not so distinctive. But the Mallard has several close relatives in which the adult males wear brown plumage very much like that of the female. The American Black Duck of eastern North America has long been recognized as distinct, but the Mottled Duck of the Gulf Coast states was not described to science until 1874. The Mexican Duck, found from central Mexico north into the southwestern U.S., was not described and named until 1886. North of the Mexican border the bird was not noticed until around 1920, when Wharton Huber described it as a brand-new species under the name “New Mexican Duck,” but its connection to the birds in Mexico was soon recognized.
If you’re lucky enough to go birding in west-central Mexico, say in the marshes around Lake Chapala, you’ll see pairs of Mexican Ducks. They’re beautiful birds, patterned in rich dark browns. The ones in the southwestern U.S. may look a bit different. Birds classified as Mexican Ducks can be found year-round in southern Arizona, New Mexico, and western and southern Texas, but many of them show some evidence of past interbreeding with Mallards. In Arizona, where I lived for many years, “normal” Mallards breed in the northern part of the state and locally elsewhere. Birds classified as Mexican Ducks breed in the southeastern part of the state. Whenever I spent time watching the latter birds, I would notice some males with patches of green on their heads, or females looking paler than expected, suggesting the influence of Mallard genes.
Ornithologist John Hubbard did a careful study of Mexican Duck and Mallard specimens in the 1970s, with examples all the way from Canada to central Mexico. Based on appearance, he found plenty of evidence of interbreeding between the two—especially in the southwestern U.S., where their ranges approached most closely. On that basis, the AOU checklist committee concluded in 1983 that the Mexican Duck was just a subspecies of the Mallard. That has been its official status ever since.
However, the committee has received proposals to split the Mexican Duck three times in the last decade—in 2010, 2018, and again this year—with increasing levels of evidence each time. This year, the proposal was accepted.
What was the reasoning? Partly, it involved looking at the situation with related species. Yes, Mexican Ducks do interbreed frequently with Mallards, but the same is true of American Black Ducks and Mottled Ducks. In just about any winter flock in the northeastern states in winter, with enough scanning, you can find Mallard x American Black Duck hybrids. Feral Mallards often hybridize with Mottled Ducks in Florida and elsewhere. So for the sake of consistency, if those species are considered separate, the Mexican Duck should be as well.
The most recent proposals also included much more DNA analysis. These genetic studies confirm that Mallard, American Black Duck, Mottled Duck, and Mexican Duck are all very close relatives, but Mexican Duck is no closer to Mallard than to any of the others in this complex. So a consistent treatment would suggest lumping all four of these into one species—which doesn’t seem justified—or else restoring the Mexican Duck to its status as a full species. The committee went for the latter interpretation, so now birders traveling to the Southwest can make a point of seeking out this special duck to add to their lists.
Northwestern Crow: On the Fence for 160 Years
If you visit the coast of southeastern Alaska, British Columbia, or a short distance south into Washington, you might notice that the crows there look a little smaller than American Crows elsewhere. Some of their calls also sound a little low-pitched and rough, and these birds seem to spend a lot of their time right along the water’s edge. Up to this year, they were treated as a separate species, the Northwestern Crow. But people have raised doubts about the validity of this species ever since it was first described to science in 1858.
Even Spencer Baird, who originally described and named the species, seemed uncertain. He wrote that it was so much like the American Crow “as to be only distinguishable by its inferior size and habits. Indeed, it is almost a question whether it be more than a dwarfed race of the other species.” Other ornithologists who wrote authoritative texts in the late 1800s, from Elliott Coues to Robert Ridgway, treated these northwestern birds as part of the widespread American Crow. But when the first AOU Checklist was published in 1886, it included this bird as a full species, under the name “Northwest Crow.” By the time of the fourth edition of the AOU Checklist, in 1931, it was treated as just a subspecies of the American Crow, but in the fifth edition in 1957 it was once again listed as a full species. Clearly more research was needed.
Such research appeared soon thereafter. In a 1961 monograph about crows, David W. Johnston reported that his field studies and analysis of specimens found no dividing line between American and Northwestern Crows—they interbred freely in western Washington and British Columbia, and there was a wide stretch inhabited only by intermediate birds. When I first visited the Seattle area as a teenager in the 1970s, expert local birders flatly told me that the Northwestern Crow didn’t exist. But for some reason, Johnston’s research wasn’t enough to convince the checklist committee of the AOU.
So the Northwestern Crow remained on the official bird lists. Birders still went to the Pacific Northwest coast to look at small crows so we could add this sketchy bird to our life lists and our year lists. But many of us felt uneasy about it.
New research, headed up by ornithologist David Slager, has finally put the controversy to rest. Slager and his coworkers did a thorough genetic analysis of crows all along the coast from southern Alaska to southwestern Washington. They found there was a stretch more than 550 miles wide in which ALL the crows had intermediate DNA. Furthermore, the DNA revealed no first-generation hybrids—typically what you find if two species only occasionally interbreed, or if they've just recently come in contact. Instead, the mixing and interbreeding in the hybrid zone had been going on for a long time. So the crows are not operating as separate species, and shouldn’t be classified as such.
Still, if you visit southeastern Alaska, take a few minutes to watch the crows along the shoreline. Notice that they’re small, and that they have some funny callnotes. They don’t count as a new species on your life list, but they have a long history of causing confusion for scientists and birders, and we should salute them for forcing us to pay more attention to the endless variations in nature.