It’s not often that you see a bird that doesn’t match anything in the field guides—even in Sibley’s Bible of bird identification. But there it was, hanging out with a raft of overwintering common goldeneyes and Barrow’s goldeneyes at the Bayward end of Lake Merritt: a midsized duck with a dark head (showing a purple gloss when the sun hit it) and a backswept crest, a dark back, and pale sides with two vertical hash marks.
I did find a similar bird illustrated in a treatise on North American waterfowl: a hybrid between the common goldeneye and hooded merganser. Local birders have speculated that one of the parents of the Lake Merritt duck was a Barrow’s goldeneye; Barrow’s has a purple gloss to the head, common a green gloss. But there’s more of a range overlap between the merganser and common goldeneye, and more records of that hybrid combination. In any case, one goldeneye or the other must have gotten together with a hooded merganser to produce the anomalous duck.
Ducks are prone to that kind of thing. You’ve seen the genetic scrambles among flocks of domestic ducks: part Peking white, part Muscovy, part wild mallard. And hybridization happens with some frequency in the wild. Years ago at Coyote Hills, I saw the mallard-gadwall cross that Audubon had described as a distinct species, the “Brewer’s duck.” It not only looked odd, it sounded odd.
“Mack?” it said. Hybrid ducks are often fertile; only mandarin ducks are incapable of producing fertile offspring with another species. Conservationists are concerned about mallards genetically swamping some of their closer relatives: black ducks on the east coast, mottled ducks in the South, koloas in Hawai’i.
But ducks aren’t alone. A few years ago ornithologists in the South Bay spotted an “avostilt,” the apparent offspring of an American avocet and a black-necked stilt. Shorebirds like stilts and avocets rarely hybridize, but it happened at least once. Some eastern and western species pairs of birds have hybrid zones where their ranges overlap.
A friend recently asked me if the Lake Merritt goldeneye-merganser cross (goldanser?) was a new species. No, although hybridization sometimes leads to the formation of a new species among plants. The phenomenon does raise questions about the way we define species, though.
Back when Darwin was pondering how species originate, naturalists had a kind of Platonic notion of what a species was. There was some essence of, say, mallardness, and any variation from that deviated from the ideal type. And as Darwin found, there was a whole lot of variation. He was hard put to draw a distinction between species and varieties.
Some 90 years after The Origin of Species was published, Ernst Mayr—who died this February at the age of 100, still cranking out books—came up with a better idea. Mayr defined “species” in population terms. According to his Biological Species Concept, species are groups of interbreeding natural populations that are reproductively isolated from other such groups. This made a lot of sense and was widely accepted by biologists, although it didn’t work for organisms like self-cloning sea anemones or parthenogenic whiptail lizards, and wasn’t the best fit for plants. Mayr was careful to point out that the concept applied to wild creatures with a free choice of mates. Cage a tiger and a lion together and you may get a hybrid “liger,” but this would be an improbable outcome even in the small area of India where lions and tigers coexist.
Ducks don’t seem to fit, though. Duck species, which will hybridize at the drop of the hat, are clearly not reproductively isolated from each other. There seem to be limits to the process, because we haven’t wound up with just one generic type of duck; but the species boundaries do seem fuzzy.
In the 1980s, a South African entomologist named Hugh Paterson proposed a rival definition, the Recognition Species Concept: a species is a population of biparental individuals with a shared mate recognition system. Fertility can be part of the system—at the level of egg recognizing sperm—but it’s not essential to the definition. It can also be a matter of the organism recognizing another individual as an appropriate mate, through simple visual cues or the elaborate song-and-dance routines that birds have evolved.
The nice thing about the Recognition Species Concept is that it’s field-testable. Peter Grant has spent years working with the Darwin’s finches in the Galapagos Islands, as described in Jonathan Weiner’s fine book The Beak of the Finch. Most of the 14 finch species look much alike to the casual human observer. But Grant’s experiments showed that male finches could differentiate between stuffed models of their own species and those of a very similar neighbor species. It was harder to run the tests with live female and dummy male finches because the females’ mates kept attacking the dummies.
With male ducks, the visual cues—the sharp dark-and-light patterns, the colors, the crests—are obvious. But mistakes do get made.
Ducks of different species tend to have broadly similar courtship displays; maybe that confuses the issue.
What interested me most about the goldanser was that it was not just a mosaic of goldeneye traits and merganser traits. Males of both the parent species have white head markings, but this duck’s head was all dark, and its crest was unlike either a merganser’s or a goldeneye’s. Goldeneyes have standard duck bills, while mergansers have narrow, saw-edged bills for snagging fish. The hybrid’s bill was intermediate in shape. I saw it eating mussels along with the goldeneyes, and it seemed to have no trouble handling them—and its equipment had been functional enough to allow it to survive to adulthood.
When I last saw them, the goldeneyes were gearing up for the mating season with head-pumping displays. How a female goldeneye or female merganser would respond to the goldanser remains an open question. ?