Wild Neighbors: When Is a Tanager Not a Tanager?

By Joe Eaton
Thursday December 17, 2009 - 08:54:00 AM
Hawai’i o’o: not a honeyeater.
John Gerrard Keuleman
Hawai’i o’o: not a honeyeater.

This summer, when I wasn’t paying attention, the western tanager was determined not to be a true tanager. This was not exactly a demotion, as was the reclassification of Pluto from bona fide planet to small planet-like object, or whatever it is now; more like a lateral transfer. Still, I expect this move came as a shock to a lot of birders. 

The taxonomic shuffle was part of the 50th Supplement to the American Ornithologists’ Union Checklist, an annual updating of the nomenclature of New World and Hawai’ian birds. As with Pluto, I believe there is an actual vote on proposed changes. Sometimes the Supplement contains splits—newly recognized species—and bird-listers rejoice. Sometimes it contains lumps—mergers of old species—and listers mourn. Sometimes, as happened this year, birds get relocated from one family or order to another. 

In part, this reflects the growing sophistication of the tools available to scientists who try to sort out the natural world. Linnaeus, the Great Classifier, could only rely on anatomy. If two species of birds, or other animals, were similar in plumage, beak shape, and other characters, they were assumed to be closely related. For Linnaeus, a pre-Darwinian, this didn’t imply common descent-more like some kind of filing system in the mind of God. 

Later field studies helped define species limits. But the true revolution came with new biochemical and genetic techniques that revealed evolutionary relationships. With birds, the last big reorganization followed data from DNA-DNA hybridization studies, which provided a rough index of genetic similarity. That was when mockingbirds and mynahs were determined to be close kin, and the New World vultures and condors were moved from the order of raptors to the order of storks and other wading birds. 

But that wasn’t the last word. More recent work with mitochondrial DNA—the other genome, the one that passes intact from mother to child—has called some of the hybridization studies into question. Two years ago, the vultures and condors became raptors again, although no one is quite sure where to put them. 

The problem with the old school of taxonomy was that unrelated creatures often evolve similar body forms. Think of sharks, tunas, dolphins, and the long-extinct reptiles called ichthyosaurs, all with torpedo-shaped bodies propelled by fins or flukes. Think of the marsupial wolves, cats, moles, anteaters, and gliders of Australia, each a rough facsimile of a placental-mammal counterpart. The phenomenon is called convergence, and for years it was the bane of classifiers. 

Case in point: one of this year’s changes in the AOU supplement involves an unfortunately extinct group of Hawai’ian songbirds: four species of o’o, variations in black and yellow, and the larger, streaky-plumaged kioea. Because all of them had longish decurved bills and fed on nectar, they were placed with an anatomically and ecologically similar family of birds, the honeyeaters of Australia and the South Pacific. Other honeyeater species occur as close to Hawai’i as Samoa and Fiji, so getting the ancestral population there didn’t seem too much of a stretch. 

But then a team led by the Smithsonian’s Robert Fleischer pulled out a bunch of museum specimens and did DNA comparisons with other songbirds. O’os turned out not to be honeyeaters after all. Their closest relatives were revealed to be, of all things, waxwings and silky flycatchers (like our native phainopepla.) The founding population may have reached the islands from North America, to be shaped over millions of years into honeyeater look-alikes. 

Some similar kind of convergence must account for the resemblance between the North American tanagers of the genus Piranga—the western, scarlet, summer, and hepatic tanagers—and the true tanagers of the New World tropics. Mitochondrial DNA analyses show the Piranga tanagers to be more closely related to the black-headed and rose-breasted grosbeaks, and a little more distantly to cardinals and lazuli buntings. So they were booted out of the tanager family and reassigned to the cardinal-grosbeak family. To see a real tanager now, you’ll have to visit the rain forest exhibit at the California Academy of Sciences—or, if your budget allows, Costa Rica. 

The change was foreseeable. As far back as 2000, published research suggested that the Pirangas didn’t fit genetically with the tropical tanagers. But those of us who don’t keep up with the technical journals wouldn’t have been aware of that. 

Field guides will have to recognize the reclassification, although that won’t be a big deal for most North American books. I suppose we can still call them tanagers. Names like “finch,” “grosbeak,” and “bunting” are still shared by birds from completely different families. On the other hand, maybe we could have a contest.