Wild Neighbors: Building a New Tree for the Birds

By Joe Eaton
Thursday July 03, 2008 - 10:18:00 AM

Some people follow soccer; I follow taxonomy. A slower game, but it has its regards. 

Taxonomy is the science of classification of living things. People since Aristotle have been trying to put all the animals and plants and other organisms into some sort of reasonable order. The modern version derives from the 18th-century Swedish botanist Linnaeus, who started with plants and went on to develop a whole Systema Naturae. It owes its longevity to its neat fit with the ideas of Darwin and Wallace about common descent, although Linnaeus himself was a creationist. Every species goes into a box, that box into a bigger box, and so on.  

I will borrow my associate Matt the Cat for illustrative purposes: a member of the species Felis sylvestris, the genus Felis, the family Felidae, the order Carnivora, the class Mammalia, the phylum Chordata, the kingdom Animalia, the domain Eukarya. (And those aren’t the only divisions: Matt is also a placental mammal, a vertebrate, and a craniate, the last meaning that he has a head.) 

Linnaeus never heard of domains, of course; that’s recent. And taxonomists keep changing the basis for classification. It used to depend on physical structures; now it’s mostly genetics, or some combination of genetics and morphology. The most influential figure in the field has to be the German entomologist Willi Henning, founder of cladistic analysis with its forbidding terminology. (A clade is just a lineage, a group of organisms sharing a common ancestor.) 

And every now and then a group of scientists decide to have a go at a fairly large clade: modern birds, say, or flowering plants. Plant taxonomy is wild these days. They’ve broken up the lily family, like a telephone monopoly, into a couple dozen smaller families. Just last fall I learned in a plant taxonomy course that most asters aren’t asters any more. They’re in a new genus with a longer name. 

Now it’s the birds’ turn. In an article just published in Science, a group of scientists—the lead author is Shannon Hackett of the Field Museum, and the group includes Rauri Bowie and John Harshman from UC Berkeley—propose a new phylogenetic tree for this well-studied group. They used a huge data set: 19 loci from 15 different chromosomes in the chicken genome, representing three different kinds of genes, for 171 species. The results, which they justifiably call robust, suggest that it’s time to revise the field guides again. 

The last big avian reshuffle was the work of Charles Sibley (no relation) and Jon Ahlquist in the 1980s, using a technique called DNA-DNA hybridization. That led to some surprises, including the New World vultures and condors being split off from the hawks, eagles, and Old World vultures and grouped with the storks. Later, other taxonomists claimed that ducks and chickens, broadly speaking, diverged early from all other lineages; only ostriches and their kin were more basal. The first edition of the National Geographic bird guide began with grebes and loons, odd clumsy birds with a prehistoric look. For the fifth (current) edition, ducks and chickens come first. 

The Hackett et al. phylogeny keeps that basic divison between ducks and chickens and everything else. There are interesting rearrangements further along the tree, though. New World vultures are reunited with other diurnal raptors, a change already endorsed by the American Ornithological Union, the College of Cardinals of bird taxonomy. Grebes are confirmed as the sister group of flamingoes. Some of the orders are pretty much dismantled. The Gruiformes is down to cranes, rails, and bustards, plus, counterintuitively, cuckoos. A few families stand out as isolates: tropicbirds, sandgrouse, and the very odd South American hoatzin, which had been placed all over the map. 

What interested me most was the top of the tree. I wasn’t surprised to see the passerines, somewhat misleadingly called song birds (not all of them sing, and some non-passerines do) or perching birds (ditto), up there. They’re a recent group in evolutionary terms, and an enormously diverse and successful one. And they include some of the brainiest birds: the crows and ravens. 

But they’re not alone. The parrots are their sister lineage, which makes a kind of sense. And sister to the combined passerine-parrot clade is the falcon family. Falcons? Admirable birds, of course, very good at what they do, but you don’t think of them as having a lot in common with songbirds and parrots. 

That would mean that the raptorial way of life—the killer beak, the gripping talons, the acute vision—evolved twice, on separate branches of the avian tree. More than that, actually, if you consider owls. And then there are the shrikes, songbirds working their way toward raptorhood. 

The new tree is controversial, and I’m sure people are gunning for it already. From one approximation to the next science lurches on.