It’s been about a year since I wrote here about how swiftly, how dramatically, our conception of the world of birds is changing these days.
I was at the last annual AOU meeting then, and at the time, much of the buzz still centered around a certain woodpecker in Florida. But for my money, the most exhilarating moment of the meeting was delivered by scientists who were using mtDNA and nuclear data to shed light upon a group of birds known as nine-primaried oscines -- tanagers, cardinals and grosbeaks, New World sparrows, New World blackbirds, and wood-warblers.
The methods the scientists described sounded highly technical and their discussions were loaded with jargon, but their conclusions were thrilling.
Among them: that our Scarlet, Western, Hepatic, and Summer Tanagers aren't really tanagers; that five genera currently assigned to the New World sparrows are probably better placed elsewhere in the songbird tree; that two genera now placed among the tanagers appear to belong within the sparrows; and that Olive Warbler and the chats don’t properly belong with the wood-warblers.
I remember thinking then how lucky I was to be present at just the moment when an established worldview was swept away and we were asked to look at the avian tree of life, or a part of it, in a new way.
I have that feeling again today.
Because today, scientists who have spent the last five years examining DNA not just from the nine-primaried oscines but from all major living groups of birds are publishing their findings, and they are every bit as thrilling as those announced last year. Indeed, they promise not only to send the writers of biology textbooks and field guides back to their drawing boards but to shake our understanding of evolution.
The scientists, from institutions in the U.S., Scotland, and Australia, are collaborators in a large-scale, cooperative research project known as Early Bird, a part of the National Science Foundation’s ambitious Assembling the Tree-of-Life initiative.
Their objective: to determine the evolutionary relationships among all major groups of birds.
Their report, which appears in the June 27, 2008, issue of the journal Science, is sure to cause a stir. In an accompanying article, Joel Cracraft, an evolutionary biologist at the American Museum of Natural History in New York City, calls the work “the most impressive paper in the higher level phylogeny of birds to come along in a long time.... It will be used by avian systematists and nonavian systematists for a very long time.”
Here’s the citation:
The paper is available online, but only subscribers have access to the full text. The rest of us will have to make do with the abstract.
In the paper, the scientists report, as expected, that they found genome-wide support for two well-known branches at the base of the avian tree of life: one where tinamous and ratites (that is, kiwis, ostriches, and the like) split off from all other birds, and another where chickens, ducks, and their allies split from Neoaves, the group that contains 95 percent of the bird species on the planet today.
Land birds make up the largest part of Neoaves, and Passeriformes, or perching birds, make up a lion’s share of the land birds. This was expected, too.
What wasn’t expected was an apparent sister relationship between Passeriformes and Psittaciformes (parrots). That’s right, parrots. Just imagine -- songbirds and parrots descending from a common ancestor.
And it gets better.
According to the new phylogeny, the falcon family (Falconidae) and the hawk and osprey family (Accipitridae) should no longer be placed together in the order Falconiformes. Rather, Accipitridae shares ancestors with Cathartidae (New World vultures), while Falconidae appears to be closely related to perching birds and parrots.
“This relationship varied slightly among analyses and gene-jackknifing,” the scientists write, “yet the close relationship between passerines with parrots and/or falcons appeared consistently.”
The biologists also uncovered evidence that flamingos, a group alternately described as a suborder of wading birds, waterfowl, or shorebirds, are in fact a sister taxa of grebes (Podicipediformes), and that grebes share ancestors not with loons, but with tropicbirds (Phaethontidae), sandgrouse (Pteroclididae), mesites (Mesitornithidae), and pigeons and doves (Columbiformes).
One of the scientists’ other important findings is that several well-accepted orders appear not to be monophyletic -- that is, that the species in those orders appear not to have originated from common ancestors. Some eyebrow-raising examples:
-- Tinamiformes (tinamous) are found within Struthioniformes (ostriches and allies)
-- Apodiformes (hummingbirds and swifts) are found within Caprimulgiformes (nightjars and their allies)
-- And Piciformes (woodpeckers) evolved from Coraciiformes (kingfishers, hornbills, rollers, and allies)
And that flighted tinamous belong in the same group as flightless ostriches, says ornithologist Shannon J. Hackett of Chicago’s Field Museum of Natural History, one of the lead authors, “can change the way people look at the evolution of flight.”
The broad structure of the reconfigured avian tree of life suggests diversification along general ecological divisions, such as water birds, shorebirds, and land birds. “However, adaptations to these environments clearly arose multiple times,” write the scientists, “because many aquatic birds were not part of the water bird clade (e.g., tropicbirds, flamingos, and grebes) and terrestrial birds were found outside of the land bird clade (e.g., turacos, doves, sandgrouse, and cuckoos).”
Sushma Reddy, Bucksbaum Postdoctoral Fellow at the Field Museum and another lead author, summed up the Early Bird scientists' findings in two succinct, sweeping statements: “First, appearances can be deceiving. Birds that look or act similar are not necessarily related. Second, much of bird classification and conventional wisdom on the evolutionary relationships of birds is wrong.” -- C.H.