Of all the birds on Earth, perhaps none is more unique than the South American oilbird, which 90 million years ago hopped onto its own branch of the evolutionary tree and has been on it ever since.

The oilbird perches atop a new analysis of avian distinctiveness: how old each species is, and whether they have close relatives.

By this metric, the most distinct species are truly one-of-a-kind. "It's millions of years of evolutionary information that's distinct to the species," said evolutionary biologist Walter Jetz of Yale University.

The new analysis, led by Jetz and published April 10 in Current Biology, used a computer model that turned genetic data, the fossil record and previously-proposed taxonomic trees into a new evolutionary tree of birds.

Just because a species isn't distinct, of course, doesn't mean it's not special: If yellow warblers vanished, for example, North America would be a far less cheerful place. But at least there are other other warblers. If the giant ibis or California condor goes extinct, though, there's nothing else remotely like it.

"Those millions of years of evolutionary history would be lost," said Jetz.

Jetz emphasized that distinctiveness isn't just a matter of coloration, anatomy, and other aesthetics. It's also reflected in habits and behaviors and life histories, perhaps even in cognitive abilities and molecules. After 80 million years of evolution, oilbirds probably make some pretty interesting proteins.

Some evolutionarily distinctive birds, including ostrich and osprey, are relatively common. Others, however, such as Christmas Island frigatebirds and the great ibis, are imperiled. We have a special obligation to protect these species, says Jetz.

His team is trying to use these data to come up with a conservation strategy, identifying key habitats where with relatively little effort, humans could save 60 percent of this evolutionary distinctiveness. They've also developed a website where you can look at the distinctiveness of birds in your own area.

Evolutionarily distinct birds often don't occur in biodiversity hotspots, said Jetz, and aren't appreciated as exceptional by birdwatchers and nature-lovers. He hopes that will change.

"Just looking at an oilbird, I feel like I've gone back 60 million years. You say to yourself, 'You're a bird from a different time!,'" said Jetz. "I wish that as many people as possible, as many future generations, get to experience this, too."