Study offers new clues to extinction of passenger pigeon

Martha, the last of her kind, resides in a glass case at the Smithsonian’s National Museum of Natural History, perched on a thin branch. She’s a passenger pigeon, Ectopistes migratorius, and in the final years of her life, before her death in 1914 at the Cincinnati Zoo, she achieved fame as the last survivor of a species once so populous that its flocks could darken the noonday sky.

Martha is small and gray, with flecks of blue and green iridescence on the back of her neck. She is looking sharply to the right, as if looking over her shoulder — as if a bit wary. (You’re not being paranoid when you’re the only one left.)

“Some people find her a little plain looking,” said Helen James, the Curator of Birds, who can put her hands on more specimens of passenger pigeons, older and unheralded, stored upstairs in the museum’s ornithology collection.

How the passenger pigeon died out is hardly a whodunit. Humans exterminated them through ruthless and efficient hunting in the late 19th century. We’ve driven plenty of species to extinction, but the case of the passenger pigeon is one of the most perplexing. This had been the most abundant bird in North America and possibly the world. A single flock could contain more than a billion birds. Scientists still wonder: Why didn’t some pigeons survive in remote areas?

A new study of the passenger pigeon’s genome, published recently in the journal Science, dives into the debate over this famous extinction. The paper argues that the passenger pigeon, contrary to what some scientists have said in recent years, did not suffer wild fluctuations in population before humans wiped them out. Rather, the population was stable for thousands of years, even during periods of dramatic climate change, the new paper states.

The new study also bolsters research showing that the passenger pigeon didn’t have very much genetic diversity across its vast population. “We have this very large population size but not very large genetic diversity,” said Gemma Murray, an evolutionary biologist at the University of California at Santa Cruz and the lead author of the paper.

The researchers studied the nuclear DNA of four passenger pigeon specimens, and also looked at the mitochondrial DNA of another 41 specimens. They compared the genetic markers to the extinct bird’s relative, the bandtailed pigeon.

The study concluded that much of the bird’s genetic code shows signs of strong natural selection, and simultaneously a low level of genetic drift or “neutral” mutations — the kind of changes that may not have any obvious adaptive advantage in the short run but could serve as a hedge in the future if the ecosystem changed.

The new study does not contend that the low level of genetic diversity led to the demise of the passenger pigeon. That’s an extra leap. It may be that even a bird with tremendous genetic diversity, and nimble when ecosystems change suddenly, could not have withstood the onslaught of human predation.