Passenger pigeon population had booms and busts

Passenger pigeons, so abundant during the early 19th century that skies darkened with passing flocks, may often have been nothing special in numbers during much of their last million years.

DNA from the extinct species, coaxed from toe pads of three museum specimens, suggests that population numbers fluctuated in the long term. The breeding population could have been at times only roughly several hundred thousand or even just tens of thousands of birds, says Chih-Ming Hung of National Taiwan Normal University in Taipei.

That’s a surprise. A survey of various other animals suggests that the size of the breeding population, like that pulled from the pigeon DNA, is typically about one-tenth of the whole population, Hung says. Yet the estimates of breeding populations in the new study are only one ten-thousandth of the 3 billion to 5 billion birds estimated in 19th century eyewitness reports.

Hung doesn’t dispute those huge 19th century estimates. The DNA tells a story of bird numbers that soared and sank over time, he and his colleagues argue June 16 in the Proceedings of the National Academy of Sciences. He would need more analysis, he says, to see if an extreme crash during the most recent Ice Age would have produced such a skewed ratio in the last century. Or, in a more controversial scenario, he proposes that passenger pigeons may have had population booms and busts repeatedly on much shorter time scales.  

Hung became curious about genetic trends of the passenger pigeon after a pivotal conversation several years ago regarding the 100th anniversary this September of the death of Martha, a passenger pigeon in the Cincinnati Zoo and the last known individual of the species.

Museum specimens’ toe pads provide the best picture yet of passenger pigeon genetics, Hung says. From the amount of variation the researchers found in the specimens’ DNA, they could work out an approximate number for the birds that were passing on their genes, called the effective population size.

Over the last million years, Hung and his colleagues found, the typical number of breeding birds could have averaged something like 330,000. Another method found lower numbers but a similarly small order of magnitude: 170,000 at the population’s height to perhaps 50,000 at its worst. The ups and downs over deep history fit with the timing of glacial cycles and with computer simulations of the niches available for the birds as climate changed.

Genetic tests can’t detect population ups and downs at the scale of a mere century. But Hung and his colleagues speculate that pigeon populations might have fluctuated in the short term too, perhaps shrinking drastically during times of skimpy acorns.

In theory, a species that surges in mind-boggling numbers certainly can go extinct quickly, says entomologist Jeffrey A. Lockwood of the University of Wyoming. The Rocky Mountain locust (Melanoplus spretus) is an example that savaged wide swaths of cropland during its booms but abruptly went extinct at the end of the 19th century when farmers took over the very specific habitats it needed to breed.  

But passenger pigeons couldn’t have boomed in huge numbers quickly, says conservation biologist Stanley Temple of the University of Wisconsin–Madison. The birds tended just one nest a year and raised one chick. “There is absolutely no way these birds could rapidly increase their numbers,” he says. “It would take them probably centuries to increase their population even tenfold, let alone several orders of magnitude.”

Hung and critics agree that natural cycles, either short- or long-term, do not mean that the passenger pigeon would have eventually cycled into oblivion on its own. David Blockstein, senior scientist at the National Council for Science and the Environment in Washington, D.C., has described how intensive shooting at breeding colonies contributed to the species’ demise by disrupting its reproduction. And ecologist Stuart Pimm of Duke University says, “The basic cause for the passenger pigeon’s decline was the destruction of the Eastern forests.”  The message of the paper, Hung says, is that “the passenger pigeon had repeatedly recovered from population lows over the course of its history.” Then came 19th century humans.