November 18, 2017
Scientists Offer New Insights On Why Passenger Pigeons Went Extinct

New research has offered some additional information on why the passenger pigeon went extinct over a century ago, not long after it was considered North America's most common bird.

In the 19th century, passenger pigeons were a ubiquitous presence in North America, with its numbers estimated at around 5 billion, and flocks even being capable of reaching 300 miles long and darkening the daytime sky. According to the Daily Mail, an increase in hunting activity was blamed for the species' drastic decline throughout the 1800s, and it didn't take long for the last of its kind to die alone in captivity. The last known surviving passenger pigeon, a female bird named Martha, died at the Cincinnati Zoo in 1914.

Currently, Martha remains preserved at the Smithsonian's National Museum of Natural History, where she remains an ostensible reminder of how excessive hunting could kill off even the most common of species. But a team of scientists believes that there are other explanations as to why passenger pigeons went extinct, as DNA analysis tells an interesting supporting story of how the birds' numbers went from 5 billion to zero at such a rapid pace.

"We were hoping that we could get to the bottom of why they went extinct so quickly — why it was that this giant population of birds suddenly became extinct, entirely extinct, over the course of just a couple of decades," said researcher Beth Shapiro of the University of California, Santa Cruz.

According to NPR, Shapiro and her colleagues were able to gain insights on why passenger pigeons went extinct by asking museum curators to let them take tissue samples from the preserved birds in their collections. By analyzing a small piece of skin from the bottom of one of the birds' toes, the researchers generated a complete genome sequence from several different passenger pigeons. They then compared their DNA to that of the band-tailed pigeon, a species mainly found across North America's west coast and considered to be the passenger pigeon's closest living relative.

As Shapiro explained, population size is the main difference between passenger pigeons and band-tailed pigeons. That allowed her team to investigate the evolutionary implications of extremely large populations, considering that band-tailed pigeons live in considerably smaller groups.

After analyzing the nuclear DNA of four specimens and the mitochondrial DNA of 41 others, the researchers concluded that natural selection might have played a role in the passenger pigeon's extinction, with the birds well-adjusted to living in large populations, but not in situations where their numbers are much smaller. At the same time, the scientists observed a low level of "neutral" genetic mutations the could have helped the passenger pigeon survive in the long term, giving it a form of insurance in the event of a changing ecosystem, the Washington Post wrote.

"It's known that they collaborated in finding food, and they also collaborated in rearing young," study lead author Gemma Murray, also from the University of California, Santa Cruz, explained.

"And so these sorts of behaviors are the sorts of things which might work really well when your population size is large and dense. But when hunting had a big impact on their population, and their numbers went down hugely in the 19th century, maybe those things didn't work anymore."
The UCSC study also suggested that passenger pigeon populations were quite large and stable until the time that people started hunting the birds in the 19th century. This countered an earlier paper that suggested the species' population fluctuated, thus making passenger pigeons more susceptible to a hunting-related extinction.

Although the new research doesn't dispute the existing theories of why passenger pigeons went extinct, the findings may be instrumental in aiding modern-day conservation efforts, according to Smithsonian National Museum of Natural History curator of birds Helen James, who was not involved in the study. Speaking to NPR, James said that changes to the ecosystem could also affect abundant species, as this could lead to their "sudden disappearance," and a ripple effect that could jeopardize other species as well.

[Featured Image by Susan Walsh/AP Images]