Passenger Pigeon DNA Shows How Large, Stable Populations Can Quickly Go Extinct
The DNA of a long-gone bird reveals that evolutionary success can be short lived, providing a cautionary tale for species today — even humans.
The passenger pigeon was once the most abundant bird in North America, numbering between 3–5 billion. John Muir and many other naturalists were fascinated by these large, intelligent birds that often would passager, according to the French, or pass by, while migrating.
Muir devoted five full pages to passenger pigeons in his autobiography, mentioning that "the air was literally filled with pigeons" and the "continued buzz of wings" lulled him to sleep.
Over the course of Muir's lifetime, however, passenger pigeons died by the millions. In 1914 — the year of his own death — only one was left. Martha, as she was called, died on September 1 of that year at the Cincinnati Zoo, marking the species's extinction.
What lives on is a cautionary tale, according to a new genetic study on the birds published in the journal Science. Their DNA provides evidence that even large, stable populations can be at risk of extinction if rapid environmental change occurs.
"When passenger pigeon populations were large, there was natural selection for advantageous genes and against deleterious genes — both forms were highly efficient," senior author Beth Shapiro of the University of California, Santa Cruz told Seeker. "However, selection always works within a particular environment."
"In this case," she continued, "that environment was living in huge flocks. When the flocks became suddenly tiny — thanks to human hunting — these adaptations to life in large populations were suddenly of no use to the birds."
Shapiro, lead author Gemma Murray, and their international team were granted access to collections of passenger pigeons in museums. To cause minimal damage, the researchers extracted DNA from toe pads or bone samples of 84 of the birds.
The researchers then extracted DNA from four band-tailed pigeons. The band-tailed pigeon — a sociable bird with a mellow coo — is common in forests of the Pacific Coast and Southwest and is the closest living relative of the passenger pigeon.
A comparison of the two species' nuclear genomes revealed that the once-large population of passenger pigeons allowed for faster adaptive evolution than what is seen in band-tailed pigeons. High-diversity regions of passenger pigeon underwent stronger and faster genetic selection to remove harmful mutations and to maintain advantageous genes.
In short, passenger pigeons were almost perfectly adapted to their habitat and way of life.
The researchers were surprised to see that passenger pigeon populations were large even throughout the last ice age.
“This meant that these birds must have had both a very broad diet and a tremendous capacity to adapt to the enormous ecological changes that occurred as climate warmed into the present day," Shapiro said.
The apparent idyllic life of the passenger pigeon took a drastic turn for the worse when humans — first Native Americans, then Europeans — arrived in the Americas.
Because the meaty birds were close-knit and existed in large flocks, hunters could easily kill many at a time with little effort. By the 19th century, pigeon hunting intensified with growing demand for what was then considered to be cheap and good eats.
Passenger pigeons that survived the blood bath were reduced to living in small, isolated populations.
"Perhaps it was harder for them to find food, find a mate, and to do what it meant to be a passenger pigeon," Shapiro said. "If the decline had been slower, it is possible that passenger pigeons would have gradually adapted to their new ecological state."
Humans did not just hunt the birds. They also destroyed their habitat.
“The deforestation that was going on in the 19th century,” said Murray, “would have also had an impact, since passenger pigeons lived in forests and woodlands and ate nuts from trees."
With little time to adapt to the sudden changes, passenger pigeons went into a gradual decline from about 1800 to 1870, followed by a rapid decline between 1870 and 1890. The last confirmed passenger pigeon in the wild was thought to have been shot in 1901.
Passenger pigeons and band-tailed pigeons co-existed, but did not interbreed, Shapiro said. She explained that the Rocky Mountains, which seem to be a barrier for band-tailed pigeons today, kept the two species apart.
Band-tailed pigeons may have survived, Murray said, because they form much smaller flocks and are less of a social species than passenger pigeons. “These features might make band-tailed pigeons more resilient than passenger pigeons to extinction, since they might allow them to live more successfully in smaller and more isolated populations," she said.
Murray and her colleagues hope to find out if the DNA dynamics of passenger pigeons could be at play today in other birds and particularly in animals of any kind with very large population sizes.
Humans could, of course, fall into that category. Other studies over the past few decades show that for a population size of about 7 billion, our species has relatively low genetic diversity. While we are in no imminent danger of suffering the passenger pigeon's fate, the cautionary tale of the two pigeons — the passenger and the band-tailed — shows that even large, stable populations are at risk of extinction if environmental change outpaces adaptive evolution.