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.
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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.