The passenger pigeon. JOEL SARTORE/National Geographic Creative

Passenger pigeons were once the most abundant bird in all of North America. Now, there are none left. The turn of the 18th century saw their numbers diminish so rapidly that five billion birds vanished in two centuries. A new study published has shown that not just hunting but genetic reasons played an important role in the biggest extinction on this planet.

For scientists to wrap their heads around the speed of the extinction is no easy process. When Martha, the last surviving passenger pigeon died in the Cincinnati zoo in 1914, the science community set forth to solve the largest disappearance in the animal world.

"Why did little tiny populations of this bird not somehow survive in some refugial forest somewhere?" Beth Shapiro, member of the study said in a release on the University of California Santa Cruz website. "Why did they just go from billions to none?"

Passenger pigeons were impressive birds capable of some mean aerial fetes. They had the ability to sustain a continuous flight for about 300 miles without stopping. They were so abundant across the continent that it was famously known that a flock of passenger pigeons flying overhead could block out the sun. Where did it go wrong for this relative of the consummate city survivor, the feral pigeon?

This new study, published in the journal Science, found that their huge numbers and uncontrolled growth also played a crucial role in the bird’s disappearance from earth.

The team of researchers has now zeroed in on the fact that the pigeons evolved so quickly that their numbers grew exponentially on an annual basis. They started propagating across the areas they inhabited. However, they also lost some useful traits that would let them survive in smaller groups. When the season hunting began to increase, they weren’t able to adapt to that kind of circumstances.

“When we looked at rates of adaptive evolution and purifying selection in both species, we found evidence that natural selection had resulted in both a faster rate of adaptive evolution in passenger pigeons and a faster purging of deleterious mutations,” coauthor Gemma Murray said. “That is exactly what you would expect to see if the selection is causing the differences in genetic diversity,” she added in the press release.

The study says that in the early 1800s, this species started experiencing a vast falling due to a surge in hunting.

The team used tissue samples from preserved specimens of passenger pigeons across the world in museums. They found similarities between the evolutionary pattern between passenger pigeons and modern-day pigeons. This might mean a threat to them if scientists don’t find a solution soon — besides slowing down the hunting.

“Our results suggest that even species with large and stable population sizes can be at risk of extinction after a sudden environmental change,” Beth Shapiro added in the release.

As part of evolutionary changes seen in the species, their defenses dropped drastically in smaller groups. Shielded by the security of their evolutionary security of large groups, when they fragmented they did not know how to survive. Thus, being unable to protect themselves in front of predators. It only took them a few decades to disappear.

According to Professor Shapiro, the main reason for a species’ disappearance will always be hunting. She said that passenger pigeons could still be flying around today if not for the skilled human hunters, but evolutionary changes also played a role in their disappearance.