At their peak, a passing flock could darken the sky for hours. The downbeat of their wings would cause a chilling draft. By some estimates, a single discharge from a gun could bring down 30 birds or more.

The last passenger pigeon, a female named Martha, died at the Cincinnati Zoo in 1914, the last of a population that once numbered six billion birds.

“Most of what we would imagine to be common backyard birds barely break population estimates of 180 million,” said John Fitzpatrick, the director of the Cornell Lab of Ornithology, a branch of zoology that studies birds. There is no bird alive today that even begins to approach the passenger pigeon’s once overwhelming population size.

It is difficult to imagine that a bird so populous would become extinct in the span of a few decades. For people at the turn of the 20th century, the passenger pigeon seemed an endless resource for food and sport. Hundreds of millions were killed and shipped to cities on the East Coast, where they were sold for pennies on the dollar. At the time, passenger pigeons were the cheapest and most abundant form of protein available.

Today, a century after their extinction, passenger pigeons stand as a warning of the so-called thoughtlessness of man. But it appears that society has failed to heed this warning. In the time since the passenger pigeon’s passing, countless other species have been driven to extinction, in part, by the actions of man. Ornithologists and conservationists alike are now hoping to use this morbid centenary as an opportunity to raise the call for action to further protect threatened species.

Similarly, the death of a species has brought about much discussion on the issue of “de-extinction”—the process of using modern technology to attempt to revive lost species, including the passenger pigeon. It’s a complicated process with a relatively low success rate. For this reason, some scientists, Fitzpatrick included, are of the mindset that de-extinction is impractical and a distraction from more effective conservation efforts.

Carl Zimmer, a science author and Yale University lecturer, feels that the feasibility of de-extinction really depends on the species in question. In April 2014, Zimmer wrote a feature article for National Geographic exploring the possibilities and problems presented by de-extinction. Zimmer explained in a phone interview that he hasn’t seen any noticeable evidence of de-extinction having a harmful effect on conservation efforts.

“[De-extinction] would be relatively easy to do for small species where good biological samples have been preserved,” Zimmer said. His writing has brought him in touch with scientists on the cutting edge of this movement, and he has become well versed in the practicalities of the technique. As far as reviving the passenger pigeon goes, Zimmer believes that the solution is unclear.

We don’t really know that it would be impossible for the species to come back,” Zimmer said. “A lot of forest has been regrown on the East Coast.” Zimmer’s comment refers to oak forests, which were the historic food source and nesting habitat of the passenger pigeon.

Though the passenger pigeons’ numbers were heavily reduced by hunting and trapping, they were also impacted by habitat fragmentation, which began to occur as old-growth forest was cleared for agricultural use. By the end of the passenger pigeon’s existence, much of the old-growth forest that it relied on had been decimated as well.

In the century since, large tracts of forest have been brought back, though they will never exist at the same size they did during the passenger pigeon’s time. If they were to be revived, passenger pigeons may fulfill an ecological role they left behind.

The only thing that would prevent the hypothetical return of the passenger pigeon, according to Zimmer, is climate change.

“We’re creating a fundamentally different environment with climate change,” Zimmer said. “In 100 years [the environment] is not going to be the same.” Zimmer’s fear is one shared by many, including American University Professor Chris Tudge. Tudge holds a doctorate in zoology and has taught courses on birds.

“A lot of our bird species fly to the tropics for the winter and don’t breed down there, but feed down there and then come back up to breed,” Tudge said. “If you shift climactic bands, then there might be cases when [the birds] don’t actually have to leave North America to reach the same sort of climactic band where they were living in the tropics.”

Tudge recognizes that with changing climates, there is a possibility migration patterns would be disrupted. If a natural cycle begins earlier than it has historically, the birds may arrive after it has already ended. Without the fuel to continue their migration, birds die and populations collapse. It’s difficult to say whether our current climate would support the migration patterns of an extinct species such as the passenger pigeon, but Tudge believes their successful reintroduction would be unlikely.

Though we may have lost the passenger pigeon, there is hope for species that were similarly abundant but are now rare. Fish species, such as Atlantic cod and blue-fin tuna, which have been exploited as food source, are now receiving attention from conservation groups working to avoid a catastrophe on the same scale as the passenger pigeon extinction. If we play our cards right, these species may continue to persist for centuries to come.