The last passenger pigeon, which was once the most common bird species in the U.S., died in 1914. Terence Faircloth, Flickr

Reasons to raise the dead

Scientists are moving ahead with de-extinction, in the hopes bygone animals can aid landscape restoration

When the flocks arrived, people stumbled out of their houses to marvel. Until the mid-19th Century, mind-boggling numbers of passenger pigeons crisscrossed eastern North America, descending on an area perhaps once a decade. What naturalist Alexander Wilson assumed was a tornado was instead the “loud rushing roar” of millions of wingbeats as the passenger pigeons approached. 

At times, landscapes across the region were plunged into darkness for hours, even days, as the vast flocks passed over, and men shooting into the sky could take down a hundred birds with a single bullet. Bird droppings fell like snow. When the pigeons formed breeding colonies – like penguins do – every tree within 250 square kilometers of forest could be crowded with 10 to 50 nests.

“Passenger pigeons were once the most abundant bird on the planet,” says Ben Novak, the lead scientist at U.S. genetic rescue non-profit Revive & Restore. “It’s estimated that they represented one quarter of all living birds in North America, by biomass.” Novak once calculated the mass of a 3-billion-strong flock that passed over Toronto in 1860 was equivalent to that of 180,000 African elephants. Imagine that many pachyderms pushing through a forest all at once, Novak says, and you have some idea of the kind of ecosystem engineers these birds were. 

It would have been chaos. By sheer force of numbers, they would break branches or even entire trees off at the trunk; eat every berry, nut and seed; and then “deposit literal tons” of dung, he says. “There are records of guano at the base of a single tree several feet deep.” 

All this disturbance was good for forests, Novak says. Eastern North American ecosystems evolved with passenger pigeons and came to rely on their occasional visits to create room for new generations of trees, clear out flammable undergrowth and fertilize the soil – as though the birds were storm and wildfire at once. 

Now, those billions of pigeons are gone. Deforestation and an orgy of commercial hunting in the 19th century caused the population to collapse in just a few decades. The very last bird, Martha, died in 1914. 

The species’ loss has had dramatic effects on the forests it once fertilized and the predators (like the peregrine falcon) it once fed, says Novak. Many birds and animals feed and raise young in regenerating forest habitat, and trees like oaks and American chestnuts require disturbances to complete their life cycle. 

Without passenger pigeons, eastern U.S forests are less productive and less diverse, and require expensive fire management – which is the main reason Revive & Restore is trying to bring the species back from the dead. “The big target for recreating passenger pigeons is to create a biological force that restores forest dynamics, so that humans don’t have to keep doing it,” says Novak.

De-extinction is no longer the stuff of science fiction. Several groups of scientists around the world are working on developing genetic technologies they hope will enable them to resurrect new versions of extinct species – which at the moment include the woolly mammoth, the Tasmanian tiger and the passenger pigeon – that could play the same ecological role as their ancestors and contribute to landscape restoration. As of this year, they’ve raised millions of dollars in funding. But not everyone is convinced we can truly bring these animals back – or that we should.

Mammoths may have been hunted to extinction. Wikimedia Commons
Mammoths may have been hunted to extinction. Wikimedia Commons

How to make a mammoth

To make a mammoth, Texas company Colossal Biosciences plans to sequence the species’ genome and identify where it differs from that of its closest living relative, the Asian elephant. Then, the company’s scientists will use CRISPR gene-editing technology to alter the elephant DNA until it resembles the mammoth’s and create an embryo that carries the modified genes. The hybrid mammoth-elephant fetus would need to gestate for nearly two years in an African elephant “surrogate” mother or an artificial uterus (which has not yet been invented).

If and when that baby animal takes its first stumbling steps, it would not be a true mammoth – rather an “Artic elephant,” as Colossal co-founder Ben Lamm calls it, with mammoth traits like small ears, shaggy hair, a domed forehead and curving tusks. 

In August 2022, Colossal announced it will also work with Andrew Pask from the TIGRR Lab at the University of Melbourne on reviving Australia’s thylacine – a dog-sized marsupial predator also known as the Tasmanian tiger. Along the way, they’ll likely invent new reproductive technologies, like artificial pouches, that Pask says will aid marsupial conservation more broadly. Other scientists, however, have called the idea fairytale science, suggesting the thylacine was too evolutionarily distinct to convincingly bring back.

Birds have their own challenges. To recreate the passenger pigeon, Novak aims to start with the band-tailed pigeon, a close relative from the western United States. For various reasons, birds’ eggs themselves can’t be gene-edited. Instead, Novak’s team plans to take band-tailed pigeon primordial germ cells – the cells destined to become sperm and eggs – and insert passenger pigeon genes into the DNA until the genome resembles that of the extinct animal.

When injected into the embryos of a surrogate species (likely the domestic pigeon), the modified cells will migrate to the reproductive organs. In theory, when those genetically altered birds grow up and breed, the chicks that hatch from their eggs will look and behave like passenger pigeons – not their parents.  But while this technique has been used successfully in domestic chickens, so far, no one has pulled it off in wild birds. 

Thylacines in the Washington D.C. National Zoo in 1902. E.J. Keller, Smithsonian Institution
Thylacines in the Washington D.C. National Zoo in 1902. E.J. Keller, Smithsonian Institution

Ecosystem engineers

Assuming they succeed, the scientists say they’re not interested in recreating extinct species just for the glory or so that people can gawk at them in a zoo. Mammoths, thylacines and passenger pigeons were all chosen for resurrection because they played important roles in their ecosystems – and because their environment still exists (more or less) and stands to benefit from the animals’ return. 

“We want [mammoths] back in their natural habitat in the Arctic,” says Lamm, “and we want thousands of them.” Hopefully, they would act like the original mammoths did – consuming and disturbing massive amounts of vegetation, transporting nutrients in their dung, compressing the snow and thus preventing the permafrost from melting, and helping to recreate the mammoth steppe.  

Breeding a thousand almost-mammoths is one thing, but how many almost-passenger pigeons will we need for them to become an ecological force? A million? A billion? Novak doesn’t expect he’ll see the full effect of his efforts in his lifetime. “It’s going to take many, many decades of releasing birds from captivity to try and keep the wild flock growing faster.” But he thinks that a few hundred thousand birds might be enough to begin to create local-scale disturbance. “We’ll get to see the window into the future.”

If Pask and Colossal do manage to re-wild a viable population of almost-thylacines into Tasmania’s forests, it could be “a massive win for conservation,” says Euan Ritchie, an ecologist at Deakin University in Melbourne.  Thylacines were Tasmania’s sole apex predator, he says, and as the re-introduction of wolves into Yellowstone National Park in the U.S. has proven, “the effects of an apex predator being present or absent can actually be quite profound, and manifest in ways that many of us wouldn’t even realize until we start looking.”

Resurrected almost-thylacines could help keep herbivore populations in balance, Ritchie says, and make populations healthier by picking off sick and weak individuals. The predator’s absence is one reason Tasmanian devils might be suffering so much from an infectious face cancer, Pask adds. “The longer a sick animal survives in the population, the more it spreads the disease.” 

Thylacines were once Tasmania's top predators. Fidel Fernando, Unsplash
Thylacines were once Tasmania’s top predators. Fidel Fernando, Unsplash

Avoiding eco-zombies

Still, ecosystems are complex, and their dynamics are still poorly understood. In a 2017 paper, Douglas McCauley from the University of California, Santa Barbara and colleagues warned that de-extinction projects risked manufacturing “eco-zombies” that might look like the lost animals but fail to fulfill their original ecological function. 

And there are plenty of environmental and ethical questions to consider before de-extinct creatures could be let loose. Can a baby mammoth learn to behave like a mammoth without a mammoth mother? Has the microbiome of the thylacine gone extinct with its host, and what role did that play in its behavior? 

Full-blown restoration of the large numbers of animals needed to have an ecological impact would inevitably bring them into contact with people, McCauley notes. A century ago, humans relentlessly hunted passenger pigeons and thylacines. Even now, people frequently come into conflict with animals when they interfere with livelihoods – elephants raiding crops, for example. Australia’s sole remaining apex predator, the dingo, is frequently poisoned, shot, trapped, or fenced out of crucial habitat by farmers, says Ritchie. “I don’t think we’ve done a brilliant job managing that. So the question is, would we do any better for thylacines?” 

In Australia, Bradley Moggridge, an environmental scientist from the University of Canberra and member of the Kamilaroi Aboriginal nation, wants to ensure Indigenous Peoples are consulted before any animals are released on their traditional lands – and that the companies and non-profits working on de-extinction will have enough money long-term to take care of the animals once they’re released.  “A lot of Aboriginal people say, you whitefellas can’t even look after the ones you got now. How are you going to look after a new one?”

Other critics worry about the welfare of the animals engineered in the lab, whether the de-extinct animals will be patented, and that the idea’s romance might rob funding from existing conservation efforts.

Novak is convinced the obstacles are surmountable, at least in the case of the passenger pigeon. Revive & Restore’s pigeon effort is part of its wider Biotechnology for Bird Conservation project, and the reproductive technologies Novak hopes to develop in the process will be used to help endangered but not yet extinct species, too. 

He doubts the passenger pigeons would become an agricultural pest; farmers now use drills to bury their seeds beyond the reach of beaks, and mitigation methods used to protect tree and fruit crops from other birds could be adapted for pigeons, he says.  

Novak also points out that conservationists have been restoring locally extinct populations for decades – like Yellowstone’s wolves – and are even filling ecological niches left by extinct tortoises with related but (non-native) species. Revive & Restore hopes to borrow other techniques developed for seabird conservation to help teach the resurrected passenger pigeons how to behave: building communal nesting sites, for instance, or using speakers to simulate the sounds of giant pigeon flocks.

The idea that it might be possible to bring back a lost wonder of nature is beguiling. But so too should be the conservation of the animals and plants with which we still share the planet. Few species have quite the landscape-scale impact of a billion-strong flock of pigeons, but each plays a role in its ecosystem. Preventing the extinctions of the future must be a priority, even as we try to reverse those of the past.



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