It is often said that the
passenger pigeon, once
among the most abundant birds in North America, traveled in flocks so
enormous that they darkened the skies for hours as they passed. The idea
that the bird, which numbered in the billions, might disappear seemed
as absurd as losing the cockroach. And yet hunting and habitat
destruction pushed the animal to extinction. Martha, the
last known passenger pigeon, died in 1914 at the Cincinnati Zoo.
Plans are afoot to bring back the bird by using a weird-science process called de-extinction. The work is being
spearheaded by Ben J. Novak, a young biologist who is backed by some big names, including the Harvard geneticist
George Church. The idea was recently promoted at a
TEDx
meeting in Washington and is being funded by Revive and Restore, a
group dedicated to the de-extinction of recently lost species. (Other
candidates include
the woolly mammoth and the dodo.)
Novak’s idea takes a page from “
Jurassic Park,”
in which dinosaur DNA was filled in with corresponding fragments from
living amphibians, birds and reptiles. Working with Church’s lab and
Beth Shapiro,
an evolutionary biologist at the University of California at Santa
Cruz, Novak plans to use passenger pigeon DNA taken from museum
specimens and fill in the blanks with fragments from the band-tailed
pigeon. This reconstituted genome would be inserted into a band-tailed
pigeon stem cell, which would transform into a germ cell, the precursor
of egg and sperm. The scientists would inject these germ cells into
developing band-tailed pigeons. As those birds mate, their eventual
offspring would express the passenger pigeon genes, coming as close to
being passenger pigeons as the available genetic material allows.
The
process is not the same as cloning. Novak’s approach would use a
mishmash of genes recovered from different passenger pigeons, resulting
in birds as unique as any from the original flocks. Most pigeons mature
and reproduce quickly enough that the de-extinction process could be
completed in less than a year. Producing a flock large enough to release
into the wild would take at least another decade.
Novak says he
is confident the procedure will work. “Essentially, the genomes of the
band-tailed pigeon and the passenger pigeon, I think, will prove to be
similar enough to easily convert one to the other,” he said.
In fact, he says, “making the passenger pigeon genome right now will be easier than making the first living passenger pigeon hatch from an egg.”
Experts say there is little question that re-creating the pigeon is technically possible. Indeed, the
genome of the woolly mammoth
has largely been sequenced using elephant DNA as a scaffolding.
Complete, working genomes of dogs, sheep, horses, cows and other species
have been artificially inserted into egg cells to produce living
organisms.
But the project still faces many challenges, among them the contamination of much of the DNA specimen.
The
hundreds of passenger pigeons in museum collections have been exposed
to heat and oxygen. Specialized equipment would be used to identify the
surviving fragments of DNA and reassemble them into working genes. It’s a
painstaking process that could take years.
But the larger
problem, say some scientists, is that even if the passenger pigeon is
re-created, it’s unlikely to be viable as a species in today’s
ecosystem. Novak’s plan is to breed the first new generations of the
bird in captivity. But eventually he hopes to release the animal into
the wild.
Such a proposition, some experts say, poses a number of
fundamental problems: There is some question as to whether today’s
forests can support a restored passenger pigeon population, and its
nesting behaviors make the bird particularly susceptible to dying out
again.
“Much of their breeding and wintering habitat is gone,”
says Scott C. Yaich of the conservation group Ducks Unlimited, and the
animal’s primary breeding-season food — beech mast, the nuts of a beech
tree — is limited.
Altered landscape
The birds “simply couldn’t be restored to a landscape that is so
radically altered from the one to which they were uniquely adapted,”
says Yaich, director of conservation for Ducks Unlimited.
But
Mark Twery,
a research forester at the U.S. Forest Service, says that though beech
bark disease has reduced beechnut production, “the overall quantity of
forested habitat is likely to be ample to support a large enough number
of pigeons for a viable population, even should people be able to
restore the species.”
Other experts say that given the nesting
behavior of the passenger pigeon, releasing a handful of birds into the
wild would be a losing proposition.
The mainstream view of
passenger pigeon ecology is that they used a reproductive strategy
called predator satiation. The recent cicada invasion is one example of
this strategy. Each cicada is individually easy to catch in its slow,
bumbling flight. But there are so many millions of cicadas in a spot at
one time that they are able to finish mating and laying eggs before
predators have had time to eat all of them. If only a few thousand
cicadas emerged at once, then most of them would probably be eaten
before they were able to reproduce. In this way, the cicada’s survival
depends on showing up in hordes.
Flimsy nests
Passenger pigeons succeeded through a similar sort of mob rule.
Individually, their behavior was borderline reckless. They built flimsy
nests, often dangerously low to the ground. The nests were built so
hastily that when bad weather would slow down construction, a female
would sometimes be forced to lay her eggs on the ground. When the young
were ready to leave the nest — after only 14 days of development — they
would spend their first few days on the ground, vulnerable to any
hungry predator.
Passenger pigeons could get away with such
behavior because of their incredible numbers. When a flock arrived at a
nesting area, predators could gorge themselves for weeks. Each pair of
nesting pigeons would produce two eggs, at least one of which usually
ended up on the ground. But even with the constant work of foxes, bears,
possums, raccoons, hawks, eagles, snakes and other meat-eaters, enough
of the young pigeons survived to fly away.
This system works great with a flock of 5 million birds. But according to
Kirk Mantay, a biologist specializing in habitat restoration, if only a few thousand pigeons show up, the whole system falls apart.
“If
you put 5,000 out there, even with good habitat, they could all still
be gone in a few decades unless you could exclude the predators somehow
and make sure that they nested right where you wanted them to go. You
just couldn’t make enough birds for it to work.”
A handful of
nests and fledglings might escape the notice of predators, but as soon
as the colony grew to a few dozen nests, the noise and scent would bring
those predators in to feast on easy meals. You would need to skip ahead
to millions of birds for the predator satiation strategy to properly
work.
Still, “I believe the passenger pigeon will survive because we have people committed to its survival,” Novak says, citing the
reintroducton of the condor into the wild in California. In that case, the birds, on the verge of extinction, were bred in captivity, then gradually released
beginning in the 1990s; there are now about 200 living in the wild.
Would
a commitment to its survival be enough to sustain the passenger pigeon?
A few specimens living in an aviary would be a historic accomplishment.
But an effort to put the passenger pigeon back into the wild would be
challenging at best.
“Habitat restoration is hard to get right
for species like turkey and quail that we know about,” says Mantay. “How
long is that going to take with something we can’t study in the wild
first?”
There may be other species that could be resurrected,
animals that can survive in smaller numbers with less habitat. The
Carolina parakeet might have a chance, with federal protection. The
woolly mammoth could do very well in a herd of a few dozen within a
large park, living at least as wild as bison in Yellowstone. As for the
passenger pigeon, science may permit us to mourn it all over again.
Landers is the author of “The Beginner’s Guide to Hunting Deer for Food” and “Eating Aliens.”
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