The Bicknell's Thrush is a bird that can only live in a few very very restricted places. It spends its summers in dense alpine forests in the Northeast of the US. In the winter, perhaps as many as 90 percent of the birds fly to the Dominican Republic. It's a bird without many options, and that makes it a poster child for what's to come.
“Our bird”—that’s what conservationists in New England call the Bicknell’s Thrush. Why do they love it so much? It’s not a particularly comely bird. It’s almost entirely indistinguishable from the much more common gray-cheeked thrush. It has a nice song, but it’s about as endearing as any other song bird you might notice in the woods. What gets the Bicknell’s thrush its the moniker is simply that you can’t find it anywhere else.
“They’ve pigeon-holed themselves into a pretty narrow ecological niche,” says Chris Rimmer, director of executive director of the Vermont Center for Ecostudies, a little research and conservation outfit that has taken up the challenge of trying to study and conserve the thrush.
That ecological niche is so small it’s almost comical. In the summer, the birds stick to “thick stands of stunted conifers on steep mountain slopes or near tree-line” according to the researcher that successfully argued the bird should be considered its own species, back in 1993. That means we’re talking about a handful of weather-beaten, high mountain peaks in the northeast of the U.S. and Southern Quebec. In the winter, the birds fly south, and nearly all of them head to the same place. Rimmer says that somewhere around 90 percent of Bicknell’s thrushes spend the winter in wet forests in the interior of the Dominican Republic.
In other words, the Bicknell’s thrush is a specialist: on both ends of its range, it lives only in a very narrow band of habitats. They don’t seem to know how to live anywhere else. “If these habitats disappear from our mountain tops,” explains Rimmer, “I don’t think the birds are going to just find a different place to go.”
Consider, now, another bird, one nobody seems to call “our bird,” though it has its aficionados: The turkey vulture.
“I think turkey vultures are just about a perfect creature,” says Katie Fallon, author of Vulture: The Private Life of an Unloved Bird. “They breed from south central Canada, throughout most of North America, Central America, and all of South America. They’re even on islands... Caribbean Islands... the Falkland Islands. They’re a bird that can be seen by almost everyone in the hemisphere.”
Turkey vultures aren’t picky. They will nest in dark crevices, abandoned buildings, the nests of other birds, mammal burrows, and even quiet spots on the forest floor, if nothing else is available. They are also shockingly efficient. When soaring, their heart-rate is nearly the same as when they are sleeping, which has even led some to suggest turkey vultures might actually take quick naps while flying. This is just one of the many delightful facts about these birds—my personal favorite is that their stomachs are acidic enough that it can neutralize cholera, botulism and anthrax.
When you add these various evolutionary talents up, you get an animal that is poised for success in virtually any habitat; basically, you have a generalist on your hands.
The world is made up of many species, and any one of them will loosely either fit the profile of a generalist or a specialist. That has always been true. What is newly true is that species are disappearing at an alarming rate, and many scientists believe we’re seeing the beginnings of something that will eventually be recognized as a mass extinction event.
And the problem is that these extinctions are not distributed equally. They’re coming for the specialists first.
“There’s really a striking common pattern that specialist species are declining everywhere,” explains Romaine Julliard, a researcher with the National Museum for Natural History in Paris, who co-authored a paper on the subject with the striking sub-title: toward a global functional homogenization? He say he found the decline “in coral fish, marsupials in Australia, and bumblebees in the UK, and some plants.”
But what’s intriguing about the trend is that the decline of specialists is “almost balanced by the increase in population size of generalist species.” Julliard has studied European birds in particular, and he found that while the abundance of specialist birds has declined 20 percent, numbers of generalists has increased by 20 to 25 percent.
We see this in our tale of two birds as well. The Bicknell’s thrush is losing habitat at both ends of its range. The high, coniferous forests are retreating upslope towards oblivion as climate change warms the Northeast, and illegal agriculture has eaten into the national parks that serve as the bird’s refuge in the Dominican Republic. There are estimated to be around 100,000 of the birds in total, and the species is on several lists of birds that the conservation community is concerned about.
The turkey vulture—in contrast with the Bicknell’s thrush—is thriving. Roadkill on our highways has created what amounts to a massive network of turkey vulture smorgasbords, crisscrossing the nation. Because the black asphalt absorbs and re-radiates heat during the day, these serpentine buffets also act as a ready source of thermal updrafts for the birds to surf along, spreading their ever-growing population to every nook and cranny of the hemisphere. Fallon says that 25 years ago the birds were estimated to number around 5 million, but today that number has risen to nearly 20 million worldwide.
This is the current trajectory we are on: The beautiful finely tuned specialists, hyper-efficient little motors built to extract calories from their own very, very specific habitats, are on the way out. As they vanish, the generalists—admittedly, marvels of flexibility and adaptation in their own right—are ascendant, rising to fill the space that’s left behind.
What’s behind this shift? According to Julliard, to date, it’s just regular old habitat loss. “Even though the climate change footprint on pressure on biodiversity is increasing and the evidence for that is increasing, it’s still likely lower than habitat degradation,” he says. In fact, a paper on extinction risk that was published in the most recent Proceedings of the National Academy of the Sciences came to the same conclusion: Large animals are most at risk from us eating them, small animals are at risk because we are destroying the places they live.
Is there anything wrong with this push toward functional homogenization? We already see the same starlings and house sparrows in almost any city anywhere in the world. Could we get to a future where the skies are full of nothing but turkey vultures, and the oceans are populated entirely by jellyfish? To me that feels like a nightmare scenario—something from Margaret Atwood’s Maddaddam trilogy, except minus the genetic engineering.
Julliard has a reminder for me: “Evolution is really a force that drives to specialization and to differentiation,” he says. Pointing out that just as soon as we stop doing all the things that make life hard on them, the specialists will start to thrive again, and given enough time, speciation of new specialists will start to pick up again.
This reassurance is thanks to one of the tenets of ecological niche theory: In a stable habitat, natural selection favors the specialist. Which means “you need really a very high pressure to maintain this homogenization,” he says.
The problem, of course, is that the time-scales involved are deeply out of whack with our human experience. The world can recover from a whole heck of a lot, but that can take millions of years, and the world we’ll inhabit in the meantime will be a deeply impoverished one in comparison. And more to the point, who knows if we’ll even be around to watch the birds that repopulate that sky, to call them our own.
Outside/In was produced this week by:
Outside/In was produced this week by Sam Evans-Brown with help from: Maureen McMurray, Taylor Quimby, Hannah McCarthy, and Jimmy Gutierrez.
Music from this week’s episode came from Blue Dot Sessions, Poddington Bear, David Szesztay, Jason Leonard and Ikimashoo Aoi.
Our theme music is by Breakmaster Cylinder.
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