The topic for today’s episode was voted on by our brainiacs at patreon.com/YBOF, though I almost ignored the poll results because someone in the comments thought I couldn’t make an episode out of the other topic, so you can bet your sweet bippy I’ll do that soon. Today’s topic was meant to be notable extinctions, but research is as research does, so it ended up being an all-bird episode, so we go.
Although extinction is a natural phenomenon, it occurs at a natural “background” rate of about one to five species per year. Scientists estimate we’re now losing species at 1,000 to 10,000 times the background rate, with literally dozens going extinct every day through habitat destruction, exploitation, and climate change. Sometimes, though, we really excel ourselves. Take for example the passenger pigeon. Possibly the most abundant bird on earth, early nineteenth century estimates put their population at somewhere between three and five billion with a B individuals, about a third of today’s North American bird population. Records in the 1830’s tell of passing flocks that darkened the skies for several days at a time. They nested in trees in such great numbers that their weight would break off branches. There were contests to shoot as many of them as possible during a certain period of time, with one winner shooting 30,000 birds. By 1900, there were none left in the wild. The last captive specimen, a pigeon called Martha in the Cincinnati Zoo, died in 1941.
The history of the passenger pigeon can teach us a lot about how and why species become extinct. Native Americans also relied on passenger pigeons for food, but by and large had learned to harvest the species to a sustainable level. It was common in some parts of North America to only eat young pigeons that were hunted at night, since this did not seem to scare away the adult birds or prevent them from re-nesting. But starting around 1500, a more aggressive variant of humans came to the continent. Were Europeans to blame for the extinction? Not entirely (though still mostly).
A 2014 study published in the scientific journal PNAS strongly suggested that humans were simply the final straw in destroying a species that was already vulnerable and headed to oblivion. The researchers asserted that despite their enormous numbers, the passenger pigeons, who population figures could vary widely, were already in trouble.
Studies of the genetic variation of the species using an investigative method called PSMC formed the background for these theory. The PSMC, or Pairwise Sequentially Markovian Coalescent, method can use the information in the genes of a single individual of a species to map the history of the species. You should therefore be able to see how the species developed over many generations, and estimate how many individuals there were at any given time, all based on a single genome. Using this method, researchers found that the number of passenger pigeons was in free fall even before the arrival of the Europeans. Although the species might not have gone extinct, left alone, it would have shrunk dramatically, maybe to only a few hundred thousand individuals.
It sounds almost too good to be true that you can come up with something so definitive based on information from just one or a few individuals. And in this case it is — at least if we believe a new study published in the journal Science. That study claims the PSMC method can’t be used on passenger pigeons. Their research provides completely different results. PSMC is based on the assumption that genetic variations occur relatively evenly all along the chromosomes that constitute the genome. In passenger pigeons, most of the genetic diversity was found at the *ends of the chromosome. The middle of the chromosome showed little variation from one generation to the next as a result of the selection on these genes.
The researchers behind the article in Science didn’t use the PSMC method, but instead used mitochondrial DNA from 41 passenger pigeons as their starting point. Science nerds and true crime buffs will know that mitochondrial is passed down from your mother, from her mother, and so on and does not contain DNA from your father. Variations in mitochondrial DNA also occur due to mutations, and happen relatively consistently over time. The study in Science analysed the entire genomes from four passenger pigeons and compared them with two genomes from band-tailed pigeons, one of their closest relatives. The final result was that the new study ended up with completely different answers about the passenger pigeons and why the species met its demise.
Scientists previously believed that the larger the population of a species is, the more genetically diverse it will be, but this proved not to be true with passenger pigeons. According to the article in Science, the large population size appears to have enabled passenger pigeons to adapt and evolve more quickly. The fact that beneficial mutations became incredibly dominant so quickly simply led to the disappearance of other genetic variants. This in turn led to the genetic diversity in the passenger pigeon being surprisingly low in relation to their numbers. This may have made the species more vulnerable to changes. The large grasshopper Melanoplus spretus from the western United States suffered the same fate. It went from a population of several trillion to zero in a few decades, possibly because farmers destroyed its breeding grounds.
While a lack of genetic diversity made the passenger pigeon susceptible to change, it was still ultimately humans that did it in. People ate passenger pigeons in huge amounts, but they were also killed because they were perceived as a threat to agriculture. As Europeans migrated across North America, they thinned out and eliminated the large oak forests that the pigeons depended on for their primary food, acorns. The advent of the locomotive was a boon to the commercial pigeon hunter, because it meant great barrels of the little birds could be loaded up and sent to other cities. The pigeons were probably dependent on a large flock size to reproduce. Their instincts didn’t work when only a few individuals remained here and there. As the species was already dying out, 250,000 birds, the last big flock, were shot on a single day in 1896. That same year, the last passenger pigeon was observed in Louisiana. It was also shot.
Shooting the last of something is sadly not rare in the annals of animals that aren’t here anymore. The culprit here is less the hunter than it is his boss, a private collector or even a scientific institution. History is littered with the stuffed and mounted carcasses of animals that were the last of their kind, bagged by overzealous collectors who didn’t stop to consider the cost of the kill. In collecting’s heyday, bagging a rare species was a point of pride for naturalists, and wealthy wildlife lovers amassed taxidermied animals the way another person might accumulate art. Famous scientists like Charles Darwin and Alfred Russel Wallace collected and preserved hundreds, thousands, even tens of thousands of specimens — most of which served a vital role in making new species known to science. But collectors, who traveled to the world’s most remote regions in search of as-yet-unknown animals, also had an Indiana Jones-like swagger. Competition to find something first was fierce, and institutions vying for new and exotic specimens meant that dozens of researchers would go tramping up mountains and into jungles to kill the same animal.
Among the most famous victims of this is the great auk, a now-extinct North Atlantic bird with a penguin’s tuxedo-like plumage and ungainly waddle. It’s population had been decimated by demand for its down feathers. The species was already teetering on the brink when naturalists and museums took an interest in it in the 19th century. Climate change during the Northern Hemisphere’s several-century cool spell known as the “Little Ice Age” had decimated the population. Humans then finished the job. The birds stood nearly three feet tall and sported thick, plumage, making them a valuable food source and even more valuable commercial product. Adding insult to injury, its clumsiness on land (and inability to fly) made it an easy target for hunters.
Paradoxically, it was the great auk’s sudden rarity that made scientists so eager to kill them. According to the Smithsonian, the great auk’s classification as endangered in 1775 led to increased demand for specimens — a single bird could be sold for $16 in the early 1800s, a full year’s wages. No longer hunted for its meat and down, the great auk and its eggs became a target for their scientific value. On July 3, 1844, a group of fishermen caught two of the birds on a remote island off the Icelandic coast. The fishermen strangled the birds to kill them with minimal damage to the body and crushed the egg they were brooding. They didn’t even get a meal out of it, which you could at least respect a little. The birds’ carcasses were sold to a chemist in Reykjavik, who stuffed and mounted the birds, then preserved their eyes and internal organs like pickles in jars of alcohol. No one on record has seen a great auk since.
The great auk was actually the logo for something I did in elementary school called the Knowledge Master Open, a kind of battle of the brains tournament. We were eliminated in the first round, dead as a dodo. The dodo, of course, being the poster child for extinction, probably because we effectively lost the species twice. (How’s that for a segue?)
Most people are familiar with the sad story of the dodo. This plump, flightless bird was so tasty and so tame that it was hunted to extinction within a century by Dutch sailors arriving on the island of Mauritius in the Indian Ocean. Fewer people realise that this story is mostly false. Were these flightless birds tasty? Probably not, since the waste pits from the early Mauritian settlements are full of animal bones from the Dutch dinner table, but there is not a single dodo bone amongst them. Dodos also weren’t as plump as you see in illustrations. The pictures our pictures are based on were probably made of overfed captive birds or poorly-taxidermied specimens. In the wild, the dodo was a much leaner bird. Were they hunted to extinction? Unlikely – Mauritius was blanketed in thick impenetrable rainforest, and dodos deep in the heartland would have been well beyond the reach of even committed hunters.
How can such an icon of human-induced extinction be so misunderstood? The answer lies in the shameful way the dodo was treated *after the last bird died about 350 years ago. “We have this continuous series of tragedies, forgetting the dodo over and over again,” says Leon Claessens at the College of the Holy Cross in Worcester, Massachusetts. But perhaps no longer. Because of the work of Claessens and his colleagues – including Julian Hume at London’s Natural History Museum and Kenneth Rijsdijk of the University of Amsterdam – science is finally giving the dodo the attention it deserves.
It is difficult to trace that evolutionary process due to the island’s acidic soil and humid tropical climate, which make for an unforgiving environment for fossils. All we can say for sure is that the dodo evolved at some point in the last 8 million years – simply because it was 8 million years ago that Mauritius, a volcanic island rather like Hawaii, first rose above the waves.
Of course, the dodo’s extinction is sadly easier to pin down than its origins. Dutch sailors probably first encountered the bird in 1598, but the sailors themselves did not make much of a contribution to the dodo’s extinction, says Claessens. “At most there were a few hundred people living in a coastal settlement.” The problem was more likely the ship rats and other animals they brought with them, which spread across the island, eating dodo eggs and outcompeting the birds for food.
The last confirmed sightings came in the 1660s. The living dodo was lost forever, but specimens of the strange bird had already been sent to Europe for scientific study. In several museums and university collections, skeletons and stuffed dodos survived. Unfortunately, Europe’s 17th-Century scientists did not realise quite how valuable their dodo specimens were. The problem was that the dodo had disappeared at the wrong time. Its extinction came long before scientists were willing to accept that species really could vanish forever. The great French palaeontologist Georges Cuvier is widely credited with alerting the scientific world to the reality of extinction, but he did not do so until 1796. This meant that 17th and 18th Century museum curators felt confident that there were more dodos out there to replace any specimens that became damaged. Specimen damage or loss was common, especially at a time when taxidermy was in its infancy and museum records were relatively crude.
There used to be a complete dodo in Oxford, but they had to discard the majority of the specimen in the 1700s. They kept just the head and a foot. The British Museum also had a dodo foot, but they plum lost track of it about a century ago. There is also a dodo skull in Copenhagen and part of a beak in Prague. That’s about it.
The dodo might have fallen into obscurity forever, if not for the work of two Victorian researchers, Hugh Edwin Strickland and Alexander Gordon Melville. In 1848 they published a monograph, The Dodo and its Kindred. Inadvertently, Strickland and Melville had kick-started a wave of dodo-mania. This arguably reached a peak when the bird featured in Lewis Carroll’s Alice’s Adventures in Wonderland, which I would argue did more to help us remember the dodo than any scientific work.
With all the advances in scientific processes and thinking, we’ve surely moved past hunting specimens for display and study, right? It’s sweet that you think that. In 2015, Christopher Filardi of the American Museum of Natural History scoured the highlands of Guadalcanal in the Solomon Islands for a bird he’d been searching for for two decades: the moustached kingfisher.
“Described by a single female specimen in the 1920s, two more females brought to collectors by local hunters in the early 1950s, and only glimpsed in the wild once,” he wrote. “Scientists have never observed a male. Its voice and habits are poorly known. Given its history of eluding detection, realistic hopes of finding the bird were slim.” But he did it. After setting nets across the forest, he and his team secured a male specimen with a “magnificent all-blue back” and a bright orange face, at which point his team collected it.
Of course, ‘collect’ means killed. While this wasn’t trophy hunting, outrage ensued. The controversy led the Audubon Society, which had previously published a piece innocently titled Moustached Kingfisher Photographed for First Time, to add an editor’s note: “This story has been updated to clarify that the bird was euthanized and the specimen collected.” A researcher on Filardi’s team, it added, “told Audubon that they assessed the state of the population and the state of the habitat, and concluded it was substantial and healthy enough that taking the specimen – the only male ever observed by science – would not affect the population’s success”.
Filardi was also compelled to write an op-ed for Audubon: Why I Collected a Moustached Kingfisher. “I have spent time in remote, and not so remote, forests of the Solomon Islands across nearly 20 years,” he wrote. “I have watched whole populations of birds decline and disappear in the wake of poorly managed logging operations and, more recently mining. On this trip, the real discovery was not finding an individual Moustached Kingfisher, but discovering that the world this species inhabits is still thriving in a rich and timeless way.” Filardi stressed that, among Guadalcanal locals, the bird is known to be “unremarkably common”. He explained how he and his team made the decision – “neither an easy decision nor one made in the spur of the moment” – to collect the bird with reference to “standard practice for field biologists”. And he said that killing one kingfisher might help save them all.
The mustached kingfisher is a fabulously brightly-colored bird. I’ll put a link in the show notes. If you podcast app doesn’t support html, at me on social media. Social media is also a great way to help your favorite podcast by sharing the show with your friends and followers.
Some extinctions feature of middleman between the humans and the wild animals. Another thing we’re quite fond of is messing up ecosystems by introducing new animals. Sometimes it’s deliberate, like introducing cane toads and rabbits to Australia, but other times, it’s more incidental, like pet cats going out and doing what cats do. Cats, be they pets or feral, have been responsible for at least 63 extinctions of mammals, birds, and reptiles in the past 5 centuries. One extinction is even blamed on a particular cat, a lighthouse keeper’s companion named Tibbles.
David Lyall was the assistant lighthouse keeper of the newly opened lighthouse on Stephens Island, an island of .5 sq. mi./1.3 sq. km near New Zealand, arriving in 1894 with sixteen other people to man the outpost and of course his cat. Lyall was also a naturalist, so he looked forward to being on the largely unexplored and uninhabited island. The term “naturalist” must have been a low bar back in the day, because he had no inkling of how much havoc introducing even a single predator could do, and cats are fantastic predators. In the book Cat Wars, authors Peter Marra and Chris Santella estimate that outdoor cats kill around 2.4 billion birds in the United States alone. A large number of these victims are killed not for food, but for the sheer fun of the hunt. Debate still rages as to whether the dead animals they bring home are their contribution to feeding the household or a sign of pity on their poor, stupid human servants.
Shortly after Lyall got settled on Stephens Island, Tibbles started bringing him such presents. He recognized most of the species dangling from the cat’s mouth, except one. This bird was small, olive on the back, pale on the breast, with body feathers edged with brown. It had a narrow whitish yellow streak above the eye, short wings, and a long, straight bill. Lyall collected a few and sent them to renowned ornithologists of the time. It was recognized as a new species and given the the scientific name Traversia lyalli.
While that’s happening, the cat population in Stephens Island grew and they began to kill the birds in alarming numbers. In Tibbles defense, she wasn’t the only cat on the island, and cats weren’t the only invasive species, but she’s the only one named in this ecological destruction. The wrens were easy pickings for the cats because they were flightless and could only run low on the ground or hop from branch to branch. There was no keel on the breast bone to anchor the flight muscles, the wings were very short and rounded and the feathers were loose, so the flight feathers were not airtight. Of the 4000 species of songbird, only five were flightless and all but one of them were species of New Zealand wrens.
The bird was originally found all over New Zealand, but predators like the Polynesian rat killed off nearly the entire population. The birds probably migrated to this isolated island during the last glaciation when it was connected to the mainland. When the sea level rose, Stephens Island became an isolated safe haven with no natural predators, until David Lyall arrived with his cat.
By February 1895, less than a year since Tibbles brought the first specimen, the wren had become impossible to be found. Their bones could sometimes be found in the horked-up pellets of the laughing owl, who were themselves decimated by cats. Lyall wrote to a prominent ornithologist, “the cats have become wild and are making sad havoc among all the birds.” Lyall’s successor as lighthouse keeper four years later made serious business of hunting the feral cat population. It would actually take 26 years to eradicate them, as anyone who’s ever tried to catch a feral cat will attest.
And that’s where we run out of ideas, at least for today. Rather than leave one more fact, I want to close the show with a plug. Hours after recording this episode, I’m picking up my niece to begin filming Science With Savannah, Age 7, a weekly podcast and YouTube show exploring zoology, physics, chemistry, you name it. Savannah’s a natural to have her own show — she has all my know-it-all-ism, but she’s still cute enough to get away with it. Thanks for spending…