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The passenger pigeon was once the most abundant bird in North America - maybe even the world. In the first half of the 19th century, estimates put the number between three and five billion, which would have been at least one-third of the total bird population of North America today.

But while flocks could block out the sky in 1833, by 1900 there were no wild birds to be found. And by 1914 there were none alive even in zoos. So what happened to make the population go from billions to zero in less than a century?

To understand how passenger pigeons died, we need to understand how they lived. Because it was kind of our fault.

Passenger pigeons were native to eastern North America and migrated between the regions around the Great Lakes in the north and the Gulf of Mexico in the south. They traveled in huge flocks with hundreds of thousands of pals.

When roosting or nesting, it wasn’t uncommon for so many birds to perch on a tree that the branches would snap under their combined weight. Sometimes birds would even perch on top of each other just for a place to sleep. Real cozy.

This strength in numbers was their main defense against predators like wolves, foxes, weasels, and hawks. The local carnivores could eat themselves sick and still not make a dent in the flock. Ecologists call this predator satiation, and it’s a pretty common defense tactic in both
plants and animals.

It’s why groups of oak trees produce almost no acorns for two years, then on the third year it’s an acorn party. It’s also why cicadas reproduce periodically, why salmon clog streams during their breeding
season, and why caribou travel in massive herds.

Now, passenger pigeons worked together to raise their young and to find food like nuts, seeds, and berries. So when humans started cutting down forests to plant crops, pigeons went after those grains too. And, as you might guess, that made them pretty unpopular with farmers.

But, really, everything changed when we started hunting the pigeons as a commercial food source. Because they were apparently super tasty. The expansion of train networks made it possible for hunters to kill and ship the birds nationwide. And it wasn’t hard. Because they traveled in dense, low-flying flocks, hunters could basically just swing a stick and hit a couple birds.

To satisfy the demand, hunters started using baited traps and controlled fires to kill more birds in one go. They even burned sulfur below pigeon nests to suffocate them, collecting the bodies as they fell from the trees.

The pigeons were apparently so tasty that no one questioned whether eating birds killed by fire and smoke was a good idea or not. Even as late as the 1870s, people thought there were so many pigeons that it wasn’t possible to hunt them too much.

But once their flock numbers dipped too far, the situation got dire. By 1900, the only surviving birds lived in zoos across the Midwest. And on September 1st, 1914, the last captive passenger pigeon, named Martha after Martha Washington, died at the age of 29.

Looking back, scientists have tried to figure out why the passenger pigeon couldn’t survive in smaller flocks — like, maybe they couldn’t defend themselves or breed successfully. In 2014, a group of scientists published a paper suggesting that passenger pigeon populations
likely fluctuated even before modern humans arrived.

In fact, they may have been in a natural decline that they had recovered from in past cycles. But then humans came in, messed around, and ended up being the last nail in the coffin. These scientists used ecological models that generally say that bigger populations mean
larger effective population size — which is a measure of how many individuals are making babies and contributing to the total gene pool.

And the neutral theory of molecular evolution suggests that the effective population size should be linked to DNA variations called neutral mutations, which don’t really affect survival. Basically, more breeding adults means more neutral mutations randomly cropping up. So, theoretically, large populations should result in high genetic diversity from bird to bird.

But — according to their DNA analyses — genetic variation between passenger pigeons was low, despite how many there were. Based on that fact, the researchers estimated that the pigeon population dipped down in cycles, which could explain why the billions of birds seen in the 1800s seemed like not-so-distant relatives.

This idea was seemingly supported by pollen records from the last 20,000-odd years suggesting that acorn production varied enough that huge bird populations couldn’t always survive. However, a 2017 study suggested that the lack of genetic diversity came from rapid evolutionary adaptation instead.

These scientists pointed out that there are plenty of populations whose size doesn’t correlate with genetic diversity. And this might have to do with how new mutations can spread. When organisms make sperm and eggs, there’s a mix-and-match process where DNA gets swapped
around called recombination. This makes it so every offspring has a unique set of genes.

But recombination involves swapping big chunks of DNA. So as an advantageous gene spreads like wildfire, whole stretches of DNA spread. That could explain why there doesn’t seem to be a lot of genetic diversity in some big populations — like passenger pigeons — despite what the neutral theory suggests.

So these researchers think that the pigeon population was always big, and that their huge drop-off in their numbers probably wasn’t normal. So it was largely our fault. It’s a gloomy thought that we caused the passenger pigeon extinction, but it did inspire the first wide-scale laws aimed at species conservation.

In the 1890s, there was almost no regulation on hunting or killing any kind of wildlife. But in 1900, this looming extinction inspired a federal law prohibiting interstate shipping of illegally hunted game. 13 years later, the Weeks-McLean Act added a ban on hunting migratory birds in the spring, when many species are breeding. It also banned imports of wild bird feathers used for fashion.

Today, we have many such laws protecting wildlife and we keep a much closer eye on endangered species — so we can hopefully prevent more stories like the passenger pigeon.

Thanks for watching this episode of SciShow, which is produced by Complexly — a group of people who believe the more we understand about the world, the better we are at being humans. Even if humans aren’t always so great.

If you want to learn more about all kinds of animals that we still share the planet with, check out Animal Wonders at youtube.com/animalwondersmontana!

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