Most of us are surrounded by wildlife, no matter where we live.

Even in cities, we share our neighborhoods with hordes of pigeons, squirrels, and raccoons. But all of those animals die eventually, right?

So why aren’t we tripping over critter carcasses on a daily basis? Nature, it turns out, is remarkably good at getting rid of the evidence most of the time — although there are some some rare exceptions that can leave us struggling to dispose of thousands of carcasses. The main reason you don’t see many dead animals lying around is that most of them don’t just drop dead on their own.

Unless an animal is at the very top of the food chain, it’s likely that it’ll meet its end at the hands — or rather, claws — of another animal. Pigeons, for example, rarely make it past five years old before being picked off by a predator like a hawk, owl, or stray cat. Since these predators eat what they kill, their victims are usually gone before you have a chance to see them.

Of course, sometimes animals do evade predators for long enough to die in some other way. They could freeze to death in a winter storm, they could starve, or they could catch a lethal disease. But even then, they don’t usually just fall over in the street.

Instead, they seek out a hiding spot when they feel themselves growing weak, crawling into someplace like a duct or a hollow tree where you’re less likely to stumble across their carcasses. And if they do die out in the open, scavengers like vultures, coyotes, and insects are super efficient at finding and eating them, with an assist from decomposers like fungi and bacteria. Nature’s cleanup crew can reduce a carcass to unobtrusive scattered bones and feathers in just a few days.

So basically, it’s not that animals aren’t dying all around you all the time; it’s just that you don’t usually notice. Sometimes, though, a large number of animals die at once -- so many that everyone notices. In spring 2015, for example, a bacterial infection killed off more than half the world’s population of saiga, an antelope species with a funny trunk-like snout.

It swept through herds in Kazakhstan, and more than 200,000 saiga died in less than a month. And in the winter of 2015 and 2016, tens of thousands of seabirds called murres washed up on the west coast of North America after unusually warm water caused the populations of the fish they eat to plummet. Events like this can overwhelm nature’s capacity to dispose of the remains.

People had to bury dead saigas in mass graves dug with bulldozers. And if it seems that things like this are in the news more and more often, you’re not imagining it. A 2015 study published in the journal PNAS found that these events are increasing in frequency, becoming more common for birds, marine invertebrates, and fish.

The researchers followed a pretty strict definition of mass die-off, only looking at events that killed at least a billion animals, 635 million metric tons’ worth of animals, or 90% of a population. The study concluded that the most common causes of mass die-offs include disease, human-caused disturbance, and toxins from things like algae blooms. These events can permanently alter food webs, and they can endanger human activities like farming, too — for example, if there’s a die-off of a crucial pollinator species.

It’s unclear why this sort of thing is on the rise, but the most likely culprits are climate change and increasing environmental degradation. Either way, scientists agree we need to do a better job of studying and tracking mass die-offs. These events are still pretty rare overall, though; it’s not going to suddenly start raining dead birds in your neighborhood tomorrow.

And the next time you do happen to run across a smelly, rotting dead animal, remember: you have predators and scavengers to thank for the fact that this doesn’t happen more often. Vultures may look kind of creepy -- but they’re doing an essential job as nature’s janitorial staff. Thanks for watching this episode of SciShow, which was brought to you by our patrons on Patreon.

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