Cats are adorable, and sometimes aloof. Occasionally, they can be allergic to you, and half of them have no reaction whatsoever to catnip. They’re also hunting machines: A recent study estimated that cats kill between 1.4 billion and 3.7 billion birds and between 6.9 billion and 20.7 billion small mammals a year in the United States alone. And, as any owner of an indoor-outdoor cat knows, some of these dead (or nearly dead) animals end up on our doorsteps uneaten. Why does your cat kill something if he's not intending to eat it? And why is he leaving that dead mouse for you?

No, it's not because he's a vicious murderer. It all comes down to a cat’s natural predatory instinct. “Cats are hunters by nature,” says Dr. Stephanie Liff, a veterinarian at Brooklyn Cares Veterinary Clinic in Brooklyn, New York. “They will go out and kill their prey and then bring it home for the rest of the ‘pack’ for sustenance, and maybe to boast—but that is really anthropomorphic and probably not a real explanation.” Even if your domestic cat has no need to hunt because you’re providing food for him, he’ll still display that behavior (just look at how he stalks his toys!).

Typically, cats learn how to hunt by watching their mothers. To teach her kittens how it's done, a mother cat will bring back dead animals, and then, later, she’ll bring home injured prey that her kittens can kill on their own. Eventually, when her kittens are big enough, she’ll take them out to hunt with her.

But even without maternal interaction—in cases where the mother cat dies, or abandons her kittens—or when domesticated cats are spayed and don’t have kittens to care for, “these cats will still hunt and are very likely to bring the prey home to their human family,” Liff says. The behavior is hardwired into their systems.

So the next time you find a dead or dying mouse in your house, try to look at it this way: Your cat is trying to take care of you by providing you food and teaching you how to hunt. Isn’t that sweet? And if you find the grisly treats disgusting, the solution is simple: Keep your cats inside. The birds will thank you!