For centuries, people have noticed that crows, ravens, jays, and related birds pay attention to the bodies of their dead, by making alarm calls or recruiting other birds to the scene. It’s easy to interpret these “funerals” in the context of grieving or human attitudes to death. But researchers like Swift and Marzluff think that crows recognize dead individuals as signs of danger, and opportunities to learn about potential threats. In one study, they showed that crows become wary of places where dead crows are found, and will harass humans or hawks who handle the corpses. But if they treat dead individuals as signs of danger, why would they approach one, much less have sex with it?

To find out, Swift needed some dead crows. Fortunately, it’s not hard to get some in Seattle. When local rehabilitation facilities can’t save the birds in their care, they donate the bodies to the local natural-history museum. When private citizens find birds that have crashed into windows or collided with power lines, they do the same. Swift procured dozens of crows from the museum’s freezers, and her colleague Joel Williams stuffed them. Then Swift drove through Seattle and its neighboring cities looking for crow nests. When she found them, she would patiently wait for the owners to leave, before placing a dead bird on the sidewalk. “People often called the police,” she says. “They see someone with binoculars and a camera near their property.”

Over three consecutive summers, Swift tested the responses of hundreds of crows to their dead peers. Most often, she found that the birds made alarm calls from afar, or rapidly dive-bombed the bodies. This fits with the idea that they generally treat dead crows as signs of danger. But in 24 percent of the cases, something overwhelmed those instincts and the birds would touch, pull, drag, or peck at the corpses. And in 4 percent of the cases, these encounters turned sexual.

“In the most dramatic examples, a crow would approach the dead crow while alarm calling, copulate with it, be joined in the sexual frenzy by its presumed mate, and then rip it into absolute shreds,” Swift wrote. “I must have gone through a dozen dead crows over the course of the study, with some specimens only lasting through a single trial.”

These crows were not trying to scavenge from the remains. Cannibalism has rarely been reported in crows, and Swift found that the birds treated dead peers differently than the carcasses of other animals such as squirrels or pigeons. Nor were the crows simply mistaking the corpses for living intruders. Swift found that they treat crows that were taxidermied in lifelike postures differently from those preserved in deathly poses.

It also isn’t the case that crows are so desperate for a mate that they’ll copulate with anything. Sure, necrophilia was more common toward the start of the breeding season, but Swift found that the crows would mount dead birds even when their living mates were nearby. In one memorable instance, a pair of crows saw a dead body and then proceeded to mate with each other.