Why are some crows committing acts of necrophilia?

The setting was romantic enough. Sunny spring day. A cherry tree blossoming a vivid pink. One party, the suitor, was dark, fetching and amorous. But the other party lay there like a corpse. It was, in fact, a corpse.

So began the first documented human observation of a crow copulating with a deceased member of its own species.

In April 2015, Kaeli Swift, a doctoral candidate at the University of Washington who studies crows, was demonstrating one of her experiments for a film crew when she left an expired crow, stuffed by a taxidermist, unattended on the ground. A nearby crow soon swooped down upon the stuffed crow, crouching low, its wings spread wide, and attempted intercourse. The move astonished Swift enough that she spent the next three springs and summers re-creating these conditions and documenting the behavior.

Swift and her co-author, Dr. John Marzluff, detail that field work in a study published Monday in the journal Philosophical Transactions of the Royal Society B. Exposed to their dead, crows may touch, attack, scavenge and attempt to have sex with the body, the authors explain. The study adds a new twist to previous observations that the birds primarily respond to crow cadavers as signs of danger.

The conduct, the researchers speculate, may be the result of hormonal fluctuations that cause some crows to become confused about how to respond to stimuli.

“It was certainly very surprising to me,” Swift said of the newly observed behavior. There is anecdotal evidence that other animals engage in necrophilia, she said. Humans have spotted dolphins, whales and squirrels getting down with their dead. “I never heard of this with crows before, and I had never seen it.”

Past studies by Swift and Marzluff of crow behavior toward corpses involved the presence of a potential mortal threat, whether a human being, a stuffed red tail hawk or both. At the sight of a dead crow near possible predators, the birds swarm in large numbers, emitting a cacophony of caws, but never touch the body. The researchers had called the activity a crow “funeral” and surmised that such gatherings are how the birds learn about and process danger.

But what they witnessed in the spring of 2015 made it clear that there’s more to how crows react to the dead than avoiding harm.

For the new study the researchers removed the predator element from the equation. After identifying the territory of an adult pair of crows in a Seattle neighborhood, they placed a cadaver on a sidewalk or other exposed area and retreated about 65 feet away.

Among the hundreds of crows observed during the study, the majority merely cawed loudly and dive bombed at the corpse without touching it. But 24 percent of the winged subjects pecked, pulled at or dismembered the body. And in 4 percent of the encounters observed, crows attempted copulation. (In one case, a crow mounted a dead pigeon used in the study for comparison.)

Kevin McGowan, a professor at the Cornell Lab of Ornithology, and not involved in the study, said he had never witnessed a sign of necrophilia in more than three decades of observing crows. But he suspects overcharged hormones are largely at play.

“There’s a huge amount of growth and disappearance of bird gonads during the off-breeding season,” McGowan said, referring to the way bird testes may expand or shrink, depending on the time of year.

That affects the hormones circulating in its system and the reaction to the stimuli the crow encounters. “So it’s not surprising that you would see incidental sexual behavior pretty much only in the breeding season.”

The new study did not record whether the individual necrophiliac crows were male or female, but Swift agreed that the time of year had a lot to do with the results. The sexual and aggressive behavior she observed was limited to the first half of the breeding season, generally in mid-June.

“I think the next line of testing would be to do hormonal profiles and see if there are endocrine differences between the birds that engage in this behavior and the birds that don’t,” she said.

For now Swift speculates that the behavior may be a blend of potential responses a crow might have to its dead — seeing corpses as a sign of danger, a source of food or a potential mate. Some crows may be incapable of umpiring their own responses to the stimuli, she suggests. So they engage in them all.