Some of them, on occasion, go a bit further than that: They have sex with the corpses.

Crows are not alone in this peculiar predilection. Scientists have witnessed isolated examples of different types of animals — from ducks to dolphins — trying to copulate with deceased members of their own species. But scientists couldn't say how common that behavior is among species, which makes it difficult to explain why animals were doing it. [The 12 Weirdest Animal Discoveries]

However, a couple of researchers who study American crows (Corvus brachyrhynchos) now have some answers. They conducted the first-ever study to observe and document the practice of corpse copulation in crows — or in any land animals with backbones, for that matter. The goal? To determine how frequently it happens and to better understand what it means, lead study author Kaeli Swift told Live Science.

"Until now, it's not something that we've ever explored in a systematic way," Swift said.

Caws for alarm

Plenty of studies have documented crows' intelligence, from their puzzle-solving prowess and tool use to their ability to remember the faces of humans that threaten them. Other research has highlighted aspects of crows' social behavior, finding that groups of crows notice and react to the sight of their dead.

Swift, a doctoral candidate in the School of Environmental and Forest Sciences at the University of Washington (UW), was documenting a crow "funeral" in 2015, when she first observed some unusual sexual activity, Swift wrote in a blog post. At the time, she and John Marzluff, a UW professor of wildlife science and co-author of the new study, were investigating the birds' organized vocal responses to finding a dead crow, which signals a potential threat to the living, Swift told Live Science.

And they saw something they had never observed before: A crow approached the corpse, mounted it and started "thrashing" in a manner that was immediately recognizable, Swift wrote on her blog.

Crows that had sex with dead crows often showed aggression toward the corpses. (Image credit: Courtesy of Kaeli Swift)

Learning from the dead

In previous research into how crows gather and communicate around their dead, Swift and Marzluff found that the birds used dead crows to learn about and avoid potential risks. This made their discovery of the new crow behavior — having sex with the dead — extremely puzzling, Swift said. If a dead crow is a danger signal, why would a living crow want to get close to it?

"Engaging so closely with a dead conspecific [animal of the same species] could expose you to disease, or parasites, or scavengers," Swift said.

For the new study, the researchers conducted a series of experiments in four Washington cities, testing 308 mated pairs of wild crows. They exposed the birds to carefully positioned taxidermic crows — and to other prepared animal corpses, such as pigeons and squirrels — to see if the crows' responses were common to a range of dead things or if they were specific to their own species.

While crows may gather and call at the sight of a dead crow, they don't usually get close to it. (Image credit: Courtesy of Kaeli Swift)

They found that the birds were more likely to caw in alarm when the corpse that they saw belonged to a crow, particularly if the stuffed crow was in a "dead" pose rather than a more lifelike posture. The birds approached dead crows about 25 percent of the time, but only 4 percent initiated sexual activity, hinting that corpse canoodling is not commonly practiced, the study authors reported.

"Clearly, most birds are not engaging in this behavior, and that suggests that there's probably some cost associated with it that makes it undesirable," Swift told Live Science.

Furthermore, the crows that mounted dead birds often demonstrated aggressive behaviors in addition to a sexual response. It's possible that the heightened stress of breeding season, combined with the sight of a dead crow, simply confuses some individuals, so they respond to a corpse with both aggression and sex, the researchers said. However, further research will be required to be able to say for sure what leads some birds to react this way, the scientists concluded.

The findings were published online June 16 in the journal Philosophical Transactions of the Royal Society B.

Original article on Live Science.