A case of homosexual necrophilia has been observed among sand martin birds, with three males seen repeatedly trying to mate with a dead male lying at the side of a road. Researchers believe the incident may have been a case of mistaken identity, with the birds believing it was a female they were mating with.

Naoki Tomita and Yasuko Iwami from the Yamashina Institute for Ornithology noticed three male Sand Martins surrounding a dead bird on a roadside. They noticed all three repeatedly tried to mate with the corpse, which was lying face down with its wings spread. One of the birds was also seen guarding the corpse in a bid to stop the other two from mounting it.

They filmed the indecent assault, which took place in 2014, showing how the birds approach the deceased and each try to copulate with it. After 15 minutes, the birds left, New Scientist reports

The scientists then collected the dead bird and realised it was a male. Publishing a study on the case in the journal Ornithological Science, they said the case of homosexual necrophilia has implications for arousal in species that are sexually monomorphic (that males and females look the same).

Riparia riparia sand martin
Sand Martins filmed engaging in homosexual necrophiliaKen Billington

Homosexual behaviour is found among birds. Indeed, around 130 species have been recorded engaging in same sex behaviours. There are lots of theories as to why this is, one being that it is evolutionary beneficial as it helps individuals defend territories and advance social status.

However, in the case of the sand martins, it was likely a case of mistaken identity and that the dead bird's posture had made the others think it was a female. "We observed three sand martin Riparia riparia repeatedly attempting to copulate with a dead bird lying face down on the ground, with its wings spread and lowered," they wrote.

"Based on our observations, we propose that the observed homosexual necrophilia may be partly explained by the absence of sexual dimorphism in this species and the posture of the dead martin. We suggest that posture is an important trigger arousing male sex drive in a sexually monomorphic species."

As for the necrophilia, it is not clear why they were trying to mate with a dead bird. This has been seen in birds – and other species – but why it happens is not known.