Now Kees Moeliker provides an even darker explanation.
Kees is curator of birds at the Rotterdamn Natural History Museum in the Netherlands. In 2003 he was awarded the Ig Nobel biology prize for his paper "The first scientifically recorded case of homosexual necrophilia in the mallard duck." His recently published book De Eendenmon ("The Duck Guy") includes a section on wildlife necrophilia. One of its key examples is those same two barn swallows.
Kees tells us, "These particular birds were not injured, in mourning or in a territorial battle. No, the fluttering swallow-on-top was engaged in one of the best photographically documented cases of necrophilia. From the pictures it is hard to tell if it was homosexual necrophilia or just heterosexual necrophilia: sexes in the barn swallow are very much alike. The less deeply coloured and slightly mottled throat and forehead of the dead swallow point towards it being a juvenile. Indicating that this was a rare case of pedophilic necrophilia.
Kees goes on to say that the pictures were taken by photographer Wilson Hsu somewhere in Taiwan in March 2004. "The talented nature photographer seems to have assumed that the live swallow tried to revive his dead 'relative.'