When zookeepers at Artis Amsterdam Royal Zoo discovered an abandoned egg on the floor of the vulture aviary, they initially placed the egg in an incubator. But presence of an empty nest in the enclosure of two male griffon vultures raised another idea: the keepers decided to place the egg in the empty nest.
The two vultures — who have been together as a couple for a number of years now — took to the egg as if it were their own. They took turns sitting on the egg until it hatched, and now the two share feeding responsibilities of the young chick the way all vultures care for baby vultures: by regurgitating food into its mouth.
Roughly translated from Danish by Google, the zoo's website notes that these kind of same-sex couples occur in nature, and that such pairings are especially common among birds.