Carlos and Fernando, male flamingos at the Slimbridge wildfowl reserve in Gloucestershire, are inseparable.
They have been together for more than five years and have even reared foster chicks.
Twice a year they perform the elaborate courtship dance usual to males and females, before building a nest.
Homosexual activity is not unknown within the animal kingdom but few people know about it, according to zoologists. Keepers at Slimbridge said it was unique among their flamingos.
Nigel Jarrett, the reserve’s aviculture manager, said: “They seem very happy. They will probably stay together for the rest of their lives.
“They are not picked on by the other birds. If anything they are afforded more respect because two males together can be a pretty fearsome prospect for the other flamingos.”
The pair have reared three generations of adopted flamingos, by making off with the freshly laid eggs of their heterosexual neighbors.
Mr Jarrett said: “They have been known to fight the heterosexual birds and steal their eggs. There is usually a ‘handbags-at-10 yards’ moment where they scrap with the couple before stealing the egg.
“They are very good parents though and behave just as the heterosexual birds do when rearing their young.”
The pair are Greater Flamingos, native to the Mediterranean and Africa, and live on algae and small fish.
As well as male flamingos that mate, there are male ostriches that only court their own gender. Film-makers recently caught female Japanese macaque monkeys engaged in intimate acts.
Male penguins have been known to pair up and engage in sexual activity, while ignoring potential female mates.
Adrian Walls, a bird keeper at London Zoo, said: “Homosexual behaviour is often seen amongst birds in captivity, but it is not often long-lived. If they go a long time without chicks, they often search out a different sex partner.”