Same-sex pairs of monogamous birds are just as attached and faithful to each other as those paired with a member of the opposite sex.
The insight comes from a study of zebra finches - highly vocal, colourful birds that sing to their mates, a performance thought to strengthen the pair's bond.
Scientists found that same-sex pairs of finches sang to and preened each other just like heterosexual pairs.
The study is reported in the journal Behavioural Ecology and Sociobiology.
Lead researcher Julie Elie from the University of California Berkeley said that the research showed that "relationships in animals can be more complicated than just a male and a female who meet and reproduce, even in birds".
Dr Elie and her colleagues are interested in zebra finches' behaviour. The birds establish life-long relationships and are highly social; males sing to their mates, the birds preen each other and pairs share a nest.
"I'm interested in how animals establish relationships and how [they] use acoustic communication in their social interactions. My observations of [them] led me to this surprising result: same-sex individuals would also interact in affiliative manners, like male-female pairs."
Dr Elie decided to look more closely at the formation of these bonds and the behaviour of finches in same-sex pairs.
First, she and her colleagues, Clementine Vignal and Nicolas Mathevon from the University of Saint-Etienne, raised young finches in same-sex groups. More than half of the birds paired up with another bird of the same sex.
The team then closely monitored the birds for signs that they had bonded fully.
Bonded birds, Dr Elie explained, perch side by side, nestled together. They also greet each other by "nuzzling" beaks.
In the next stage of their study, the scientists brought novel females to a group of bonded male-male pairs. Out of eight males that were engaged in same-sex pair-bonds, five ignored the females completely and continued to interact with their male partner.
The findings indicate that, even in birds, the drive to find a mate is far more complicated than simply the need to reproduce.
"A pair-bond in socially monogamous species represents a cooperative partnership that may give advantages for survival," said Dr Elie. "Finding a social partner, whatever its sex, could be a priority."
There are many other examples of same-sex pairing in the avian world.
In monogamous gulls and albatrosses, it gives females the chance to breed without a male partner.
"Female partners copulate with a paired male then rear the young together," Dr Elie explained.
In captivity, there have been at least two cases of male penguins forming long-term bonds when there are females available.
Perhaps the most famous of these was two male chinstrap penguins in Manhattan's Central Park Zoo, named Roy and Silo. They bonded and paid no attention to females in their enclosure for at least a year.
They even built a nest together and incubated and hatched a fertilised egg donated to them by one of the keepers.
