The male-male pairs nestled and preened each other just like male-female pairs
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.
Male king penguins have been seen to "flirt" with other males in the colony
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," Dr
Elie told BBC Nature.
"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.
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