"A pair-bond in socially monogamous species represents a cooperative partnership that may give advantages for survival," said Elie. "Finding a social partner, whatever its sex, could be a priority."
Although some have reported this as evidence of homosexuality in the animal kingdom, the lack of any observed sexual behavior means there is an important piece missing from that argument. Just because males are affectionate to each other and live together doesn't mean their "bro-mance" is homosexual.
Same sex pair bonds have been observed in birds before. Elie noted the case of female albatross, who will bond with another female, then mate with a male from another pair bond in order to raise a chick with her female partner.
"Female partners copulate with a paired male then rear the young together," Elie said.
In the zebra finch study, females were not raised together, so it is unknown if female finches will form pair bonds as well.
The famous relationship of two male penguins, Roy and Silo, at Manhattan's Central Park Zoo is another example of same-sex pair bonding in birds. The two male chinstrap penguins formed such a strong bond that they were even able to incubate and hatch a fertilized egg that a keeper placed in their nest.