"The research showed relationships in animals can be more complicated than just a male and a female who meet and reproduce, even in birds. A pair-bond in socially monogamous species represents a cooperative partnership that may give advantages for survival. Finding a social partner, whatever its sex, could be a priority," said Julie Elie, one of the researchers, to BBC Nature.
It seems that young zebra finches for example, raised by a same-sex couple, will prefer a same-sex relationship and even if they meet finches of the opposite sex.
Moreover, the same-sex couples have the same behaviors than the opposite-sex couples. They nest, sing together and perch side by side.