A new study published online in the journal Animal Behaviour takes a look at parental care and homosexuality in birds. Homosexual behavior is known to occur in over 130 species of birds, making it a very prevalent but sometimes difficult to explain occurrence evolutionarily speaking (in terms of reproduction and the propagating of one's genes). This study, which pulled together mating and behavior data from multiple studies on 93 bird species, shows that the sex that spends the least time on parental care is more likely to engage in homosexual activity. The results show that same-sex courtship, mounting and pair-bonding are prevalent -- 38% of the species participated female–female sexual behaviour and 82% participate in male–male behaviour. But homosexual behavior accounted for less than 5% of all sexual encounters in the analysis.
These results offer a possible explanation for the evolution of homosexuality. Basically, if you aren't the one taking care of the kids then you have lots of free time for other pursuits, but because you have kids you are still passing along your genes. The cause of the behavior is still unclear - neutral evolutionary by-product or adaptive function? Perhaps its to practice courtship displays, reduce social tension, show dominance, form alliances, gain access to resources, share care-taking responsibilities, or maybe they just think its fun to do when they're bored.
This is the reference:
MacFarlane, Geoff R. , Blomberg, Simon P. & Vasey, Paul L. (2010) Homosexual behaviour in birds: frequency of expression is related to parental care disparity between the sexes. Animal Behaviour: published online. (DOI: 10.1016/j.anbehav.2010.05.009)
(image from birdsoftt.com)