A new study published this week may mark the end of the theory that homosexuality only has evolutionary disadvantages, according to Nature.

The findings, based on observations of 93 bird species that are known to engage in homosexual activity, revealed that the amount of time males or females put toward parental care was proportional to how often they engage in homosexual behavior.

This means that homosexuality may not be costly for birds that have plenty of mating opportunities because of lower parenting demands, said Geoff MacFarlane, one of the study's principal researchers.

In other words, since some animals can devote more energy toward mating behavior than to raising offspring, there is wiggle room for homosexuality to become a common behavior without sacrificing evolutionary efficiency.

Previously, biologists struggled to explain how homosexuality could have evolved since it distracts animals from sexual activity that directly produces offspring. The fact that it had evolved was difficult to deny: more than 130 species of birds participate in homosexual activity. For example, among Laysan albatrosses, as many as 31 percent of all pairings are female-female. Among graylag geese, one in five pairings are male-male.

The research team reached its conclusion by scoring each bird species based upon the relative contribution of males and females to parental chores. They found that male homosexuality is more prevalent among bird species in which the female is more heavily devoted to parenting tasks (such as tending the nest or feeding and caring for chicks). Similarly, when females had more free time, female homosexuality was more frequently witnessed.

Overall, the research discovered that 38 percent of the species studied display female–female sexual behavior and 82 percent participate in male–male behavior. In total, 5 percent of all sexual encounters among all the species was homosexual in nature.

"This is one of the few studies that explains homosexual behavior from the evolutionary point of view," said Vincent Savolainen, a biologist at Imperial College London.

Although the study dispels of the theory that homosexuality is evolutionarily disadvantageous or unnatural, it cannot determine what the ultimate explanation for homosexuality is. As evolutionary geneticist Allen Moore points out, "this study suggests that when there's no cost, homosexuality can persist, which isn't the same as saying it's adaptive. It may be that when there's no parental care involved, it's like having a hobby."

Researchers have speculated, though, that homosexual behavior in birds may help them to practice courtship displays, form alliances, reduce social tension or solidify dominance.

"The next logical step is to see if similar patterns occur across other vertebrate species," said MacFarlane. His team has already reached some preliminary results that suggest the patterns are similar for primates.