[Tokra]

The act of a male animal humping another male animal has nothing to do with sex. It has to do with dominance. This is why dominant female dogs will hump male dogs. The act of mounting is a form of securing a position of prominance within the animal's society.

I'm not talking about humping or signs of dominance or submission. I'm talking about birds and mammals that mate with members of their own sex during their entire lifetime. They even build nests together (obviously there will never be anything in them). Nest building is not a sign of "securing a position of prominance" - that is an entirely different subject.

[Anitius Severinus Boethius]

I'd be interested in reading some studies on this.

It would seem odd that a male bird, since it is in their habit to build a nest, would instead seek to live in one built by another male. Might be a genetic defect. Happily for that species, that defect will be removed from the genetic pool in each instance in which it appears.

[stands2reason]

DId these animals reject available mates of the opposite sex for same-sex mates?

BTW animals that mate for life give "singletons" fewer chances to "hook up" which would logically result in more homosexual pairings.