In the "animal" world it is about procreation, almost entirely. Especially with non-mammalian species where the hypothalamus plays such an insignificant role in brain chemistry. It is the hypothalamus in mammals that determines sexual orientation and any difference in orientation that might veer from "assigned" gender. So, being isolated with another bird of the same sex would prohibit procreation, an unnatural, therefore unfortunate state for a bird. Not necessarily so for a human. Big difference.