In swans, the pair is the central social unit. The birds reinforce the unit with frequent preening and sex. Should one die, the other will usually live out the remainder of its life alone. The pair builds nests, raise cygnets and defends the territory. Two cobs, being bigger and stronger than a cob and a pen, can hold down a larger territory, and provide their cygnets with more to eat.
Such same-sex pairs represent a major fitness bonus to a pen, and pens without partners will seek out these couples, have sex with one or other of the cobs and lay eggs in their nest. She is then chased off, not being a part of the pair, and the cobs raise the cygnets themselves. Having access to more food the brood have up to ten times the survival rate of a brood with a heterosexual swan couple. From an evolutionary point of view, this is a very rewarding strategy for the cobs as well.
A same-sex lifestyle will be advantageous in some situations, but not in others. However, having a partner is a requisite for building a nest and keeping a territory, and an opposite-sex partner may not always be available when forming pairs. Thus, the ability to form a male pair is a normal part of the black swan’s social behaviour and an example of a flexible life strategy in the species.