Life-long pair-bonded male-male couples of the species Cygnus atratus will raise an egg together donated by a female.
Male-male black swan parents are more successful.
80% of gay black swan couples successfully raise their young, but only 30% of straight swan couples successfully raise their young.
Black swans (Cygnus atratus) also form stable male-male pairs that last for many years. Gay swans may even raise offspring together as a couple. A female may temporarily associate with a male-male pair, mate with them, and leave her eggs with them.
By sharing the workload more equally, male-male parents access better nesting sites and territories than straight couples.
The male couple then parents the eggs and is reported to be more successful than a male-female couple because together they access better nesting sites and territories, sharing the workload more equally than between-sex couples.
A full 80-percent of the gay couples successfully fledge their young, compared with 30 percent for straight couples. (EN19)
(EN19) L.W. Braithwaite, 1981, Ecological studies of the black swan: III. Behavior and social organization, Australian Wildlife Research 8:135-46.
Citation: Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. p. 136.
Update (5/24/23): Thanks to J. Boeheme at Science TV Australia for clarifying these papers are about black swans, not all swans.