Scientists have documented same-sex matings in over 94 bird species. One of them is the common goose, Anser anser, who pair-bond for more than a decade and live for 20 years.
The male goose becomes despondent & defenseless when his same-sex partner dies.
This is the same behavior that between-sex (male-female) goose partners display.
A male is reported to show “grief” after his partner dies, becoming despondent and defenseless, just as between-sex partners do when one dies.
Often one female & 2 males will co-parent a family together.
Geese sometimes form threesomes that are the reverse of oystercatchers: a male pair is joined by a female and the trio raise a family together.
Citations
F. Cezilly and R.G. Nager, 1995, Comparative evidence for a positive association between divorce and extra-pair paternity in birds, Proc. R. Sox. Lond., ser. B, 262:7-12.
T. Clutton-Brock, 1989, Review lecture: Mammalian mating systems, Proc. R. Soc. Lond., ser. B, 236:339-72.
Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. p. 136.