Social evolution of gulls.

In five species of gulls we have now discovered populations in which
frequencies of lesbian couples exceed estimates for our own society.
The couples -- like heterosexual pairs -- often stay paired in
successive seasons.  They defend a territory, court each other, build
a nest, and alternate sitting on the eggs.  In some couples, one
female shows such normally male behaviors as courtship feeding,
mounting, and attempted copulation, although the courtship feeding is
never as intense as in heterosexual couples, which may explain why
lesbians commonly lay smaller eggs than do heterosexual females.
(p. 199)

Trivers, R.  Social evolution.  Menlo Park; Benjamin/Cummings, 1985.
462pp.  ISBN 0-8053-8507-X.