An estimated one-quarter of all black swans pairings are of homosexual males. They steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs.
More of their cygnets survive to adulthood than those of different-sex pairs, possibly due to their superior ability to defend large portions of land. The same reasoning has been applied to male flamingo pairs raising chicks. Studies have shown that 10 to 15 percent of female western gulls in some populations in the wild exhibit homosexual behavior.
Research has shown that the environmental pollutant methylmercury can increase the prevalence of homosexual behavior in male American White Ibis. The study involved exposing chicks in varying dosages to the chemical and measuring the degree of homosexual behavior in adulthood. The results discovered was that as the dosage was increased the likelihood of homosexual behavior also increased.
The endocrine blocking feature of mercury has been suggested as a possible cause of sexual disruption in other bird species. Mallards form male-female pairs only until the female lays eggs, at which time the male leaves the female. Mallards have rates of male-male sexual activity that are unusually high for birds, in some cases, as high as 19 percent of all pairs in a population.