Biological Exuberance: Animal Homosexuality and Natural Diversity

then chasing her away once the eggs are laid, after which they begin parenting as a male couple. Other homosexual pairs chase heterosexual pairs from their nests and adopt the eggs that have already been laid. The two males incubate the clutch, hatch the eggs, and raise the chicks together. In fact, homosexual pairs are often more successful than heterosexual ones at raising chicks, in part because they have access to the best nesting sites and the largest territories, and probably because they also share incubation duties more equally. On average, 80 percent of homosexual parenting efforts are successful, compared to only about 30 percent of heterosexual ones.

A homosexual pair of male Black Swans performing the “greeting ceremony”

Both male and female homosexual pairs occur in Mute Swans. In female pairs, both birds build a nest, lay eggs (which are usually infertile), and incubate the eggs. Sometimes one female stands guard over both the nest and her mate (as does the male in heterosexual pairs) and defends their territory. If their nest is disturbed by intruders, the females may begin a second nest and lay a new clutch of eggs, while still attending to the first as best they can. Male pairs also annually build nests together on which they take turns sitting, although unlike Black Swans they do not acquire any eggs. Male Mute Swans also sometimes form homosexual pair-bonds with other species, including Trumpeter Swans (Cygnus buccinator) and Greylag Geese.

Frequency: Overall, male couples constitute 5–6 percent of all pairings in Black Swans; in a given year, an average of 13 percent of the birds are in homosexual pair-bonds, and sometimes this proportion is as high as 20 percent. Homosexual parents account for 20–25 percent of all successful families. Same-sex bonds probably occur only sporadically in Mute Swans.

Orientation: Many Black and Mute Swans in same-sex pairs are probably exclusively homosexual, since they do not engage in heterosexual copulation or pairing and usually ignore unpaired birds of the opposite sex. However, some male pairs of Black Swans—while primarily homosexual—form short-lived bisexual trios in order to mate with females and thereby father their own offspring.

Nonreproductive and Alternative Heterosexualities

Populations of both Mute and Black Swans contain large proportions of nonbreeding birds. More than half of all Mute Swans are nonbreeders (as much as 89 percent of some populations), often gathering into their own flocks separate from breeding pairs. Many birds nest only once or twice during their entire lifetimes (which can last for 15–20 years), and a few never breed. Overall, only about a fifth of all Black Swans nest in any year, and in some populations more than 90 percent of the adults do not breed. Young, sexually mature Black Swans may remain with their parents and delay their breeding for many years (as long as three to eight years in some cases). On occasion, such a youngster will form an incestuous pair-bond with its parent: male Swans have been known to mate with their mother on the death of their father. Brother-sister and parent-offspring matings also occur in Mute Swans, as do heterosexual pairings with other species of swans (such as bewick’s, whistling, whooper, and Trumpeter) and geese (e.g., Snow, Canada, and Greylag). In fact, Black and Mute Swans may pair with each other, and trios of a male Black with two female Mute Swans have also been observed. Heterosexual trios within the same species are also common: about 14 percent of all bonds in Black Swans involve two males with a female, while Mute Swan trios are usually made up of two females with a male.

In addition to such polygamous associations, several other alternative family arrangements occur. “Foster parenting” or adoption takes place frequently among Black Swans (and occasionally in Mute Swans). In some colonies, more than two-thirds of all cygnets are raised in broods that combine offspring from 2–4 families (and occasionally from as many as 30 different families). Such BROOD AMALGAMATIONS—which may have up to 40 youngsters—are attended by a single pair of adults, who are not necessarily the biological parents of any of the cygnets. Adoption also occasionally occurs when adults “steal” eggs laid near their nest by other birds, rolling them into their own nest. Single parenting is a prominent feature of Black Swan social life: often a male or female deserts its mate during incubation, and in some colonies the majority of nests are attended by single parents. Occasionally, a pair “separates” rather than divorces, with one bird taking the newly hatched young while the other remains to incubate the rest of the eggs. Among Mute Swans, the divorce rate is 3–10 percent of all pairs, and about a fifth of all birds have two to four mates during their lifetime. Some Mute Swans are nonmonogamous, courting or mating with another bird while remaining paired with their partner; some of this activity may involve REVERSE copulations (in which the female mounts the male). Many within-pair copulations are nonprocreative, since most pairs mate far more often than is required for fertilization of the eggs. Swans also sometimes engage in behaviors that are counterreproductive. A third of all Black Swan eggs, for example, are lost through abandonment of the nest by the parent (s), while 3 percent of Mute Swan parents desert their nests, and birds often attack and even kill youngsters that stray into their territory. Eggs are sometimes also destroyed during territorial disputes, and adult birds may be killed as a direct result of such attacks as well (accounting for 3 percent of all deaths).

Sources

*asterisked references discuss homosexuality/transgender

Bacon, P. J., and P. Andersen-Harild (1989) “Mute Swan.” In 1. Newton, ed., Lifetime Reproduction in Birds, pp. 363–86. London: Academic Press.

Braithwaite, L. W. (1982) “Ecological Studies of the Black Swan. IV. The Timing and Success of Breeding on Two Nearby Lakes on the Southern Tablelands of New South Wales.” Australian Wildlife Research 9:261–75.

*———(1981) “Ecological Studies of the Black Swan. III. Behavior and Social Organization.” Australian Wildlife Research 8:135–46.

*———(1970) “The Black Swan.” Australian Natural History 16:375-79.

Brugger, C., and M. Taborsky (1994) “Male Incubation and Its Effect on Reproductive Success in the Black Swan, Cygnus atratus.” Ethology 96:138–46.

Ciaranca, M. A., C. C. Allin, and G. S. Jones (1997) “Mute Swan (Cygnus olor).” In A. Poole and F. Gill, eds., The Birds of North America: Life Histories for the 21st Century, no. 273. Philadelphia: Academy of Natural Sciences; Washington, D.C.: American Ornithologists’ Union.

Dewer, J. M. (1942) “Menage a Trois in the Mute Swan.” British Birds 30:178.

Huxley, J. S. (1947) “Display of the Mute Swan.” British Birds 40:130–34.

*Kear, J. (1972) “Reproduction and Family Life.” In P. Scott, ed., The Swans, pp. 79–124. Boston: Houghton Mifflin.

*Low, G. C. and Marquess of Tavistock (1935) “The Extent to Which Captivity Modifies the Habits of Birds.” Bulletin of the British Ornithologists’ Club 55:144–54.

Mathiasson, S. (1987) “Parents, Children, and Grandchildren—Maturity Process, Reproduction Strategy, and Migratory Behavior of Three Generations and Two Year-Classes of Mute Swans Cygnus olor.” In M. O. G. Eriksson, ed., Proceedings of the Fifth Nordic Ornithological Congress, 1985, pp. 60–70. Acta Regiae Societatis Scientiarum et Litterarum Gothoburgensis Zoologica no. 14. Goteborg: Kungl. Vetenskaps- och Vitterhets-Samhallet.

Minton, C. D. T. (1968) “Pairing and Breeding of Mute Swans.” Wildfowl 19:41– 60.

*O’Brien, R. M. (1990) “Black Swan, Cygnus atratus.” In S. Marchant and P. J. Higgins, eds., Handbook of Australian, New Zealand, and Antarctic Birds, vol. 1, pp.1178–89. Melbourne: Oxford University Press.

Ogilvie, M. A. (1972) “Distribution, Numbers, and Migration.” In P. Scott, ed., The Swans, pp. 29–55. Boston: Houghton Mifflin.

Rees, E. C., P. Lievesley, R. A. Pettifor, and C. Perrins (1996) “Mate Fidelity in Swans: An Interspecific Comparison.” In J. M. Black, ed., Partnerships in Birds: The Study of Monogamy, pp. 118–37. Oxford: Oxford University Press.

*Ritchie, J. P. (1926) “Nesting of Two Male Swans.” Scottish Naturalist 159:95.