Biological Exuberance: Animal Homosexuality and Natural Diversity

while crouching slightly and raising his wings a bit at the shoulders. The House Sparrow then mounts the Cowbird, grasping his head feathers in his beak while attempting to copulate. If he shows signs of leaving or lack of interest after a single mounting, the Cowbird will immediately resume the invitation posture next to him, insistently nudging the Sparrow with his head and persistently following him until he mounts again. This may continue for an extended time, with repeated homosexual mountings (five or more) occurring in a single session.

Homosexual courtships occasionally occur in Wattled Starlings. Males sometimes select another male as the object of their attentions, displaying to him with a number of stylized postures. Among these are the LATERAL DISPLAY, in which the male turns sideways and lets his wings hang down at his side (exposing their white feathers); the FRONTAL DISPLAY, where the courting male fluffs up his belly and back feathers, raising and quivering his wings while spreading his tail; and the distinctive VULTURE POSTURE, in which he stretches his entire body vertically while puffing out his breast feathers and tightly folding his wings against his sides like a vulture. A form of plumage transvestism also occurs in this species, in which some females develop a male appearance. Most males have a special seasonal nuptial plumage, growing two pendulous wattles from either side of their beaks and losing most of the feathers from their head, thereby exposing the yellow or black skin and two fleshy, comblike growths on the forehead. This feather loss has even been described as a form of “male pattern baldness” akin to the type of hair loss found in humans, and indeed it is regulated by male hormones (as is human baldness). While the majority of females never exhibit these plumage characteristics, a few females do acquire a male appearance with feather loss, wattles, and combs.

Frequency: Brown-headed Cowbirds regularly perform the head-down display toward other species in the wild, and approximately 36 percent of such displays are directed by male Cowbirds toward male House Sparrows; however, Sparrows respond with homosexual mounting probably only sporadically. Similarly, homosexual courtship is in all likelihood only an occasional occurrence in Wattled Starlings. About 2—10 percent of female Wattled Starlings are transgendered, exhibiting fairly complete wattles and/or baldness. Other females appear to fall along a continuum of plumage characteristics, with some individuals showing only partial wattle development or incomplete baldness.

A male Brown-headed Cowbird in the “head-down” posture (right) inviting a male House Sparrow to mount him

Orientation: In all three of these species, not enough is yet known about the life histories of individuals participating in homosexual activity to determine their overall sexual orientation profiles. However, at least some male Wattled Starlings appear to preferentially select other males to court.

Nonreproductive and Alternative Heterosexualities

Although most heterosexual Wattled Starlings form monogamous pairs, occasionally males court and mate with females other than their mate. This may result in a female raising her young as a single parent if she is not paired to the male she mates with. The male sexual cycle is especially pronounced in this species, signaled by the seasonal development of wattles and baldness. House Sparrows often engage in multiple copulations during the same mating bout: a male may mount a female and achieve genital contact up to 30 times in rapid succession. In addition, promiscuous matings are quite common in this species: more than a quarter of all nests contain at least one chick sired by a bird other than its mother’s mate. Some of these are the result of forced matings that occur during COMMUNAL DISPLAYS, in which “gangs” of up to ten males chase a female, peck at her genitals, and try to mount her. Such displays and the associated sexual activity often take place during nonfertilizable periods as well. Courting male Cowbirds also frequently harass females, and on average only about 12 percent of heterosexual consortships in this species culminate in copulation. In pair-bonding populations of Brown-headed Cowbirds, about 16 percent of courtships are actually between birds not paired to one another, and some promiscuous matings occur as well. In both this species and House Sparrows, a few pairs switch mates during the breeding season, and a subset of birds (about 5—6 percent) are polygamous.

Large numbers of male Cowbirds are nonbreeders: more than half of all males in some populations are unpaired, and only a third of males actually copulate with females in some years. Brown-headed Cowbirds are also BROOD PARASITES, which means that females always lay their eggs in the nests of other bird species and take no part in raising their own young. Infanticide occurs in 9—12 percent of House Sparrow nests, often when a female who has lost her mate pairs with a new male (who pecks her young to death in order to father his own offspring). Females in polygamous trios also occasionally kill one another’s nestlings. Sometimes, however, a female whose mate has been replaced by an infanticidal male will stop laying eggs (by interrupting or delaying ovulation) in order not to lose any more young, and some replacement males adopt rather than kill their mate’s young.

Other Species

Male Sharp-tailed Sparrows (Ammodramus caudacutus), a North American species, sometimes mount other males. Adolescent male Yellow-rumped Caciques (Cacicus cela), a South American blackbird, frequently mount fledglings of both sexes. Sexual behavior toward these younger birds is usually part of an overall pattern of harassment, in which the adolescent males (often in groups) chase, peck, attack, and sometimes even knock fledglings from their perches (often resulting in their death by drowning if they fall into water). About 36 percent of such harassments (and the associated sexual behavior) involve same-sex interactions.

Sources

*asterisked references discuss homosexuality/transgender

Craig, A. J. F. K. (1996) “The Annual Cycle of Wing Moult and Breeding in the Wattled Starling Creatophora cinerea.” Ibis 138:448—54.

Darley, J. A. (1978) “Pairing in Captive Brown-headed Cowbirds (Molothrus ater).” Canadian Journal of Zoology 56:2249—52.

*Dean, W. R. J. (1978) “Plumage, Reproductive Condition, and Moult in Non-Breeding Wattled Starlings.” Ostrich 49:97-101.

Friedmann, H. (1929) The Cowbirds: A Study in the Biology of Social Parasitism. Springfield, Ill.: Charles C. Thomas.

*Greenlaw, J. S., and J. D. Rising (1994) “Sharp-tailed Sparrow (Ammodramus caudacutus).” In A. Poole and F. Gill, eds., The Birds of North America: Life Histories for the 21 st Century, no. 112. Philadelphia: Academy of Natural Sciences; Washington, D.C.: American Ornithologists’ Union.

*Griffin, D. N. (1959) “Apparent Homosexual Behavior Between Brown-headed Cowbird and House Sparrow.” Auk 76:238—39.

*Hamilton, J. B. (1959) “A Male Pattern Baldness in Wattled Starlings Resembling the Condition in Man.” Annals of the New York Academy of Sciences 83:429-47.

Laskey, A. R. (1950) “Cowbird Behavior.” Wilson Bulletin 62:157-74.

Liversidge, R. (1961) “The Wattled Starling (Creatophora cinerea [Menschen]).” Annals of the Cape Provincial Museums 1:71—80.

Lowther, P. E. (1993) “Brown-headed Cowbird (Molothrus ater).” In A. Poole, P. Stettenheim, and F. Gill, eds., The Birds of North America: Life Histories for the 21st Century, no. 47. Philadelphia: Academy of Natural Sciences; Washington, D.C.: American Ornithologists’ Union.

Lowther, P. E., and C. L. Cink (1992) “House Sparrow (Passer domesticus).” In A. Poole, P. Stettenheim, and F. Gill, eds., The Birds of North America: Life Histories for the 21 st Century, no. 12. Philadelphia: Academy of Natural Sciences; Washington, D.C.: American Ornithologists’ Union.

Moller, A. P. (1987) “House Sparrow, Passer domesticus, Communal Displays.” Animal Behavior 35:203—10.