Biological Exuberance: Animal Homosexuality and Natural Diversity

subsequent breeding seasons. These males often perform parenting duties typically associated with females in heterosexual pairs. Other males, however, are bisexual, alternating between homosexual pairings and heterosexual ones in different breeding seasons.

Nonreproductive and Alternative Heterosexualities

As mentioned above, heterosexual divorce is common in Hooded Warblers, as are a number of other variations on the nuclear family and monogamous pair-bond. About 4 percent of males form trios, mating with two females who both nest simultaneously on the male’s territory; 6 percent of males are nonbreeders, and some females remain single as well. Among paired birds, promiscuous copulations also occur very frequently: 30—50 percent of all females copulate with males other than their mates (usually neighboring males), and more than a third of all nestlings in some populations are fathered by a bird other than their mother’s mate. In addition, males sometimes adopt young birds from neighboring families whose own parents have finished caring for them; adoptive fathers typically feed these youngsters along with their own nestlings. The adopted birds are usually not genetically related to their foster fathers, i.e., they are not the result of promiscuous matings by the bird who adopts them. Single parenting is also a regular occurrence in Hooded Warblers: once their young can fly, parents usually separate and each takes care of half the brood (unless the female begins a second family, in which case the male will assume responsibility for all of the youngsters). In fact, single parenting is generally more extensive and longer- lasting than male-female coparenting in this species. Nestlings receive biparental care for only eight or nine days, while single parenting can last for three to six weeks and involves feeding rates that are three to five times higher than that of coparents. As a result of separation of mated pairs, males and females are together for only about one month out of the entire year. During the winter, the two sexes occupy largely segregated habitats, with males preferring forests and females scrub areas.

Sources

*asterisked references discuss homosexuality/transgender

Evans Ogden, L. J., and B. J. Stutchbury (1997) “Fledgling Care and Male Parental Effort in the Hooded Warbler (Wilsonia citrina).” Canadian Journal of Zoology 75:576—81.

———(1994) “Hooded Warbler (Wilsonia citrina):” In A. Poole and F. Gill, eds., The Birds of North America: Life Histories for the 21st Century, no. 110. Philadelphia: Academy of Natural Sciences; Washington, D.C.: American Ornithologists’ Union.

Godard, R. (1993) “Tit for Tat Among Neighboring Hooded Warblers.” Behavioral Ecology and Sociobiology 33:45—50.

———(1986) “Long-Term Memory of Individual Neighbors in a Migratory Songbird.” Nature 350:228—29.

*Lynch, J. F., E. S. Morton, and M. E. Van der Voort (1985) “Habitat Segregation Between the Sexes of Wintering Hooded Warblers (Wilsonia citrina).” Auk 102:714— 21.

*Morton, E. S. (1989) “Female Hooded Warbler Plumage Does Not Become More Male-Like With Age.” Wilson Bulletin 101:460—62.

*Niven, D. K. (1997) Personal communication.

*———(1993) “Male-Male Nesting Behavior in Hooded Warblers.” Wilson Bulletin 105:190—93.

Stutchbury, B. J. M. (1998) “Extra-Pair Mating Effort of Male Hooded Warblers, Wilsonia citrina.” Animal Behavior 55:553—61.

*———(1994) “Competition for Winter Territories in a Neotropical Migrant: The Role of Age, Sex, and Color.” Auk 111:63—69.

Stutchbury, B. J., and L. J. Evans Ogden (1996) “Fledgling Adoption in Hooded Warblers (Wilsonia citrina): Does Extrapair Paternity Play a Role?” Auk 113:218—20.

*Stutchbury, B. J., and J. S. Howlett (1995) “Does Male-Like Coloration of Female Hooded Warblers Increase Nest Predation?” Condor 97:559—64.

Stutchbury, B. J., J. M. Rhymer, and E. S. Morton (1994) “Extrapair Paternity in Hooded Warblers.” Behavioral Ecology 5:384—92.

CHAFFINCH

IDENTIFICATION: A sparrow-sized bird with olive-brown plumage, distinctive white shoulder bars, and (in males) blue-gray crown. DISTRIBUTION: Europe, Siberia, central Asia, North Africa. HABITAT: Forest, farmland. STUDY AREAS: Ylivieska, Finland; near Cambridge, England; subspecies F.c. coelebs and F.c. gengleri.

SCOTTISH CROSSBILL

IDENTIFICATION: A sparrow-sized bird with olive to orange-red plumage and a distinctive crossed bill. DISTRIBUTION: Northern Scotland. HABITAT: Coniferous forest. STUDY AREAS: Speyside and Sutherland, Scotland.

Social Organization

Chaffinches and Scottish Crossbills commonly associate in flocks; the mating system involves (usually monogamous) pair-bonding.

Description

Behavioral Expression: Female Chaffinches sometimes form homosexual pair-bonds with each other. In this species, usually only males sing; however, in same-sex pairs, one female partner typically sings much in the manner of a male, throwing back her head while perched conspicuously in the trees (this is probably how she attracts her female mate). Her song resembles that of males in duration, loudness, and structure, consisting of a long stream of rapidly trilled notes of descending pitch, finished off with a staccato end phrase or “flourish.” It differs from male song, however, in having not quite as ringing a tone. Like male Chaffinches, she may even COUNTERSING in response to a neighboring male’s song, the two birds “replying” to each other with alternating or syncopated song phrases. Females in same-sex pairs may also try to sing together, although one partner may only be able to produce an incomplete version of the song. The two females behave like other mated couples, and may even engage in courtship pursuits known as SEXUAL CHASES. In this activity—which is a demonstration of sexual interest—one female zigzags after her partner using a special form of flying known as MOTH FLIGHT (rapid, shallow wingbeats without the pauses or undulating quality typical of regular flight). Occasionally, juvenile males pursue other males in such sexual chases.

Homosexual couples also occur in Scottish Crossbills—but among males. A male attracts potential mates by singing high in a treetop, advertising his presence with a loud stream of notes transcribed as chip- chip-chip-gee-gee-gee chip-chip-chip. Most singing males respond aggressively to other males who enter their territory, but occasionally the displaying male does not chase away another male that is attracted to him. The two males may then pair up and associate in much the same way that heterosexual couples do, except that copulation has not been observed between male partners. They synchronize their movements, traveling together between forest clumps, sometimes one leading the other. Occasionally, two neighboring males who are each heterosexually mated become companions. The two forage together and jointly defend clumps of pine trees (their principal food source) while still maintaining their opposite-sex bonds. The two males may even visit their female mates together, attending first to one and then the other, with one male feeding his female partner while his male companion waits for him.

Frequency: In both of these species, homosexual pairs are probably only an occasional occurrence. Although no statistics concerning the incidence of same-sex pairing are