Biological Exuberance: Animal Homosexuality and Natural Diversity

even sit on the nest together, side by side or one partially covering the other. Both females also lay eggs: as a result, their nests have SUPERNORMAL CLUTCHES of 9—12 eggs, twice the number in most nests belonging to heterosexual pairs. If their nest is robbed by predators, the pair will dutifully build a second one (often in a much higher location, to make it inaccessible to most ground predators) and begin laying eggs again (as do male-female pairs when they lose a nest). However, all of the eggs they lay are typically infertile since neither female has mated with a male, and the pair usually abandons the nest after the eggs have failed to hatch. Blue Tit female pairs are unlike most homosexual pairs in birds because only one female lays eggs. As a result, nests belonging to such pairs contain not supernormal clutches, but exceptionally small clutches of 3 or so eggs (nests of male-female pairs typically contain about 11 eggs). Both females incubate the eggs simultaneously, sitting together on the nest facing in the same direction. As in Red-backed Shrike homosexual pairs, the eggs are usually infertile and the pair eventually abandons the nest.

A nesting pair of female Red-backed Shrikes in England simultaneously incubating their eggs

In Eastern Bluebirds, two males sometimes associate with one another in what appears to be a homosexual pair-bond: they travel exclusively in each other’s company (perhaps even spending the winter together), jointly inspect nest sites during the early spring, and court one another. The latter activity involves COURTSHIP- FEEDING—a behavior also seen in heterosexual pairs—in which one male offers a symbolic food gift, such as a cutworm, to the other male, often preceded by a distinctive call. Paired males might be related to each other (father-son or brothers). Occasionally, a female who has lost her heterosexual mate is joined by another female, who helps coparent her offspring. The two birds take turns feeding the youngsters and may also be assisted by one or more of their young from a previous brood.

Frequency: Same-sex pairs probably occur only sporadically in these three species: in Red-backed Shrikes, for example, female couples account for perhaps no more than 1 percent of all pairs.

Orientation: Female Red-backed Shrikes and Blue Tits are probably exclusively homosexual, at least for the time that they remain paired with their female partner, since they invariably lay eggs that are infertile (indicating no mating with males). Whether such birds ever subsequently form or have previously formed heterosexual pair-bonds is not known.

Nonreproductive and Alternative Heterosexualities

Approximately 17 percent of all sexual activity in opposite-sex pairs of Blue Tits occurs outside the female’s fertile period. Male Eastern Bluebirds also sometimes try to copulate when females are nonfertile, such as during incubation or after the breeding season is over. Other nonprocreative matings involve occasional interspecies encounters: juvenile Eastern Bluebirds have been observed being mounted by great crested flycatchers (Myiarchus crinitus). Courtship and mounting between pair-bonded Eastern Bluebirds sometimes involve considerable aggression, with males attacking their partner and violently pecking her head or knocking her over with their feet. Heterosexual copulations are also occasionally interrupted by neighboring birds that attack the mating pair. Within-pair copulation rates are quite high in Red-backed Shrikes, roughly three times a day during the female’s fertile period. In addition, nonmonogamous matings are common in these species: approximately 10 percent of Blue Tit copulations are promiscuous, involving birds that are not paired to each other. About a quarter of Bluebird nests and a third to nearly a half of all Blue Tit broods contain youngsters fathered by an outside bird. Promiscuous copulations also occasionally occur in Red-backed Shrikes, including forced matings or rapes in which males attack and mount females or violently push them off their perches. About 5 percent of Red- backed Shrike nestlings are fathered by a bird other than their mother’s mate, compared to 8—35 percent in Eastern Bluebirds, and 11—14 percent in Blue Tits.

Several other alternative parenting arrangements occur in these species. Red-backed Shrike youngsters sometimes move to adjacent territories where they are adopted by other families, and a few nonbreeding birds of both sexes help feed chicks belonging to other pairs (sometimes even assuming full responsibility for them later on). “Helper” males such as these may even replace the biological father as a model for song-learning by the youngsters. In addition, late-breeding Red-backed Shrikes sometimes become single parents when their mate deserts them; occasionally two such birds and their offspring join together to form a “blended family.” At least 15 percent of Eastern Bluebird mothers in some populations raise unrelated youngsters when outside females lay eggs in their nest. The divorce rate for Blue Tits varies considerably, from 8—85 percent (depending on the population); in addition, about a third of females and up to 20 percent of all males in some areas form polygamous trios (or occasionally quartets). Other more complex family arrangements also occur: one female Blue Tit was part of a polygamous trio with another female and a male; she also copulated promiscuously with a second male, eventually forming a new polygamous bond with both him and yet a third male, all of whom helped care for the young! Overall, however, more than 85 percent of Blue Tits never successfully breed, and about a third of all parents never have grandchildren. Red-backed Shrike parents occasionally cannibalize their own young or eggs.

Other Species

Some male Pied Flycatchers (Ficedula hypoleuca), especially younger ones, are transvestite, having the same brownish plumage that females do; these birds are sometimes courted by other males.

Sources

*asterisked references discuss homosexuality/transgender

Alsop, F. J., III (1971) “Great Crested Flycatcher Observed Copulating with an Immature Eastern Bluebird.” Wilson Bulletin 83:312.

*Ashby, E. (1958) “Incidents of Bird Life [report on Red-backed Shrikes].” The Countryman: A Quarterly Review and Miscellany of Rural Life and Progress 55:272.

Birkhead, T. R., and A. P. Moller (1992) Sperm Competition in Birds: Evolutionary Causes and Consequences. London: Academic Press.

*Blakey, J. K. (1996) “Nest-Sharing by Female Blue Tits.” British Birds 89:279-80.

*Cramp, S., and C. M. Perrins, eds. (1993) “Red-backed Shrike (Lanius collurio).” In Handbook of the Birds of Europe, the Middle East, and North Africa, vol. 7, pp. 456—78. Oxford: Oxford University Press.

Dhondt, A. A. (1989) “Blue Tit.” In I. Newton, ed., Lifetime Reproduction in Birds, pp. 15—33. London: Academic Press.

———(1987) “Reproduction and Survival of Polygynous and Monogamous Blue Tit Parus caeruleus.” Ibis 129:327—34.

Dhondt, A. A., and F. Adriaensen (1994) “Causes and Effects of Divorce in the Blue Tit Parus caeruleus.” Journal of Animal Ecology 63:979—987.

Fornasari, L., L. Bottoni, N. Sacchi, and R. Massa (1994) “Home-Range Overlapping and Socio-Sexual Relationships in the Red-backed Shrike Lanius collurio.” Ethology, Ecology, and Evolution 6:169-177.

Gowaty, P. A., and W. C. Bridges (1991) “Behavioral, Demographic, and Environmental Correlates of Extrapair Fertilizations in Eastern Bluebirds, Sialia sialis.” Behavioral Ecology 2:339-50.

Gowaty, P. A., and A. A. Karlin (1984) “Multiple Maternity and Paternity in Single Broods of Apparently Monogamous Eastern Bluebirds (Sialia sialis).” Behavioral Ecology and Sociobiology 15:91—95.

Hartshorne, J. M. (1962) “Behavior of the Eastern Bluebird at the Nest.” Living Bird 1:131—49.

Herremans, M. (1997) “Habitat Segregation of Male and Female Red-backed Shrikes Lanius