Biological Exuberance: Animal Homosexuality and Natural Diversity

nonreproductive matings may also occur after the eggs are laid or even following hatching of chicks. Overall, each pair copulates about 50-70 times per clutch of eggs produced. At least 15 percent of matings also occur after the female’s fertile period, and more than 20 percent of mounts do not involve genital contact. In addition, a large number of heterosexual copulations that take place during incubation—as well as throughout the breeding season —are nonmonogamous matings between a female and a male other than her mate. Although many pairs are monogamous in this species (about half of all females are strictly faithful), promiscuous copulations are a prominent feature of Tree Swallow heterosexual interactions. Females often solicit such copulations (sometimes from several different males) and are also able to effectively terminate unwanted promiscuous matings. They do this by flying away, refusing to lift their tail for genital contact, or turning their head and snapping or “chattering” at the male. Nonmonogamous matings frequently result in offspring: in some populations, 50-90 percent of all nests contain young that are not genetically related to their mother’s mate, and these constitute 40-75 percent of all nestlings. In some families, all the offspring are fathered by other males. The opposite situation also sometimes occurs: youngsters may be related to the father but not his female partner. This may result from mate- switching (divorce and remating), or because females occasionally lay eggs in another female’s nests (5-9 percent of all nests are PARASITIZED this way).

In some populations, 3-8 percent of males form polygamous trios in which they bond and breed with two females simultaneously. If the two females share a nest, one may help care for the other’s nestlings if her own eggs do not hatch. Many populations also have large numbers of nonbreeding birds, sometimes called FLOATERS because they do not occupy their own territories and tend to travel widely. As many as a quarter of all reproductively mature females are floaters. In addition to helping raise unrelated birds of their own species, Tree Swallows sometimes “adopt” nests belonging to other birds such as purple martins (Progne subis) or bluebirds (Sialia spp.), raising the foster young in addition to their own. More than half of all Tree Swallow heterosexual pairs do not remain together for more than one breeding season. Single parenting also occasionally occurs in this species, for example if one parent is killed or dies during the breeding season. Frequently, however, the widowed parent re-pairs with another mate. If a single parent is laying or incubating eggs, its new mate often adopts the brood, but if a single parent already has nestlings from the previous mate, the new partner often kills them (usually by pecking) in order to begin breeding himself or herself. Infanticide also sometimes occurs when a female kills a paired female’s nestlings in order to try to precipitate a divorce and mate with her partner.

Sources

*asterisked references discuss homosexuality/transgender

Barber, C. A., R. J. Robertson, and P. T. Boag (1996) “The High Frequency of Extra-Pair Paternity in Tree Swallows Is Not an Artifact of Nestboxes.” Behavioral Ecology and Sociobiology 38:425- 30.

Chek, A. A., and R. J. Robertson (1991) “Infanticide in Female Tree Swallows: A Role for Sexual Selection.” Condor 93:454-57.

Dunn, P. O., and R. J. Robertson (1992) “Geographic Variation in the Importance of Male Parental Care and Mating Systems in Tree Swallows.” Behavioral Ecology 3:291-99.

Dunn, P. O., and R. J. Robertson, D. Michaud-Freeman, and P. T. Boag (1994) “Extra-Pair Paternity in Tree Swallows: Why Do Females Mate with More than One Male?” Behavioral Ecology and Sociobiology 35:273-81.

Leffelaar, D., and R. J. Robertson (1985) “Nest Usurpation and Female Competition for Breeding Opportunities by Tree Swallows.” Wilson Bulletin 97:221-24

———(1984) “Do Male Tree Swallows Guard Their Mates?” Behavioral Ecology and Sociobiology 16:73-79.

Lifjeld, J. T., P. O. Dunn, R. J. Robertson, and P. T. Boag (1993) “Extra-Pair Paternity in Monogamous Tree Swallows.” Animal Behavior 45:213-29.

Lifjeld, J. T., and R. J. Robertson (1992) “Female Control of Extra-Pair Fertilization in Tree Swallows.” Behavioral Ecology and Sociobiology 31:89-96.

*Lombardo, M. P. (1996) Personal communication.

———(1988) “Evidence of Intraspecific Brood Parasitism in the Tree Swallow.” Wilson Bulletin 100:126–28.

———(1986) “Extrapair Copulations in the Tree Swallow.” Wilson Bulletin 98:150- 52.

*Lombardo, M. P., R. M. Bosman, C. A. Faro, S. G. Houtteman, and T.S. Kluisza (1994) “Homosexual Copulations by Male Tree Swallows.” Wilson Bulletin 106:555-57.

Morrill, S. B., and R. J. Robertson (1990) “Occurrence of Extra-Pair Copulation in the Tree Swallow (Tachycineta bicolor).” Behavioral Ecology and Sociobiology 26:291-96.

Quinney, T. E. (1983) “Tree Swallows Cross a Polygyny Threshold.” Auk 100:750- 54.

Rendell, W. B. (1992) “Peculiar Behavior of a Subadult Female Tree Swallow.” Wilson Bulletin 104:756-59.

Robertson, R. J. (1990) “Tactics and Counter-Tactics of Sexually Selected Infanticide in Tree Swallows.” In J. Blondel, A. Gosler, J.-D. Lebreton, and R. McCleery, eds., Population Biology of Passerine Birds: An Integrated Approach, pp. 381-90. Berlin: Springer-Verlag.

Robertson, R. J., B. J. Stutchbury, and R. R. Cohen (1992) “Tree Swallow (Tachycineta bicolor).” In A. Poole, P. Stettenheim, and F. Gill, eds., The Birds of North America: Life Histories for the 21st Century, no. 11. Philadelphia: Academy of Natural Sciences; Washington, D.C.: American Ornithologists’ Union.

Stutchbury, B. J., and R. J. Robertson (1987a) “Signaling Subordinate and Female Status: Two Hypotheses for the Adaptive Significance of Subadult Plumage in Female Tree Swallows.” Auk 104:717- 23.

———(1987b) “Behavioral Tactics of Subadult Female Floaters in the Tree Swallow.” Behavioral Ecology and Sociobiology 20:413-19.

———(1987c) “Two Methods of Sexing Adult Tree Swallows Before They Begin Breeding.” Journal of Field Ornithology 58:236-42.

———(1985) “Floating Populations of Female Tree Swallows.” Auk 102:651- 54.

Venier, L. A., P. O. Dunn, J. T. Lifjeld, and R. J. Robertson (1993) “Behavioral Patterns of Extra-Pair Copulation in Tree Swallows.” Animal Behavior 45:412-15.

Venier, L. A., and R. J. Robertson (1991) “Copulation Behavior of the Tree Swallow, Tachycineta bicolor: Paternity Assurance in the Presence of Sperm Competition.” Animal Behavior 42:939-48.

CLIFF SWALLOW

IDENTIFICATION: A bluish brown swallow with pale underparts, buff forehead, and a chestnut throat; tail is not forked. DISTRIBUTION: North and Central America; winters in southern South America. HABITAT: Open country, cliffs. STUDY AREAS: Near Jackson Hole (Moran), Wyoming, Lakeview, Kansas, and along the North and South Platte Rivers, Nebraska; subspecies H.p. hypopolia and H.p. pyrrhonota.

BANK SWALLOW

IDENTIFICATION: A small, sparrow-sized swallow with a slightly forked tail, brown plumage, white underparts, and a brown breast band. DISTRIBUTION: Throughout North America and Eurasia; winters to South