Biological Exuberance: Animal Homosexuality and Natural Diversity

Frequency: Homosexual pairs probably occur only occasionally in Kestrels, although no systematic study of their frequency has been undertaken. Male pairs of Griffon Vultures have not yet been fully verified in the wild; however, tandem flights between same-sex partners (both males and females) account for about 20 percent of all display flights in the wild, and some of these probably represent homosexual pairings.

Orientation: No detailed studies of the life histories of birds of prey in homosexual pairs have yet been conducted. However, at least some male Griffon Vultures in same-sex tandem flights have female mates, suggesting a possible form of bisexuality, while at least some younger females in same-sex tandems likely have no prior heterosexual experience.

Nonreproductive and Alternative Heterosexualities

In any given year, many Kestrels do not breed: about a third of all birds in some populations are unpaired, while 6-13 percent of heterosexually mated birds do not lay eggs. Male-female pairs of Griffon Vultures, too, may abstain from procreating—some pairs go for as long as eight or nine consecutive years without reproducing. Nonlaying pairs, as well as younger Griffon Vultures that have not yet begun breeding, nevertheless still engage in sexual activity, often mating with each other near the breeding colonies. Several other types of nonprocreative copulations are also prominent in these species. Both Kestrels and Griffon Vultures sometimes mate outside of the breeding season (in the autumn and winter) and during the breeding season when there is no chance of fertilization. This includes during incubation, chick-raising, or very early in the season. Outside of the breeding season, though, Griffon females may refuse to participate in copulations, attacking their male partner when he tries to mount. Kestrel males and females very often live separately during the winter (as discussed above). In addition, heterosexual mating in both species occurs at astonishingly high rates, indicating that it is not simply procreative activity: Griffon heterosexual pairs sometimes mate every half hour, while Kestrels average a copulation once every 45 minutes, or seven to eight times per day during the breeding season. Even higher rates have been recorded for some Kestrels—up to three times per hour—and it is estimated that each Kestrel pair mates as many as 230 times during the breeding season alone. Male Kestrels also sometimes court and attempt to mate with females other than their mate; they usually do not succeed, though, owing to resistance by the female and defense by her mate. Nevertheless, 5-7 percent of all broods contain chicks fathered by a bird other than its mother’s mate, and in a few cases none of the nestlings are genetically related to their caretaking father. Nonmonogamous copulations probably also occur in Griffon Vultures.

Alternative heterosexual family arrangements are widespread in Kestrels: up to 10 percent of males in some years have two female mates (they usually each have families in separate nests), while a female sometimes forms a trio with two males. Divorce is fairly common in Kestrels: about 17 percent of females and 6 percent of males change partners between breeding seasons, and sometimes a pair splits during the breeding season as well. In Griffon Vultures the divorce rate is about 5 percent. Some Kestrel males are unable to provide their mates with enough food during incubation, resulting in desertion and loss of the entire clutch (accounting for more than half of all nesting failures). Finally, cannibalism has been documented in these species: Kestrel nestlings sometimes kill and eat their siblings, while parents of both species cannibalize their own chicks on rare occasions (usually if the chick has already died).

Other Species

Same-sex pairing and coparenting have been observed in other birds of prey in captivity. Female Barn Owls (Tyto alba) that are raised together occasionally bond with one another, ignoring any available males. They may even nest together, each laying a clutch of infertile eggs that they incubate side by side. Female coparents share parenting duties and can successfully raise foster young. Courtship, pair-bonding, nesting, and coparenting of foster chicks have also been documented in a pair of female Powerful Owls (Ninox strenua) from Australia. In addition, a pair of male Steller’s Sea Eagles (Haliaeetus pelagicus)—a species native to Siberia and East Asia—courted one another and built a nest together. They even incubated and hatched another eagle’s egg and successfully raised the chick together.

Sources

*asterisked references discuss homosexuality/transgender