Bruce Bagemihl - Biological Exuberance

During early spring through summer, Kestrels associate as mated pairs that each have their own territory; there is also a significant subpopulation of nonbreeding birds. Outside of the mating season, males and females are often segregated from each other and largely solitary: sometimes only one member of a pair—typically the female—migrates, though males that migrate often travel farther than females. During the winter, males and females also tend to occupy separate habitats, with males generally in more wooded areas. Griffon Vultures are much more social and tend to nest in colonies containing 15-20 pairs, sometimes as many as 50-100. As in Kestrels, mated pairs often last for many years.

Behavioral Expression: In Kestrels and Griffon Vultures, two birds of the same sex—usually males—occasionally bond with each other and become a mated pair. Male Kestrels in a homosexual pair often soar together in the early spring, performing dramatic courtship display flights that reinforce their pair-bond (these displays are also found in heterosexual pairs). One such display is the ROCKING FLIGHT, in which the two partners fly at an immense height and rock from side to side, using flicking wingbeats. Another display is the slower WINNOWING FLIGHT, in which the wings beat with shallow, almost vibrating strokes, giving the impression that only the tips are moving or “shivering.” Both displays are accompanied by distinctive calls, such as the TSIK CALL—a series of clipped notes sounding like tsick or kit —and the LAHN CALL, a sequence of high-pitched trills transcribed as quirrr-rr quirrr-rr. The two males sometimes display together, or one male might soar while the other sits on his perch. Same-sex partners also copulate with each other, making the distinctive copulation call sounding like kee-kee-kee or kik-kik-kik; homosexual mounts last for 10-15 seconds (comparable to heterosexual matings).

Male Griffon Vultures in homosexual pairs also mate with each other repeatedly beginning in December (the onset of the mating season), and such pairs may remain together for years. The two males sometimes build a nest together each year—typically a flat assemblage of sticks on a crag, two to three feet across. Like Kestrels, pairs of Griffon Vultures perform a spectacular aerial pair-bonding display called TANDEM FLYING. The two birds spiral upward to a great height on a thermal, then glide downward in a path that will bring them extremely close to each other, “riding” for a few seconds one above the other, until they separate again. Although most tandem flights are by heterosexually paired birds, Vultures of the same sex also engage in this activity.

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.

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).

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.

*asterisked references discuss homosexuality/transgender

Blanco, G., and F. Martinez (1996) “Sex Difference in Breeding Age of Griffon Vultures ( Gyps fulvus ).” Auk 113:247–48.

Bonin, B., and L. Strenna (1986) “The Biology of the Kestrel Falco tinnunculus in Auxois, France.” Alauda 54:241–62.