Biological Exuberance: Animal Homosexuality and Natural Diversity

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BIRDS OF PREY AND GAME BIRDS

KESTREL

IDENTIFICATION: A small falcon (12-15 inches) having chestnut plumage spotted with black, and a gray head and tail in males. DISTRIBUTION: Throughout Eurasia and Africa. HABITAT: Variable, including plains, steppe, woodland, wetlands. STUDY AREA: Nivå, Denmark; subspecies F.t. tinnunculus.

GRIFFON VULTURE

IDENTIFICATION: A large vulture (wingspan up to 9 feet) with a white head and neck and brown plumage. DISTRIBUTION: Southern Europe, North Africa, Middle East to Himalayas. HABITAT: Mountains, steppe, forest. STUDY AREAS: Berlin Zoo; Jonte Gorge and other regions of the Massif Central Mountains, France; Lumbier, Spain; subspecies G.f. fulvus.

Social Organization

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.

Description

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.