Biological Exuberance: Animal Homosexuality and Natural Diversity

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BLACK-RUMPED FLAMEBACK

IDENTIFICATION: A medium-sized, red-crested woodpecker with a golden back, black rump, and black-and-white patterning on the face and neck. DISTRIBUTION: India, Pakistan, Sri Lanka. HABITAT: Woodland, scrub, gardens. STUDY AREA: Near Chittur, India; subspecies D.b. puncticolle.

ACORN WOODPECKER

IDENTIFICATION: A red-capped woodpecker with a striking black-and-white face, black upperparts, white underparts, and a black breast band. DISTRIBUTION: Pacific and southwest United States, Mexico through Colombia. HABITAT: Oak and pine woodland. STUDY AREAS: Hastings Natural History Reservation (Monterey) and near Los Altos, California.

Social Organization

Acorn Woodpeckers have an extraordinarily varied and complex social organization. In many populations, birds live in communal family groups containing up to 15 individuals—typically there are as many as 4 breeding males and 3 breeding females in such groups (though nonbreeding groups also occur—see below). The remaining birds in a group are nonbreeding “helpers” that may share in the parenting duties. Within groups, the mating system is known as POLYGYNANDRY, that is, each male mates and bonds with several females and vice versa. In other populations, monogamous pairs as well as other variations on polygamy occur. Little is known about the social organization of Black-rumped Flamebacks, although it is thought that they form monogamous mated pairs.

Description

Behavioral Expression: Male Black-rumped Flamebacks sometimes copulate with each other. One male mounts the back of the other (as in heterosexual mating), bending his tail down and thrusting it under the belly of the other male to make cloacal (genital) contact. Reciprocal mounting may occur, in which a male copulates with a male that has just mounted him. A bird involved in same-sex mounting activity may adopt a distinctive posture, in which his body is perpendicular to the branch he is perching on and his wing tips are arched toward the ground and hanging below his feet. Males also sometimes drum against a tree trunk prior to homosexual mounting.

Acorn Woodpeckers participate in a fascinating group display that involves ritualized sexual and courtship behavior, including homosexual mounting. At dusk, the members of a group gather together prior to roosting in their tree holes. As more and more birds arrive, they begin mounting each other in all combinations—males mount females and other males, females mount males and other females, young Woodpeckers mount older ones and vice versa. The mounting behavior resembles heterosexual mating, except it is usually briefer and cloacal contact is generally not involved (although genital contact does sometimes occur). Reciprocal mountings are common, and sometimes two Woodpeckers will try to mount the same bird simultaneously. Following the display, group members fly off to their roost holes to sleep. Ritualized mounting may also occur at dawn when the birds emerge from their roost holes. Because many group members are related to each other, at least some of this mounting is incestuous. Female Acorn Woodpeckers often coparent together, both laying eggs in the same nest cavity. Such “joint nesters” are often related (mother and daughter, or sisters), but sometimes two unrelated females nest and parent together as well. Joint-nesting females may continue to associate even if they happen not to breed in a particular year.

Frequency: Homosexual behavior in Black-rumped Flamebacks probably occurs only occasionally; however, heterosexual mating has never been observed in this species in the wild, so much remains to be learned about the behavior of this Woodpecker. The mounting display of Acorn Woodpeckers—including homosexual mounting—is a regular feature of the social life of this species at all times of the year, occurring daily in most groups. More than a third of all female Acorn Woodpeckers nest jointly, and about a quarter of all groups have joint-nesting females; 14 percent of these joint nests involve unrelated females as coparents.