Biological Exuberance: Animal Homosexuality and Natural Diversity

Frequency: Among Jackdaws in the wild, approximately 5 percent of trios include bonding between the two female partners, while about 10 percent of widowed females form homosexual pairs. Overall, though, same-sex bonds probably represent no more than 1 percent of all pairs/trios. Homosexual activity occurs sporadically in Black-billed Magpies and Ravens as well: about 1 percent of nesting pairs of Magpies, for example, consist of two males. Although homosexual copulations between Magpies have not yet been recorded, heterosexual matings are also infrequently observed (only 9 male-female copulations, for example, were recorded during one 300—hour study period).

Orientation: Homosexual behavior is most prevalent among younger Black-billed Magpies, some of whom also court members of the opposite sex; many of these birds go on to form heterosexual attachments when they become adults, but most are nonbreeders during the time of their same-sex activities. Some adult males, however, continue to court younger males or pair with other adult males. In Jackdaws and Ravens many different forms of bisexuality occur. Some females are simultaneously bonded to both males and females, and such trios may develop out of either an initial heterosexual or homosexual pair-bond. Some females are sequentially bisexual, developing same-sex bonds only after losing their male partners. However, those Jackdaw females who enter into homosexual pairs as nonbreeders may have no prior (and perhaps no subsequent) heterosexual experience.

Nonreproductive and Alternative Heterosexualities

Black-billed Magpies and Jackdaws participate in a number of nonprocreative heterosexual activities. Birds sometimes copulate long after fertilization has taken place—even during incubation or chick-rearing—while courtship and mounting may occur well in advance of the breeding season in Magpies. In both species, birds form pair-bonds as juveniles, long before they are able to breed. Several alternatives to the monogamous pair and nuclear family are also to be found in these species of Crows. In Black-billed Magpies, for example, courtship and copulations with birds outside of the pair-bond can be more frequent than within-pair matings. In addition, some pairs get divorced: about half of all males and just under two-thirds of females change mates, and some may pair with up to three different partners over the course of their lives. Other Magpies, however, do remain in lifelong, faithful pair-bonds. About 6—10 percent of adult Jackdaw pairs, and a third of all juvenile pairs, get divorced. Polygamous trios also sometimes develop in Magpies (1—2 percent of bonds), but are fairly common in Jackdaws, comprising about 14 percent of all bonds. These usually result from an unpaired female joining an existing male-female pair; unlike the bisexual trios described above, she usually bonds only with the male of the pair, not the female. Occasionally, she may oust the former female and start a new family with the male; frequently, however, the newcomer female does not breed even if she remains in the trio for several years.

Nonbreeding couples also occur in Jackdaws and Ravens, as well as a significant population of single, nonreproducing birds. The latter are found in Magpies as well, where they make up 20—60 percent of the population and may be solitary or form their own flocks. In addition, about half of all Magpies that survive to adulthood leave no descendants (even if they try to breed). Some nonbreeding Jackdaw pairs—or couples who have tried unsuccessfully to breed—end up harassing breeding pairs by invading their nests, fighting with them, and even attacking and occasionally killing their young with vicious pecks. Nearly a third of all breeding pairs in some populations are harassed in this way. Cannibalism of young and eggs by neighboring breeding pairs also sometimes occurs in Jackdaws. Magpies and Ravens occasionally adopt unrelated youngsters when they pair with a bird that has lost its mate. Magpie families also sometimes foster-parent chicks as a result of the extraordinary phenomenon of “egg transfer”: in some populations, up to 8 percent of nests contain a foreign Magpie egg, but not as a result of being laid there. Rather, another Magpie has physically carried its own egg in its beak to the new nest, placing it there to hatch and be raised by the host parents. Stealing and cannibalism of young or eggs has also been reported among Black-billed Magpies: about 7 percent of nests are robbed by other Magpies. In addition, at least 30 percent of nestlings die of starvation as a result of competition from their siblings, or from direct attacks or cannibalism by their nest-mates.

Other Species