Unusual Bird Families

on nature focus The Unusual Married Life of the Handsome HARRIS' HAWK

HARRIS' HAWKS (PARABUTEO UNICINCTUS) exhibit a most unusual homelife. In the spring, when other desert birds pair off in the tradi-tional manner, these hawks are likely to form trios with two males courting and mating with the same female. The three hawks then stay together to feed the brood that results from the eggs the female lays.

In the lingo of biologists who study this sort of thing, the hawks are engaged in polyandry, a rare practice among birds. The more typical monogamy among birds exists perhaps because in most species males with a tendency to share the same mate would leave few descendants to carry on the practice in the future. In the case of Harris' hawks, the usual menage a trois consists of a primary or alpha male who does most of the mating with the female and a secondary or beta male who mates relatively infrequently and therefore presumably sires few offspring. The polyandrous beta male would seem to be much better off finding a female of his own, but he does not.

For some reason, polyandry persists among Harris' hawks, and biologists now believe the practice actually helps beta males to leave more descendants than any other option avail-able to them.

Part of the basis for this lifestyle may stem from the hawks' special fondness for jackrabbit meat. Jackrabbits can weigh three times as much as a single Harris' hawk, making them difficult for solitary birds or even pairs to hunt with success. But when a group hunts together, one bird can flush a rabbit from shel-ter while the others wait nearby to join in on the pursuit and pounce. Once a jackrabbit has been downed, there is plenty of food for all the cooperators and for any young back in the nest. Thus the diet of the hawks means that trios can feed their offspring more reliably than can a single pair of adults.

But the question remains: Why does a secondary male help bring jackrabbit meals to a batch of offspring fathered largely or entirely by the alpha male in the trio?

A second key to polyandry in this species may lie in the unbalanced gender ratio of Harris' hawks. For reasons still unknown, adult males outnumber adult females-another most unusual phenomenon. The excess of males means that not all of them are going to find a mate with whom they could set up monoga-mous housekeeping.

The younger, less competitive males are therefore faced with two options: to not breed at all or to join a pair as a secondary male. Non-breeders obviously have no chance to leave descendants that year, whereas secondary males may father an offspring or two, while also establishing a long-term relationship with a female. If the alpha male should die, the secondary male may ascend to alphahood and be rewarded reproductively for his past sacrifices. The beta male's sons will inherit his tendency to accept secondary status. They will help to perpetuate polyandry in this handsome hawk-a fact that would spread dismay among jackrabbits, if only they knew about it. AlH