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feels, and is, safer than if it lurked obscurely amid dense vegetation. In the midst of a compact flock, foraging in trees or on the ground, birds spend more time eating, less looking around or up to avoid being surprised by predators, than do individuals on the outskirts of the crowd, or those foraging alone. In these aggregations, each individual tries primarily to protect itself and increase its intake of food; but by the combined action of all, all are safer and better nourished. Because no individual willingly exposes itself to save its companions, or deliberately helps an adult not its mate to food, these foraging groups are sometimes called ''selfish herds." But is it not enough that by acting in concert the whole party benefits? Why should one individual court death to save another adult of its species? Large animals, well armed with horns, hoofs, or fangszebras, horned ungulates, baboonsmay save companions

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or dependent young by confronting powerful predators; small, weak creatures could only sacrifice themselves.

Protection from predators is the most widespread mode of mutual aid among animals. The startled cries of a bird that spies a hawk alert others within hearing, of the same or different species. These notes, frequently voiced by birds with or without dependent young, have puzzled evolutionists who believe that animals should consistently behave in ways conducive to individual survival and fitness. Why should a bird that first notices an approaching raptor draw attention to itself by its voice, when it might discreetly hide, leaving less alert companions exposed to attack? Perhaps among the birds warned by its cries are its mate or independent young, in which case its behavior is more readily explicable. But a bird may sound the alarm when no related individual is within hearing, and birds of other species may be saved by this timely warning. To be sure, the individual giving the alarm call may on another occasion profit by that of some other bird; its service may be requited. Except in the context of parents with young, alarm cries have proved difficult for evolutionists to reconcile with their theories, but they obviously benefit the caller's species by promoting the safety of its membersand often those of other species.

The great majority of birds breed in monogamous pairs, of which the male feeds and guards the young, or frequently shares with his mate all the activities of the nest, including building, incubating, and brooding the nestlings. In a minority of species, females are so well able to rear their families without help that the males are released from all domestic chores. By staying aloof, they decrease activity that might reveal nests to hostile eyes. Instead of remaining alone, the emancipated males of a number of species gather in courtship assembles, which attract females whose developing eggs need to be fertilized. Among northern birds, this mating system is followed by certain sandpipers and grouse, including the Capercaillie, Black Grouse, and Ruff in northern Eurasia, and the Sage Grouse and prairie chickens in North America. These are precocial birds whose chicks leave the nest soon after they hatch and pick up their own food under maternal protection and guidance.

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In the tropics, courtship assemblies are found chiefly among altricial birds, with broods of rarely more than two nestlingsin the New World, mainly hummingbirds and manakins; in the Australasian region, birds of paradise, larger and louder-voiced birds, which tend to be stationed farther apart, in expanded, or "exploded," leks.

A subtle balance of cooperation and competition prevails in a courtship assembly. Its members cooperate to establish a mating station that becomes well-known and accessible to the females in the vicinity because, the habitat remaining favorable, it is in the same place year after year, while the calls or wing sounds of the participants advertise its presence. At the same time, these males compete for females, whom they attract by vocalizations that,