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Monk Parakeets, and Sociable Weavers build compound nests in which each pair occupies a separate chamber, and anis make a simple nest in which several pairs raise a compound family, another mode of cooperation is much more widespread among birds: a single breeding pair is assisted by one or more nonbreeding helpers. At the bulky nests of sticks built by the big, raucous Brown Jays in Caribbean Central America, the mated pair is usually aided by younger birds that are readily distinguished by their bills, colored black and yellow in the most diverse patterns. Once I watched seven grown jays, including the parents and five helpers, feed and guard a brood of three nestlings; every nest that I studied had at least one young assistant. These helpers sometimes fed the incubating female. In a population of Florida Scrub Jays, Glen E. Woolfenden (1975) learned that nearly all the yearlings helped their parents to feed and defend nestlings and fledglings, and about half the two-year-old jays did so. Breeding pairs assisted by their older, nonbreeding offspring produced substantially more young than did pairs without helpers.

Beneath heavy rain forest in the Caribbean lowlands of Costa Rica, long burrows in the ground, which in one case sheltered a brood of three nestlings, were attended by three or four White-fronted Nunbirds, all bringing food in their vivid orange-red bills. In England, a cozy nest of Long-tailed Tits is often attended by

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one or two adults in addition to the parents, all bringing food in perfect harmony. In the highlands of Guatemala, the beautiful, lichen encrusted, downy pouches of Bush-Tits attract from one to three unmated birds wearing the black facial masks of the males. These helpers sometimes brood as well as feed the four nestlings. As a reward for their services, they are permitted to sleep with the parents and young in the swinging pouch, during the cold nights of high altitudes.

In Australia, breeding pairs of Superb Blue Wrens are often assisted by extra males, usually their offspring of earlier years. Ian Rowley's (1965) careful studies showed that pairs with helpers raised nearly twice as many nestlings per nest as unassisted pairs, and about 50 percent more nestlings for each attendant adult. In many other resident birds of warmer regions, similar breeding groups, consisting of a mated pair with one or several unmated helpers, have been discovered; and the list of avian species that practice such cooperation continues to grow as tropical birds are more extensively studied (Brown 1987, Skutch 1987, Stacey and Koenig 1990).

Among the many modes of mutual aid among birds, none is more surprising than that practiced by the unhatched chicks of certain nidifugous species. The slight sounds made by quail chicks, as they break out of the eggshell, stimulate somewhat younger chicks in other eggs in the same nest to pip their shells sooner than they would otherwise do, with the result that all hatch more or less simultaneously and can be more promptly led off to the feeding ground by their parents. This doubtless unconscious cooperation contrasts strongly with the behavior of the young of certain raptorial birds, whichsometimes when only a few days oldmurder, and perhaps afterward devour, their slightly younger or weaker nest mates, thereby removing competition for food (Johnson 1969).

Sometimes a lactating female mammal will suckle an orphaned young, of her own or even another species; as when a Blue Wildebeest who had lost her calf gave milk to a motherless Burchell's Zebra. But helpers do not fit into the mammalian system of reproduction as readily as they do into the avian system and, with the

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exception of a few cases reported for jackals and wolves, cooperation in rearing the young is largely limited to protecting the mother and her offspring. Among elephants, zebras, wild horses, and horned quadrupeds of various kinds, the males defend the females and young. When threatened by wolves or other predators, Muskoxen of the Arctic tundra form a circle