Rheas are distinguished from ostriches by their three-toed feet (those of the ostrich have two), their lack of fine plumes, and their brownish colour. The birds frequent open, treeless country and evade predators by running. They are omnivorous and can utilize a wide variety of plant and animal foods. Rheas do not form lasting pairs, and males incubate the eggs and raise the young. Moreover, the birds are polygamous, i.e., the male broods the eggs of several females laid in one nest. The females lay up to 50 eggs—about 13 cm (5 inches) long—in a shallow, grass-lined nest dug by a male in the ground. He then incubates the eggs, and the chicks hatch in about 6 weeks and are herded about by the male for another 6 weeks thereafter. Rheas frequently associate with deer or guanacos, forming mixed herds like those of ostriches, zebras, and antelopes.