Kenyahs, Kayans, and Kajangs live in highly stratified societies. Aristocrats ( ipun urna or keta'u ) are politically dominant and allied with each other through marriage over an area that crosses tribal and linguistic boundaries. Their wealth is in gongs, beads, and jars. They control the use of bird's nest caves (where swallows' nests, used for food, abound), and because they own slaves they are able to grow much more food than can commoners. Middle-class commoners (farmers and craftsmen) are known as panyun. Slaves ( lupau or lepen ) are the descendants of prisoners of war. Within the village, each longhouse has a headman, who is an aristocrat. In villages having more than one longhouse, one of the longhouse headmen is also a village headman. There is no political unity above the level of the village, although in the past large war parties composed of the men of several villages were organized. Kayan headmen also receive free agricultural labor from village members.