HomeGlossary Behaviour Lek A display ground where males gather and perform to attract females. Grouse, manakins, cocks-of-the-rock and many birds-of-paradise breed at leks. BehaviourRelated termsFull glossary Anting Rubbing ants on the feathers, or lying on an ant nest, probably to control parasites. Irruption An irregular mass movement of birds beyond their usual range, often when food runs short. Mobbing Small birds noisily harassing a predator such as an owl or hawk to drive it away. Brood parasite A bird that lays its eggs in the nests of other species, which then raise its young. Kettle A group of raptors or storks circling together in a rising thermal. Fallout Large numbers of migrants landing at once, usually when bad weather meets a migration.