HomeGlossary Behaviour Kettle A group of raptors or storks circling together in a rising thermal. At migration bottlenecks such as Veracruz or Batumi, kettles can hold thousands of birds. BehaviourRelated termsFull glossary Philopatry The tendency to return to breed at or near the place of birth. Irruption An irregular mass movement of birds beyond their usual range, often when food runs short. Lek A display ground where males gather and perform to attract females. Brood parasite A bird that lays its eggs in the nests of other species, which then raise its young. Anting Rubbing ants on the feathers, or lying on an ant nest, probably to control parasites. Mobbing Small birds noisily harassing a predator such as an owl or hawk to drive it away.