HomeGlossary Behaviour Philopatry The tendency to return to breed at or near the place of birth. High philopatry makes some island and seabird populations especially vulnerable. BehaviourRelated termsFull glossary Kettle A group of raptors or storks circling together in a rising thermal. Anting Rubbing ants on the feathers, or lying on an ant nest, probably to control parasites. Fallout Large numbers of migrants landing at once, usually when bad weather meets a migration. Lek A display ground where males gather and perform to attract females. 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.