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 Brood parasite A bird that lays its eggs in the nests of other species, which then raise its young. Lek A display ground where males gather and perform to attract females. Kettle A group of raptors or storks circling together in a rising thermal. Mobbing Small birds noisily harassing a predator such as an owl or hawk to drive it away. Irruption An irregular mass movement of birds beyond their usual range, often when food runs short. Fallout Large numbers of migrants landing at once, usually when bad weather meets a migration.