The cuckoo is the only 'brood parasite' to breed in Britain (3). Individual females prefer certain foster birds, and lay eggs that closely mimic those of the foster species, 50 of which are known (3). A female will establish a territory encompassing a number of potential foster nests, and carefully observe activity, waiting until the nests are at the right stage. She then swiftly takes her chance, swooping down, ejecting an egg and laying one of her own (3). The unsuspecting host bird then incubates and feeds the impostor, who removes other eggs and young from the nest and often grows much larger than its foster parent (3). Female cuckoos usually lay fewer than 12 eggs in 12 different host nests each year (3). Cuckoos feed mainly on insects, spiders and worms (3).