Terns are normally monogamous, although trios or female-female pairings have been observed in at least three species. Most terns breed annually and at the same time of year, but some tropical species may nest at intervals shorter than 12 months or asynchronously. Most terns become sexually mature when aged three, although some small species may breed in their second year. Some large sea terns, including the sooty and bridled terns, are four or older when they first breed. Terns normally breed in colonies, and are site-faithful if their habitat is sufficiently stable. A few species nest in small or dispersed groups, but most breed in colonies of up to a few hundred pairs, often alongside other seabirds such as gulls or skimmers. Large tern species tend to form larger colonies, which in the case of the sooty tern can contain up to two million pairs. Large species nest very close together and sit tightly, making it difficult for aerial predators to land among them. Smaller species are less closely packed and mob intruders. Peruvian and Damara terns have small dispersed colonies and rely on the cryptic plumage of the eggs and young for protection.