The common tern breeds in colonies which do not normally exceed 2,000 pairs, but may occasionally number more than 20,000 pairs. Colonies inland tend to be smaller than on the coast. Common terns often nest alongside other coastal species, such as Arctic, roseate and Sandwich terns, black-headed gulls, and black skimmers. Especially in the early part of the breeding season, for no known reason, most or all of the terns will fly in silence low and fast out to sea. This phenomenon is called a "dread".