Swallows and martins are distinctive, familiar birds with long wings, short, broad bills and a streamlined shape that aids their specialised feeding technique of catching insects on the wing. Long-tailed species are generally termed "swallows" and short-tailed species "martins", though the distinction is unscientific and not 100% applicable. Species occur on all continents except Antarctica. They tend to nest either in cavities (excavated or existing), or in distinctive nests built mainly of mud, sometimes on buildings. Most hirundines in temperate areas are summer visitors, migrating to warmer areas for the non-breeding season.