Gulls are birds in the family Laridae. They are most closely related to the terns (family Sternidae), auks and skimmers, and more distantly to the waders. Most gulls belong to the large genus Larus. They are in general medium to large birds, typically grey or white, often with black markings on the head or wings. They have stout, longish bills and webbed feet. Most gulls, particularly Larus species, are ground nesting carnivores, which will take live food or scavenge opportunistically. The live food often includes crabs and small fish. Apart from the kittiwakes, gulls are typically coastal or inland species, rarely venturing far out to sea. The large species take up to four years to attain full adult plumage, but two years is typical for small gulls. Gulls — the larger species in particular — are resourceful and highly-intelligent birds, demonstrating complex methods of communication and a highly-developed social structure. Certain species (e.g. the Herring Gull) have exhibited tool use behaviour. Many species of gull have learned to co-exist successfully with man and have thrived in human habitats. Others rely on Kleptoparasitism to get their food. Two terms are in common usage among gull enthusiasts for subgroupings of the gulls: Large white-headed gulls for the 16 Herring Gull-like species from Great Black-backed Gull to Lesser Black-backed Gull in the taxonomic list below White-winged gulls for the two Arctic-breeding species Iceland Gull and Glaucous Gull Hybridisation between species of gull occurs quite frequently, although to varying degrees depending on the species involved (see Hybridisation in gulls). The taxonomy of the large white-headed gulls is particularly complicated. In common usage, members of various gull species are often called sea gulls or seagulls. This name is used by laypeople to refer to a common local species or all gulls in general, and has no fixed taxonomic meaning.