The Atlantic Puffin and the Northern Fulmar are two unrelated species that are both considered seabirds.
A seabird, or marine bird, is any species of bird that is adapted to life in a marine environment.
The term does not refer to a taxonomic entity, as it is applied to species belonging to various orders and families.
Birds that are usually considered seabirds include the members of the orders Suliformes (gannets, boobies, cormorants, darters), Procellariformes (petrels) and Sphenisciformes (penguins), as well as the families Alcidae (auks) and Laridae (gulls and terns).
Due to their aquatic lifestyle, most seabirds have well-developed oil glands, allowing them to waterproof their plumage. Cormorants and darters, which lack the oil gland and have to dry themselves in the sun after swimming, are a notable exception.
Seabirds generally feed at sea or near the coast, eating marine creatures such as fish, molluscs, crustaceans and plankton, though their hunting methods vary greatly.
Petrels are surface feeders, plucking small animals from the water surface while flying or swimming like ducks. Most gulls are also surface feeders, though they may also feed on land as they are highly adaptable.
Plunge divers such as the Northern Gannet dive into the water at high speed to catch their prey, remaining submerged rather briefly before resurfacing.
Penguins and auks are pursuit divers, able to fish by swimming underwater using their wings and feet, and to stay submerged for several minutes - up to half an hour for the Emperor Penguin. Whilst surface feeders and plunge divers are often skilled flyers, highly maneuverable and capable of covering great distances in flight, plunge divers are generally poor fliers -if not entirely flightless- due to their adaptation to swimming. Penguins evolved from flying birds that gradually lost the ability to fly, specializing in a terrestrial and marine lifestyle. Their wings, known as flippers, are narrow and stiff, and their bones, unlike those of other birds, are not hollow in order to prevent bouyancy. All living auk species, which, unlike penguins, have terrestrial predators, are capable of flying, though their flight is often clumsy due to their small wings. Landing is particularly awkward, with the bird sometimes crashing into the nesting cliff.
Seabirds are socially monogamous and many of them mate for life. The majority of seabirds nest in colonies and have a tendency to re-utilize the same nesting site each year. Some, such as the Herring Gull, live along coasts all year long, while others, such as the Guillemot, only come to land to breed, spending the rest of the year at sea. Some, such as the Arctic Tern, are long-distance migrators.