Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu. Although most birds are characterised by flight, the ratites are flightless, and several other species, particularly on islands, lost this ability. Flightless birds include the penguins, Ostrich, kiwis, and the extinct Dodo. Flightless species are vulnerable to extinction when humans or the mammals they introduce arrive in their habitat, for example the Great Auk, flightless rails, and the moa of New Zealand. Birds are a very differentiated class, with some feeding on nectar, seeds, insects, rodents, fish, carrion, or other birds. Most birds are diurnal, or active during the day. Some birds, such as the owls and nightjars are nocturnal or crepuscular (active during twilight hours). Many birds migrate long distances to utilize marginal habitats (e.g., Arctic Tern) while others spend almost all their time at sea (e.g. the Wandering Albatross). Common characteristics of birds are the ability to fly using feathered wings, a bony beak with no teeth, the laying of hard-shelled eggs, high metabolic rate, and a light but strong skeleton. Birds are among the most extensively studied animal groups, with hundreds of academic journals devoted to their study.