Birds, warm-blooded vertebrates of the class Aves, are unique for their feathers. Feathers and other specialized body structures make birds the best fliers of the animal kingdom. However, some species, such as penguins, ostriches and kiwis are flightless. While most migratory birds use flying as a means of transportation, flightless birds walk, swim, or even float. California quail walk up and down mountains to reach their seasonal homes. Greenland's murres swim and Adelie penguins have been known to hitch a ride on passing ice floes.