Birds are warm-blooded like mammals but lay eggs, have feathers and wings, and there are over 9,000 species worldwide. Males sing primarily to mark territory and attract mates, though some species like northern cardinals both sexes sing, and singing can scare away other males while attracting females. Bird songs vary and young birds remember their parents' unique songs. Common types of birds include backyard birds, ground birds, hawks/eagles/vultures, hummingbirds, owls, seabirds, shorebirds, swimming birds, wading birds, and woodpeckers.