Birds have feathers, are warm-blooded vertebrates, and, like mammals, have a four-chambered heart. Bird characteristics also include forelimbs modified into wings, hard-shelled eggs, and keen vision. Their sense of smell is not highly developed, and their hearing range is limited. Although some groups are nocturnal, most birds are active during the day. They are more related to reptiles than mammals.
verifiedCite
While every effort has been made to follow citation style rules, there may be some discrepancies.
Please refer to the appropriate style manual or other sources if you have any questions.
Select Citation Style