Birds are warm-blooded vertebrates that have more in common with reptiles than mammals. Like mammals, they have a heart with four chambers, forelimbs modified into wings (like bats) and lay a hard-shelled egg.
Since they do not have a good sense of smell or hearing, the most important of their senses is sight, which provides birds with almost all the information they need about their environment.
Most birds are active during the day and sleep at night, but some, like owls for example, sleep during the day and hunt at night. More than 1 000 extinct bird species have been identified from fossil remains.