What are the unique biological characteristics of birds?

Birds are warm-blooded vertebrate animals belonging to the class Aves, distinguished by feathers, toothless beaked jaws, hard-shelled eggs, and lightweight skeletons. They evolved from theropod dinosaurs and represent the only living dinosaurs today. They possess unique respiratory and digestive systems adapted for flight, though secondary loss of flight has occurred in several species.

What we know

• Birds are warm-blooded vertebrate animals characterized by feathers, toothless beaked jaws, hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. [3]
• Birds are the only surviving members of Dinosauria, having evolved from earlier theropods and primitive avialans. [3]
• Modern birds diversified dramatically around the time of the Cretaceous-Paleogene extinction event 66 million years ago. [3]
• Male birds of many species display secondary sex characteristics such as brighter coloration or external ornaments, which are products of sexual selection. [5]

Where to go next

• what caused itHow did feathers evolve in dinosaurs?
• whenWhat caused the Cretaceous-Paleogene extinction event?
• how it workedHow do hummingbird metabolisms support hover flight?
• compared withWhat are the closest living relatives of birds?
• the bigger pictureWhat defines life scientifically?
• an unexpected connectionHow do male birds share traits with horned antelopes?Both male birds and horned antelopes develop conspicuous secondary sex characteristics driven by sexual selection to signal fitness to rivals and mates.

Sources

• [1]life (Wikidata Q3) · CC0 1.0
• [2]Life · CC BY-SA 4.0
• [3]Bird · CC BY-SA 4.0
• [4]Hummingbird · CC BY-SA 4.0
• [5]Secondary sex characteristic · CC BY-SA 4.0