What Is a Bird? Definition, Traits & Examples

Birds are feathered, egg-laying vertebrates — living dinosaurs. Key traits, flight adaptations, and why penguins and ostriches still count as birds.

Global Animal Guide · July 10, 2026

Bald eagle in flight — Photo: Andy Morffew from Itchen Abbas, Hampshire, UK · CC BY 2.0 · source · credits

Quick answer

Birds are vertebrates with feathers, toothless beaked jaws, hard-shelled eggs, and a high-metabolism endothermic lifestyle. All living birds descend from theropod dinosaurs. Flight is common but not required — penguins, ostriches, and kiwis are flightless birds.

Last updated: July 2026.

Birds are feathered vertebrates that lay hard-shelled eggs. Most fly; some swim or run instead. They are living theropod dinosaurs.

Defining features

Feathers (unique among living animals)
Beaks without teeth
Hard-shelled eggs
Lightweight skeletons with air spaces in many flyers
Efficient lungs with air sacs
Endothermy

Flight is optional

Wings evolved for flight, then were co-opted: penguin flippers, ostrich balance aids, kiwi vestigial stubs. Calling only flying species “real birds” is incorrect.

Ecological roles

Birds pollinate, disperse seeds, control insects, scavenge, and shape habitats. Migratory species link continents — see why birds migrate.

Frequently asked questions

What makes a bird a bird?

Feathers are the clearest living trait. Birds also lay hard-shelled eggs and have beaks without teeth.

Are penguins birds?

Yes — flightless birds specialised for swimming.

Are birds dinosaurs?

Yes — birds are the only surviving dinosaur lineage.

How many bird species are there?

Around 11,000 living species, depending on taxonomic authority.