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July 26, 2026Sources cited
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Bird

Birds are a group of warm-blooded vertebrates in the class Aves, characterized by feathers, beaks, hard-shelled eggs, and a high level of adaptation for flight in many species.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

Birds are a group of warm-blooded vertebrates in the class Aves, characterized by feathers, beaks, hard-shelled eggs, and a high level of adaptation for flight in many species. They occur worldwide and include more than 11,000 living species, ranging from tiny hummingbirds to large flightless birds such as ostriches.

Characteristics

Birds are vertebrates distinguished by feathers, beaks, and laying hard-shelled eggs. Many species have adaptations for flight, although some are flightless and have evolved different forms of locomotion.

Classification

Modern birds are placed in the crown group Aves, or Neornithes, and are commonly divided into Palaeognathae and Neognathae. These groupings reflect major evolutionary lineages within living birds.

Diversity and distribution

Birds live throughout the world and include more than 11,000 known living species. They range from tiny species such as the bee hummingbird to very large birds such as the common ostrich.

Study of birds

The scientific study of birds is called ornithology. Reference works and databases on birds commonly cover taxonomy, distribution, behaviour, migration, breeding, and conservation status.

Key facts

  • Birds are the only living animals with feathers.
  • Most bird species are capable of flight, but several lineages are flightless.
  • Modern birds are generally classified within the crown group Aves, also called Neornithes.
  • Bird species occupy a wide range of habitats on every continent.
  • Birds vary greatly in size, ecology, and behaviour, from passerines to large flightless species.
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Canadian context

Birds are an important part of Canada’s wildlife, with many native, migratory, and breeding species recorded across the country.

Frequently asked questions

What is a bird?
A bird is a warm-blooded vertebrate in the class Aves, usually identified by feathers, a beak, and egg-laying reproduction.
How many bird species are there?
There are more than 11,000 living bird species, though exact counts vary by source and taxonomy.
Do all birds fly?
No. Many birds fly, but some, including ostriches and other flightless species, do not.
What is the smallest bird?
The bee hummingbird is generally recognized as the smallest living bird.
What is the largest bird?
The common ostrich is the largest living bird.

References

  1. Avibase — https://avibase.bsc-eoc.org/
    Supports: Bird taxonomy, species counts, and global bird database scope.
  2. Wikipedia — https://en.wikipedia.org/wiki/Bird
    Supports: General overview of birds, classification, species count, size extremes, and living bird lineages.
  3. Audubon — https://www.audubon.org/bird-guide
    Supports: Bird diversity in North America and species guide context.
  4. RSPB — https://www.rspb.org.uk/birds-and-wildlife/a-z
    Supports: Bird species listing and diversity context.