Avian Taxonomy: How Birds Are Classified

Avian taxonomy is the scientific system used to classify and organize birds based on shared characteristics and evolutionary relationships. It helps ornithologists understand how different bird species are related, how they evolved, and how to identify them accurately. Modern bird classification combines traditional anatomy-based methods with advanced genetic research.

The Taxonomic Hierarchy

Birds belong to the class Aves, within the phylum Chordata. Taxonomy follows a structured hierarchy:

Kingdom

Phylum

Class

Order

Family

Genus

Species

For example, the American robin is classified as:
Class: Aves
Order: Passeriformes
Family: Turdidae
Genus: Turdus
Species: Turdus migratorius

This binomial (two-name) system was developed by Carl Linnaeus in the 18th century and remains the foundation of modern naming conventions.

Major Divisions of Birds

All living birds are placed in Class Aves, but they are divided into two major groups based on evolutionary lineage:

1. Paleognathae

This group includes large, mostly flightless birds such as ostriches, emus, and kiwis. They have primitive skeletal features, particularly in the palate structure of the skull.

2. Neognathae

This is the much larger group, containing nearly all modern bird species. It includes everything from songbirds to raptors and waterfowl.

Major Orders of Birds

There are over 40 recognized bird orders, but a few dominate in diversity:

Passeriformes (Perching birds) – The largest order, comprising more than half of all bird species. Includes sparrows, warblers, finches, and crows.

Accipitriformes – Hawks and eagles.

Falconiformes – Falcons and caracaras.

Anseriformes – Ducks, geese, and swans.

Strigiformes – Owls.

Piciformes – Woodpeckers and toucans.

Passeriformes alone accounts for over 6,000 species, making it the most diverse vertebrate order on Earth.

Modern Phylogenetics and DNA

Historically, birds were grouped based on visible traits such as beak shape, foot structure, and feeding behavior. However, DNA sequencing has revolutionized avian taxonomy. Genetic studies have revealed surprising relationships—for example, falcons are more closely related to parrots and songbirds than to hawks.

Organizations like the American Ornithological Society and the International Ornithologists’ Union regularly update official bird lists based on new genetic findings. As a result, species are sometimes split into multiple species or merged based on evolutionary evidence.

Why Avian Taxonomy Matters

Understanding bird classification is more than academic. It helps:

Clarify evolutionary history

Improve conservation planning

Standardize bird identification worldwide

Track biodiversity changes

For birdwatchers, taxonomy explains why certain species share similar behaviors or habitats. For scientists, it provides the framework for studying evolution, migration, and adaptation.

In essence, avian taxonomy is the roadmap of bird diversity—constantly refined as new discoveries reshape our understanding of the avian tree of life.

 

This taxonomic scheme tells us that the American Robin, belonging to the Order Passeriformes, is a passerine (songbird), along with about half of the birds in the world. Some of these birds, along with the robin, belong to the Turdidae, the thrush family - containing not only thrushes, but solitaires, chats, bluebirds, and the Redwing, Fieldfare. and Eurasian Blackbird. Further narrowing the classification, the robin has the genus name of Turdus, as do the true thrushes and various other species such as solitaires, cochoas, ant-thrushes, and bluebirds. Now we are down to migratorious, which tells us the specific bird, the American Robin, and no other. Turdus migratorious is the binomial scientific name, belonging to no other creature. This taxonomic scheme puts every species in its place relative to other species. The official definition of a species is that it is reproductively isolated from all other species. That's a pretty good definition, but it doesn't always hold. We'll discuss that in another blog.

For many hundreds of years, the similar appearances of birds made naturalists think they were related. Sometimes they were but often they were not. The below share many similarities - size, bright colors, behavior, diet, etc. but the sunbird belongs to the songbird order, Passeriformes, while the hummingbirds are in the Caprimulgiformes, the nightjar order. They are no more closely related to each other than they are to ostriches!

Notice that family names (Turdidae, Sylviidae, Paridae, Gaviidae) are capitalized and end in -idae while order names (Passeriformes, Anseriformes, Podicipiformes, Apodiformes) are capitalized and end in -iformes. Scientific and common names are further explained in Names.

There are such things as subspecies, versions of species that differ in color, size, song or some other aspect but that can interbreed. Their scientific name is a trinomial. One example is  the Tundra Swan which as two subspecies: theTundra Swan (Whistling), Cygnus columbianus columbianus and the Tundra Swan (Bewick’s), Cygnus columbianus bewickii. 

Note that "species" is always "species", whether we are discussing one or more than one species. Never "specie", the word for a coin.