Birds belong to the class Aves and are a group of endothermic vertebrates that have feathers, wings, and lay hard-shelled eggs. There are around 10,000 living species of birds that inhabit ecosystems across the globe. Birds are found on every continent and have adapted to fly, swim, walk, run, hop or stay grounded. Their unique features and adaptations allow them to thrive in aquatic, terrestrial, and aerial environments.
Birds are considered avian dinosaurs and are believed to have evolved from theropod dinosaurs during the Jurassic period around 150-200 million years ago. All modern birds are descendents of a common ancestor to the superorder Neornithes. Birds are unique in their high metabolic rates, efficient respiratory systems, light skeletal systems, toothless beaked jaws, and amazing flying capabilities.
Classification
Birds are placed in the kingdom Animalia, meaning they are complex multicellular organisms that acquire nutrients by ingesting food, grow, move, reproduce and possess sensory abilities. More specifically, they belong to the subphylum Vertebrata, or vertebrates, which are animals that possess a backbone and spinal column.
The class that birds belong to is Aves or aves. All modern birds are members of the superclass Tetrapoda, which are vertebrates with lungs and four limbs. Birds are further divided into 2 subclasses based on anatomical differences:
Subclass Neornithes
This subclass contains all living birds today that have the ability to fly. Neornithes translates to “new birds” and this group emerged during the Cretaceous period around 100 million years ago. They are characterized by a lightweight skeleton, toothless beak covered in keratin, excellent vision, feathers, wings, and a four-chambered heart.
Neornithes are further classified into 2 infraclasses:
- Paleognathae – flightless birds like ostriches, emus, kiwis
- Neognathae – all other birds capable of flight
Subclass Enantiornithes
This subclass is composed of several primitive and extinct lineages of birds that lived alongside dinosaurs in the Cretaceous period. They went extinct alongside non-avian dinosaurs around 66 million years ago. Enantiornithes means “opposite birds” which refers to the orientation of their shoulder joints. They were some of the earliest birds capable of powered flight.
Major Bird Taxonomy Groups
There are around 30 orders of modern Neornithes birds with diverse adaptations and evolutionary origins. Below are some of the major groups:
Passeriformes – Perching Birds
This order contains over half of all bird species, with around 6,000 species including songbirds, parrots, falcons, crows. They are characterized by feet with 3 toes pointing forward and 1 toe pointing backward which allows them to perch on branches. Other features are vocal capabilities and small size.
Examples: sparrows, warblers, finches, crows, owls
Galliformes – Landfowl
This order has over 290 species of heavy-bodied ground feeding birds like turkeys, grouse, chickens, pheasants, and quail. They are omnivorous, feed on the ground, have short and thick necks, small heads, and short and wide bills.
Examples: turkeys, chickens, pheasants, quail
Anseriformes – Waterfowl
This order contains ducks, geese, swans that frequent water environments. They have aquatic adaptations like webbed feet, bills with lamellae to filter feed, and waterproof plumage. There are around 150 species in this order.
Examples: ducks, geese, swans
Charadriiformes – Shorebirds
Shorebirds have around 370 species of gulls, auks, plovers that inhabit marine coasts and shorelines. Their bills are adapted for probing mud and sand. They have long legs, short tails, long pointed wings, and webbed feet.
Examples: gulls, terns, auks, plovers
Accipitriformes – Birds of Prey
This order has around 240 species of eagles, hawks, kites, ospreys. They are carnivorous with excellent vision and flight capabilities to hunt prey. Their feet have large talons for grasping and killing.
Examples: eagles, hawks, kites, ospreys
Struthioniformes – Flightless Birds
This order contains large flightless birds like ostriches, emus, rheas, kiwis. They have reduced wings, flattened breastbones, large bodies, and long powerful legs. There are around 60 species, most of which live in the Southern Hemisphere.
Examples: ostriches, emus, rheas, kiwis
Pelecaniformes – Tropicbirds
Pelicans, herons, ibises make up this order of water and marine birds. They have four webbed toes, long bills used for spearing fish, and extendable throat pouches. There are around 120 species.
Examples: pelicans, herons, ibises
Columbiformes – Pigeons and Doves
There are over 300 species in this order known for their small heads, dense plumage, small bills, and gentle nature. They feed on fruit, plants and walk on land able to perch in trees.
Examples: pigeons, doves
Cuculiformes – Cuckoos
The cuckoo family has around 140 species of slender arboreal birds that have zygodactyl feet with two toes facing forward and two facing backward. This allows them to perch vertically on branches.
Examples: cuckoos, roadrunners
Coraciiformes – Kingfishers
This order has rollers, kingfishers, bee-eaters characterized by large heads, long bills, and short legs. There are over 200 species that occupy trees near water and prey on insects, small reptiles and fish.
Examples: kingfishers, bee-eaters
Piciformes – Woodpeckers
Woodpeckers make up this order of climbers that frequent tree trunks. Adaptations like sharp claws, stiff tail feathers, and chisel-like bills allow them to drill holes in wood. There are around 400 species worldwide.
Examples: woodpeckers, flickers, toucans
Evolutionary History
Birds evolved from feathered theropod dinosaurs like archaeopteryx during the Middle to Late Jurassic period over 150 million years ago. They adapted powered flight capabilities around 100 million years ago and diversified rapidly alongside flowering plants. Below is a timeline of major events in bird evolution:
Jurassic Period (200-145 million years ago)
- Origin of birds from small feathered theropod dinosaurs like archaeopteryx
- Development of wishbone and wings with flight feathers
- Birds like archaeopteryx had teeth, claws, long tails, and limited flying abilities
Cretaceous Period (145-66 million years ago)
- Enantiornithes were abundant and widespread
- First modern birds emerge (Neornithes)
- Diversification of early lineages into waterbirds, landbirds, etc.
- Extinction of Enantiornithes
Paleogene Period (66-23 million years ago)
- Rapid adaptive radiation of most modern bird orders and families
- Gigantic flightless birds dominate
- Highly predatory toothed birds exist
- Birds dispersed worldwide
Neogene Period (23-2.6 million years ago)
- Songbirds (oscines) become abundant and diversify
- Continuedspread and diversification of modern bird groups
- Extinction of large terror birds
The evolutionary history of birds is dynamic with periods of transition, adaptation, and proliferation into new environments. Their unique adaptations have allowed birds to exploit flight and disperse across the planet.
Conclusion
Birds belong to the class Aves which emerged as feathered theropods over 150 million years ago and diversified into tens of thousands of species. All living birds belong to the subclass Neornithes and are characterized by adaptations like feathers, wings, light skeletons, beaks and toothless jaws. Major orders like passerines, shorebirds, birds of prey, galliformes, and waterfowl occupy critical niches worldwide. The evolutionary history of birds is marked by transitions from toothed birds like archaeopteryx to modern flying birds. Their unique adaptations demonstrate how flight evolved to allow dispersal into diverse habitats and ecological niches. Understanding the classification and diversity of birds provides insight into vertebrate evolution and adaptations over the past 150 million years since the age of dinosaurs.