What Is A bird? What Type Of Animal Is A Bird & How Birds Evolved

What Is A Bird?

A bird is a vertebrate animal belonging to the class Aves. A bird has feathers, wings and a toothless beak, and most birds can fly. All birds are descended from dinosaurs, and today, most biologists consider birds to be dinosaurs. The term ‘avian dinosaurs’ can be used to differentiate birds from other dinosaurs.

Because birds are descended from dinosaurs, they may be grouped with reptiles (because dinosaurs are reptiles). However, in most cases the term ‘reptiles’ is used to describe all reptiles apart from those of class Aves (i.e., birds).

The reptilian origin of birds becomes apparent when you consider that the closest living relatives of birds are crocodilians – animals such as crocodiles, alligators and caimans.

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Characteristics Of Birds

There are around 10,000 living species of birds. Although birds vary considerably in terms of size, appearance and behavior, all share a certain set of characteristics.

All birds belong to the class Aves.

Some characteristics of birds are listed below:

Birds are vertebrates, which means that, like other animals such fish, amphibians, reptiles and mammals, they have a backbone.

Birds, like crocodilians and mammals, have four-chambered hearts. Like mammals, birds are warm-blooded.

Like crocodilians and most other reptiles, birds lay eggs with hard shells that can survive out of water.

Most birds can fly, and many bird characteristics are connected with the ability to fly. Even flightless birds, such as ostriches, emus and rheas, are descended from flying birds.

Bird Blackbird
Birds have feathers, wings, and beaks without teeth.

The bones of a bird’s skeleton are pneumatized (filled with air cavities) for lightness.

Birds have feathers, which, as well as being used in flight, have numerous other uses, including temperature regulation, camouflage, and protection. (It’s now known that feathers evolved before the ability to fly.)

Birds have beaks, and don’t have teeth. The expression “as rare as hen’s teeth”, which is used to describe something rare or non-existent, stems from the fact that hens don’t have teeth.

Many birds build nests and take good care of their young. These are characteristics that are also seen in crocodilians, and which may also have been seen in dinosaurs.

While other animals may possess one of more of these avian characteristics, only birds have them all. For example, bats, like birds, can fly, yet bats are mammals, not birds. Insects can fly and lay eggs, but insects don’t have backbones, and are therefore not birds.

Are Birds Dinosaurs?

tyrannosaurus rex
Tyrannosaurus rex was a theropod - just like every bird!

Today, most scientists consider birds to be dinosaurs. Specifically, birds are theropod dinosaurs belonging to the group Maniraptora.

Maniraptora are a group of dinosaurs that appeared during the Late Jurassic Period. Not all maniraptorans could fly, but it is thought that all had feathers.

Birds evolved from flying maniraptorans such as Archaeopteryx, which lived around 150 million years ago.

Archaeopteryx is an example of a transitional fossil, a fossil that has characteristics of both an earlier group of species and a later group of species that are descended from the earlier group.


Archaeopteryx had characteristics of both birds (feathers and wings) and dinosaurs (teeth and a long, bony tail). Its discovery allowed biologists of the time, such as Thomas Henry Huxley, to make the connection between birds and dinosaurs.

The discovery of Archaeopteryx was also important because it provided evidence for Charles Darwin’s theory of evolution.

Are Birds Reptiles?

Bird Reptile

Using a modern (cladistic) approach to animal classification, birds can be considered to be reptiles. This is because birds are directly descended from reptiles (dinosaurs).

However, in general use, the term ‘reptile’ excludes members of Aves (birds).

The closest living relatives of birds in the animal kingdom are the members of Crocodilia – a group that includes animals such as crocodiles, alligators and caimans.

Are Birds Crocodiles?

Birds are not crocodiles, but crocodiles and other members of the order Crocodilia are their closest relatives in the animal kingdom.

Birds and crocodilians are descended from a common ancestor – an archosaur that lived in the Triassic period.

At some point during the Triassic Period, the archosaurs split into two main groups: one group included the ancestors of dinosaurs (and therefore birds); the other included the ancestors of today’s crocodilians.

The archosaurs branched off from the ancestors of today’s turtles, snakes and lizards before splitting into dinosaurs / birds and crocodilians. Therefore, crocodiles are more closely related to birds than they are to other reptiles such as snakes and turtles.

Types Of Birds

The common ostrich is the largest type of bird.

  • You can find out more about the different types of bird on this page: Types of Bird

All birds are descended from dinosaurs, and all have many characteristics in common. However, some birds look very different to others.

Birds range in size from the tiny bee hummingbird (Mellisuga helenae), which lives in Cuba and is just over 6 cm (2.36 in.) long, to the common ostrich, which lives in Africa and is 2.8 meters / 9.19 ft.) in height.

Despite their difference in size and appearance, both the bee hummingbird and the common ostrich possess characteristics that make them birds, rather than other types of animals.

The ostrich belongs to a group of birds known as ratites, most of which are flightless. Other ratites include the rheas from South America, and cassowaries from Southeast Asia and Australia.

Other flightless birds include penguins. Found on the cold coastlines of the southern hemisphere, penguins have numerous adaptations for living in this inhospitable habitat. With their short, powerful wings, penguins can “fly” through the water in pursuit of fish and other prey.

It is the ability of birds to be able to fly that sets them apart from most other animals. Although birds are not the first animals to be able to fly (both insects and pterosaurs were able to fly millions of years before dinosaurs / birds), they have mastered the art.

Some birds – such as the Arctic tern or wandering albatross – undertake migrations that take them thousands of miles every year. Hummingbirds have evolved a unique wing mechanism that allows them to hover, and to travel sideways and backwards.

The peregrine falcon and other predatory birds, such as the saker falcon and golden eagle, can dive at speeds approaching 200 mph / 320 kmph, and swifts such as the white-throated needletail and common swift can reach speeds of over 100 mph in horizontal flight.

What Type Of Animal Is A Bird: Further Reading

Love birds? Test your knowledge of local species with our bird quiz apps:

You can find out more about birds on the following pages:

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