Are Birds Mammals? (All You Need To Know)

Are Birds Mammals? (All You Need To Know)

All living plants and animals on earth are classified into various categories based on their form and structure, as well as their evolutionary history. This fascinating branch of science is known as taxonomy and it helps scientists name the different species and understand how they are related.

The last common ancestor of birds and mammals was thought to live over 300 million years ago, and the differences between these two animal classes today are plain to see. So can birds be mammals? What are the differences between the two?

Birds are not mammals, although both are chordates from the Animalia kingdom. In fact, birds belong to their own class known as the Aves while mammals are from the class known as Mammalia. Birds evolved from a group of dinosaurs known as theropods over 150 million years ago. This is a little later than the mammals, which first emerged 200 million years or more before today.

The different evolutionary histories of birds and mammals can be demonstrated pretty neatly on a cladogram, a sort of family tree of life forms. Birds and mammals are on distinct branches, but they share many common ancestors if you go back far enough.

Time and countless generations have separated modern birds and mammals dramatically from those long-extinct common ancestors and this taxonomic distinction means that no bird is a mammal and vice versa.

Modern birds and mammals may have many differences, but we also have a lot in common. Read along as we dive into what makes mammals like us different from our feathered friends.

Birds are not mammals, and instead, are classified as Aves

Birds are not mammals, and instead, are classified as Aves

How are birds different from mammals?

Birds differ from mammals in many ways. Some of these differences are plain to see, while others require more careful observation of their behaviors. Some differences can’t be seen without taking a close look at how their skeletons and organs differ.

Continue reading to learn more about how birds differ from mammals.

Physical differences


One of the most obvious differences between birds and mammals is that our feathered friends have wings. All birds have wings, including flightless species like ostriches and penguins. Bird wings are modified forearms, with the hand bones fused together. Bats are the only mammals that have wings, although some mammals like flying squirrels can glide using membranes between their limbs.

European Bee-Eater with spread wings

European Bee-Eater with spread wings


Another clear difference between birds and mammals is the bill. Birds have toothless, keratinized bills instead of toothed jaws like mammals. Bird bills are interesting structures that come in a huge range of shapes and sizes, from the tiny conical bill of the common redpoll to the enormous colorful bill of the toucan.

Birds lack teeth and heavy bony jaws because the weight would make flying that much more difficult. The bird bill is more than just a tool for eating, however. Their bills are used for all sorts of tasks like cleaning themselves, spearing fish, digging holes, and even making sounds.

Mammals do not have bills, although there is one exception. The duck-billed platypus from Australia is an interesting animal. These unusual mammals have a soft, duck-like bill and no teeth.

Close up of a perched Common Redpoll

Close up of a perched Common Redpoll


Birds are the only living animals that have feathers. Feathers are unique structures that birds use for insulation and flight. Birds have many different types of feathers on different parts of their bodies. They can be placed into two main categories, however, vaned feathers, and down feathers.

Mammals differ from birds in that they have fur or hair rather than feathers. Fur is a great insulating and protective covering and is present on all mammals for at least part of their lives.

Close up of a Bluethroat

Close up of a Bluethroat

Wings, bills, and feathers are not the only anatomical differences between birds and mammals of course. The following body structures are all seen in birds and not mammals:


Another obvious difference between birds and mammals is how these two groups move. Most birds can fly and all species are bipedal, which means they walk upright on two legs. Many birds can swim using their legs, and some can even swim at high speeds using their wings for propulsion.

Mammals are just as diverse in their means of locomotion as birds, although the vast majority of them cannot fly and are quadrupedal, which means they walk on four legs.

Hyacinth Macaw in flight

Hyacinth Macaw in flight

Reproductive Differences

Birds differ radically from mammals in that they lay eggs, rather than give birth to live young. Both birds and mammals make use of internal fertilization, but birds lay their eggs long before their chicks are ready to take their own first breath.

This strategy allows female birds to unburden themselves of the weight of their babies earlier, and often allows their partner to assist in incubating the eggs. Parental care differs between different bird species, but many male birds make great fathers, and some even do all the incubating and care on their own!

Mammals give birth to live babies. The development of the babies differs hugely, however, with some baby mammals being blind and helpless, while others can even walk and run on their first day! Most male mammals are absent fathers that do not feed or care for their babies in any way.

Moorhen sat on nest

Moorhen sat on nest

What makes a mammal?

Mammals are warm-blooded vertebrates with hair and fur. The most unique and characteristic feature of mammals is their ability to produce milk from their mammary glands. No other group of animals can feed their young in this way.

Mammal brains also differ from bird brains and those of other animals in that they have a neocortex, a part of the brain used in many higher functions. Ossicles are another unique set of features found within the mammal skull. These three tiny bones of the middle ear are used for hearing.

Similarities between birds and mammals

There are probably many more similarities between mammals and birds than there are differences! Both groups are hugely diverse and successful, living all over the planet.

Birds and mammals are the two most intelligent classes in the animal kingdom, and both have the ability to regulate their body temperatures using their metabolism. We share similar organ systems to birds, although there are important differences in the reproductive and gastrointestinal tract.

Both birds and mammals exhibit high levels of parental care which are rare in the animal kingdom, although there are examples of great animal parents from many different classes.

Great Horned Owl perched in a tree

Great Horned Owl perched in a tree

Is there a bird that's a mammal?

Birds and mammals are technically very different types of animals. Their last common ancestor was thought to exist over 300 million years ago.

There are no birds that can technically be called mammals, although one quirky bird has earned the title of ‘honorary mammal’.

The kiwi bird of New Zealand is an unusual flightless bird that forages slowly on the forest floor after dark. Their elongated feathers even look like fur, complete with long bristly whiskers.

Kiwi bird of New Zealand

Kiwi bird of New Zealand

Do birds produce milk?

Birds are one of the few examples of animals that feed their young. They share this advanced parental care with the mammals, but the two groups differ in what and how they feed their babies.

Birds do not have mammary glands so they cannot be said to produce milk. Birds usually feed their babies appropriate food items that they have collected, but some birds go a step further. Pigeons and doves, for example, produce something called crop milk. This is not true milk but rather a high protein fluid that both parents produce and feed to their chicks for their first two weeks or so.

Birds feed their babies with their mouths while baby mammals drink from their mother’s mammary glands. Some mammals do feed their young in a similar way to birds once they have been weaned, however.

A wolf feeding regurgitated meat to begging young pups can be compared to a bird feeding its chicks. Both male and female birds can feed young chicks, but only the mother mammal is able to produce milk.

Greater Flamingo feeding its young crop milk

Greater Flamingo feeding its young crop milk

Bird and Mammal FAQs

Are chickens mammals?

Chickens are birds from the Phasianidae family. These familiar game birds are no more closely related to mammals than any other bird species.

Are penguins mammals or birds?

Penguins are birds. They might seem very different from the regular songbirds we see out in our yards or neighborhoods every day but they share the same common ancestors as all other birds.

Do Mammals lay eggs?

Birds lay eggs, which is one of the key differences between them and mammals which give birth to live young. Some remarkable Australian mammals can lay eggs, however. The duck-billed platypus and the four living species of echidna are extremely unusual in that they lay eggs.

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