Are Birds Mammals? Find Out In Our Definitive Guide!

Birds are not mammals; birds belong to the class Aves and are more closely related to reptiles than to mammals. Mammals belong to the class Mammalia. Birds are the only living animals to have feathers, whereas mammals are the only animals to have hair.

All living birds are directly descended from dinosaurs; a bird is far more closely related to an alligator than it is to a bat or any other mammal!

Although both birds and mammals do share a common ancestor, the ancestors of birds split from the ancestors of mammals over three hundred million years ago.

With such a vast amount of time (and evolution) separating birds and mammals, the answer to the question "Is a bird a mammal?" is definitely "no!".

To fully understand why a bird is not a mammal, we need to take a closer look at both groups of animals...

Is A Bird A Mammal?

Is A Bird A Mammal Infographic

A bird is a member of the class Aves, a group of animals in the animal kingdom.

You can find out more about birds on this page: Birds: The Ultimate Guide

All of the members of Aves (i.e. all birds, both living and extinct) have certain things in common. These shared characteristics are what make birds birds.

If an animal doesn’t have all of these characteristics, then it belongs in a different group.

Superb Fairywren
All birds, including this superb fairywren from Australia, are members of the class Aves.

Likewise, a mammal is a member of a class of animals called Mammalia. To be a member of Mammalia–and therefore to be a mammal–an animal must have mammalian characteristics.

Mammal Sumatran Tiger
Mammals are members of the class Mammalia. This is a Tiger, a member of the cat family, Felidae.

Although there are some similarities between birds and mammals, there are also many differences between the members of each group. Read on to discover what those differences are...

Brief Guide To Animal Classification: Classes, Orders, Families, Species, Etc.

Bald Eagle
A bald eagle is a member of the family Accipitridae, which contains eagles, hawks, kites and several other birds of prey.


Both Aves and Mammalia are a "class" of animal. A class is one of several groups into which biologists sort animals (and other organisms, including plants and fungi). The process of sorting organisms into groups is known as classification.

You can find out more about animal classification on this page: Animal Classification.


Within a class, animals are further sorted into groups of animals that are even more closely related to each other. These groups are known as "orders".

The biggest order within Aves is Passeriformes. Over half of all living birds belong in this group. Members of the order Passeriformes are known as passerines. Passerines are also known as "perching birds" or "songbirds".


You may have heard of people talking about an animal "family". An animal family – such as Canidae, the dog family – is a subgroup of animals within an order.

The largest bird family is Tyrannidae, the tyrant flycatcher family. It contains over 400 bird species.

Bird And Mammal Classification

Biologists classify animals into the above groups by studying the family trees of the animals involved.

Usually, species that are closely related share physical and behavioral characteristics. Animals that are only very distantly related (such as birds and mammals) share relatively few characteristics.

It is the presence (or lack of) certain characteristics that allow biologists to determine to which group an animal belongs.

The characteristics shared by animals in the class Aves (birds) are different to those shared by the animals in the class Mammalia (mammal).

This is because 300 million-odd years of evolution separates birds and mammals; over this time they have become very different animals, and this is why a bird is not a mammal.

Birds Vs Mammals

Despite birds being different to mammals, there are some similarities between the two groups of animals.

This probably explains why so many people ask "are birds mammals?".

Let’s look at the similarities and differences between birds and mammals...

Adelie Penguin Facts For Kids
Even flightless birds such as this Adélie Penguin are definitely NOT mammals!

Similarities Between Birds and Mammals

Although birds and mammals are very different types of animal, they do have certain characteristics in common:

  • Vertebrates: both birds and mammals are vertebrates, which means that they have backbones.
  • Endothermic (warm-blooded): both birds and mammals are endothermic (warm-blooded). This means that they are able to regulate their own body temperatures, and maintain a constant body temperature. For example, warm-blooded animals are able to warm themselves up when cold (e.g. by shivering); they don’t need to move to a warmer location or bask in the sun like cold-blooded animals such as lizards).
  • Four-chambered hearts: the hearts of both birds and mammals have four-chambers. This is a more efficient system than the three-chambered hearts of amphibians and most reptiles.
  • Breathe air with lungs: both birds and mammals breathe air, although the lungs of birds and mammals are different. Birds have extremely complex respiratory systems, with air sacs as well as lungs.

Differences Between Birds and Mammals

Although they share certain characteristics, birds and mammals belong to different orders, and there are many differences between the two groups.

Bird Characteristics

A member of the class Aves (i.e. a bird) has the following basic characteristics that are not shared by mammals:

  • Feathers: birds’ bodies are covered by a number of different types of feathers. Feathers help with flight, and also with insulation (keeping the bird warm).
  • Beaks and no teeth: birds don’t have teeth, and have to swallow their food whole, or cut it up into manageable bits with their beaks. A bird’s beak is often highly-specialised according to its diet.
  • Egg-laying: all birds lay eggs with hard shells. Some mammals (called monotremes), such as the platypus, lay eggs, but monotreme eggs have softer shells, similar to those of many reptiles.

Birds are avian dinosaurs! All birds are descended from dinosaurs. This explains why birds are related to crocodilians such as alligators and crocodiles; both birds and crocodilians are the descendants of archosaurs which lived in the Mesozoic Era!

Mammal Characteristics

A member of the class Mammalia (i.e. a mammal) has the following basic characteristics that aren't shared by birds:

  • Hair: mammals are the only group of animal to have hair (or fur); if an animal has true hair, then it’s a mammal. (Hair is made of keratin, a naturally occurring protein out of which hooves, horns and fingernails are also made.)
  • Give birth to live young. All mammals – apart from the five monotreme species – give birth rather than laying eggs.
  • Feed their young with milk. All female mammals feed their young with milk. This highly nutritious liquid is produced by structures called "mammary glands", from which mammals get their name. No other type of animal can produce milk.

You can find out more about what makes a mammal a mammal here: What is a Mammal?

Bird Adaptations For Flight

The ancestors of all birds, even this flightless emu, were able to fly.

Being able to fly is not a defining characteristic of birds: bats are mammals, and they can fly, and insects were flying hundreds of millions of years before the first birds even appeared!

Bat Flying Mammal
It's not just birds that can fly: a bat is a flying Mammal

(In case you weren't sure, insects are animals – you can find out why on this page: Are Insects Animals?)

However, all birds – even flightless birds such as penguins and ostriches – are descended from birds that could fly, and their bodies reflect the adaptations required for flight.

The most obvious adaptation for flight is a bird's wings. They are the bird's forelimbs. By flapping their wings, most birds can take off, glide, and power themselves through the air.

Most birds have lightweight skeletons with hollow bones to aid flight. Birds have high metabolisms (the amount of energy required to live), and have to eat large amounts of food. This is mainly because flying requires a great deal of energy.

In addition to lungs, birds have a number of air sacs inside their bodies. This helps to move air in and out of a bird faster. This too is a necessity due to the high metabolism of birds.

Is A Bird A Mammal: Conclusion

On this page we’ve answered the question "Are birds mammals?", finding that birds are not mammals, being of a different class in the animal kingdom, and separated by over 300 years of evolution.

We’ve found that Aves and Mammalia are both classes within the wider animal kingdom. We’ve found that a member of Aves is a bird, and that a member of Mammalia is a mammal. We’ve also looked at the differences (and similarities) between birds and mammals.

So, the next time that someone asks you ‘Is a bird a mammal?’, you’ll be able to say ‘No, a bird is NOT a mammal!’

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12 thoughts on “Are Birds Mammals? Find Out In Our Definitive Guide!”

  1. Crocodiles/alligators are reptiles and armadillos aren’t the most direct example there is. But yes, I suppose.

    • Hi,

      That’s a great question!

      Alligators are not descendants of dinosaurs. Although alligators and dinosaurs are both reptiles, their ancestors split into two separate evolutionary branches millions of years ago.

      You can find out more on this page: Are Dinosaurs Reptiles?

      Armadillos, despite looking a bit reptilian with their scaly armour, are mammals, not reptiles. Armadillos are definitely not descended from dinosaurs!

      We hope this helps. If you have any more questions, feel free to ask!


      The Active Wild Team

  2. I think that dinosaurs aren’t really in a class because (a) they don’t exist anymore, and (b) birds and reptiles are descendents of dinosaurs.

    • Just because a species is extinct does not mean it’s classification no longer exists. Would we say this of species that have gone extinct in more modern times? No we wouldn’t.

      Your second comment has validity.

      Birds and reptiles can be classed as dinosaurs, but dinosaurs cannot be classed as birds or reptiles. Evolution only works in one direction.

    • Hi, thank you for your comment.

      Dinosaurs are reptiles. They, along with all other reptiles, belong to the class Reptilia.

      There were reptiles living on Earth for many millions of years before dinosaurs appeared, so reptiles are not descended from dinosaurs.

      Birds are descended from dinosaurs, but are usually put in their own class: Aves.

      We hope this helps! If you, or anyone else, has any questions, feel free to ask!

      The Active Wild Team

  3. So a bird is a dinosaur. but is a dinosaur a mammal? or is it a bird? Or is it a reptile?! What is a dinosaur then? I would think it’s a bird, but I am pretty sure they feed their baby milk also. we don’t have enough proof to me that a dino is a bird a mammal or whatever. please reply if I am just dumb. Jurassic park got meh confused

    • Evolution only goes one way, so a bird could be classified as a dinosaur, but a dinosaur would not be classified as a bird. Hope this clears things up.

      Kind regards.

    • Hi!

      Great questions!

      Birds are descended from dinosaurs. Dinosaurs are reptiles (not mammals), and today, many scientists regard birds as being reptiles!

      HOWEVER, birds are usually put in a group of their own (Aves), rather than being thought of as reptiles.

      Only mammals feed their young with milk.

      You can find out more about this subjects on these pages on ActiveWild:

      Mammal Facts
      Reptile Facts
      Bird Facts
      Dinosaur Facts

      We hope this helps!

      The Active Wild Team

  4. But don’t pigeons produce milk? What about how feathers and fur descend from a common substance and therefore are classified as the same? A chicken was born without a beak and instead with a dinosaur like face, with your classifications, that bird is either a dinosaur or a mammal. I am curious about the validity of your points. Can someone please explain? Thank you.

    • Hi,

      Thank you for your email. You bring up some great points.

      Pigeon ‘milk’ is not really milk, although it serves the same purpose (feeding the young). It is a thick substance that forms in the pigeon’s crop – a food storage area behind the bird’s throat.

      Mammalian milk is formed in a female mammal’s mammary glands. Mammary glands are modified sweat glands. No other type of animal produces milk.

      Yes, mammals share similarities with birds – they have the same (distant) ancestors – buts it’s the differences that separate the two groups!

      Birds are the only remaining descendants of dinosaurs. In fact, many scientists today regard birds as being dinosaurs!

      You can find out more about this here: Dinosaur Evolution

      I hope this helps!


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