Which Birds Are Mammals?

Which birds are mammals? It seems like a strange question, doesn’t it? Because you know mammals are the only animals with milk-producing glands and when you think of a bird, no picture of a breastfeeding offspring flashes in your mind. Birds don’t do that. Birds lay eggs, most of them can fly, and they have feathers. These are not the characteristics you find in mammals. But then, the animal world is a strange one and full of surprises. For example, it may have caught you caught off guard when you first learned that a whale is also a mammal.

In this article, we will answer all your questions so that you will know which birds are mammals? So read on!

Are Birds Classified as Mammals?

Before dwelling on the topic, we must know the basic terms we will use.

What are Mammals?

Mammals are the animals that directly give birth to their offspring. It’s reasonable to assume that, as humans, we understand the gist of what mammals are — at least the basics.

However, very few of us have given much thought to the specialized form of a bird. To begin with, while mammalian silhouettes vary greatly, the basic structure of the bird body is pretty consistent and, in general, quite different from that of a mammal.

Even when only the most basic mammal form is considered, with four similarly sized limbs and a tail, there is a noticeable structural difference between mammals and birds.

But to surprise you, certain creatures are monotremes and are called egg-laying mammals.

What are Birds?

Birds are the only living dinosaurs on the planet today, descended from a feathered theropod. They are a warm-blooded group of animals belonging to the Aves or avian class, bipedal vertebrate creatures with feathers on their backs. The world’s bird population ranges from 50 to 430 billion.

Lightweight and hollow bones make the skeleton of the bird. Their toothless beaks and egg-laying reproduction system differentiate birds.

Almost all birds can fly, but not all have evolved with this ability. Even though some birds cannot fly, all birds have wings.

Aves class

All birds are members of the Kingdom Animalia, Phylum Chordata, and Class Aves. While this may appear to be an arbitrary and artificial classification, it demonstrates the relation of birds to one another because they share many characteristics. 

These characteristics are as follows:

  • Vertebrates: They are classified as members of the Phylum Chordata because they have a backbone. Unlike most other vertebrates, birds have a lighter skeletal structure filled with hollows, gaps, and air sacs to keep them lightweight and allow them to fly more efficiently.
  • Feathers: To serve as body insulation, all birds have evolved feathers made of keratin and other proteins and light-reflecting pigments. Different types of feathers, such as plumes, crests, and streamers, can also be ornamental. Other feather types aid in flight control, whereas some feathers, such as down, are solely for insulation.
  • Wings: One of the most distinguishing features of birds is their wings. Even birds that cannot fly have vestigial or adapted wings or flippers that they can use for swimming, threat displays, or courtship dances. Wing size and shape differences between species depending on how the bird flies and wing markings can help identify bird species.
  • Warm-blooded: All birds are endothermic, generating their internal body heat and do not rely solely on their surroundings to keep warm. While many birds will sun themselves to help regulate their body temperature, sunning serves multiple functions and is not exclusively for body temperature regulation.

Birds and Mammals: Similarities

Birds are more closely related to reptiles than mammals, but they share many characteristics with mammals.


Animals such as birds and mammals do not rely on body heat to keep them warm. They are both warm-blooded, which means that they can maintain their temperature without the aid of an external source. Similar caloric requirements by weight and the ability to remain active in colder temperatures are some of the other commonalities drawn from this similarity. Being warm-blooded allows birds and mammals to thrive on any landmass.


Mammals have four-chambered hearts, but you might be shocked to learn that birds do as well. Birds can only fly because they expend a substantial quantity of energy. An effective circulatory system is required to generate a significant amount of energy. That is why birds need a four-chambered heart to pump enough blood to generate enough energy.


Vertebrates are those animals that have backbones and skeletons made of bone, such as mammals and birds. Hollow bones with a crisscrossed matrix are found only in birds, whereas mammals’ bones are solid. Birds have hollow bones that are light enough to fly but strong enough to handle the stress of taking off and landing.


Blood from birds and mammals contains both erythrocytes and leukocytes, which are blood cells. Hemoglobin, a protein containing iron, is responsible for carrying Oxygen in both types of red blood cells. Birds and mammals both have erythrocytes, but the erythrocytes of mammals lack a nucleus. Therefore, to maintain a healthy immune system, both leukocytes are necessary.

Caring for Children

Both birds and mammals have a lot in common when caring for their newborns. From species to species, the amount of time depends on how old one is when they first can take care of themselves—female mammals lactate to provide milk for their young. In contrast, birds feed their young ones beak-to-beak.

Birds and Mammals: Differences

Skin’s Outer Layer

First and foremost, feathers are a noticeable difference between birds and mammals. Because birds need to keep their bodies light and aired, they have feathers. On the other hand, mammals have an outer coat of fur or hair to keep themselves warm. Endothermic animals, such as mammals, can adjust their body temperatures.

Structure of Bones

Because birds need to be able to fly, they must have a very light bone structure, while mammals have dense bones instead of other animal groups. Birds’ hollow bones are a significant factor in this. They can fly because of their hollow bones, which allow them to keep their weight in the lower half of their bodies. However, the bones of mammals are thicker. The hollowing out of the bones of birds allowed them to soar to greater altitudes. They would not be able to fly if their bones were as dense as those of mammals.

Method of Feeding

They’re called mammals because of their mammary glands, and the mammals use these glands to produce milk and provide it to their offspring. There are no mammary glands in birds. Hence they cannot make milk. Therefore, birds supply their young with externally sourced food that is easy to digest. Look at birds, and you’ll see that they feed their young partially digested or easy-to-digest food.


Most mammals give live birth while birds lay eggs. Comparing the reproductive systems of mammals and birds reveals a wide range of fascinating contrasts. Only mammals give birth to their offspring right before their eyes and feed them. When it comes to animals, birds are the only ones who reproduce through the production of eggs. When the eggs are ready to hatch, the chicks are cared for in their nest by the females.

Anatomy of the Mouth

If you look at the two, you’ll be able to discern the next significant difference between them. For example, the teeth of mammals are powerful, which aid in the digestion of food mechanically. The bird’s beak, not its teeth, punctures the air. There are no teeth in the beak. Gastroliths aid in the mechanical digestion of food. They ingest little stones called astrocytes to break down food into smaller pieces. Mammals and birds have different digestive systems, as reflected in their oral morphology.

Capacity for Sound Processing

Compared to other vertebrates, birds have an enormous edge in auditory processing ability. They have the highest resonant frequency for every square centimeter of their body surface. However, mammals cannot produce a sound with a high resonance frequency. The higher resonance frequencies are out of reach for mammals. Hence this means that mammals usually cannot make a sound.

Are Birds Classified as Mammals?

So now that you know the basics, let’s get to the main question. Are birds classified as mammals? The answer is no.

Different Class

Birds are not classified as mammals because mammals belong to the Mammalian class, and birds belong to the Aves class. These are different groups of species. The primary way we classify animals depends on their physical characteristics, and animals in the Aves class differ significantly from mammals in this regard.

Different Characteristics

Ability to lay hard-shelled eggs, a beak, feathers and wings are all required for a bird. Animals classified as mammals have mammary glands (the structures from which the class gets its name) and hair, give birth to young offsprings and produce milk in their females.


It is essential not to focus too much on the flight itself, as several bird species, such as penguins, have modified wings specialized for movement through the water. But, of course, many birds cannot fly. Meanwhile, mammals with wings can fly, such as bats. There are some flightless birds, but they don’t make them mammals. However, this is not the only way birds and mammals differ internally to aid in flight.

Respiratory System

The unique avian respiratory system is another fascinating structural difference between birds and mammals. Flying requires a lot of energy. Try flapping your arms up and down ten to fifteen times; it’s exhausting. Many bird species cannot glide, so they must always do this to stay above ground. Hummingbird wings can flap up to twelve times per second. How else can they possibly keep up with all of this effort?


Monotremes defy mammalian classification by adopting bird-like habits. Although they are mammals, they lay eggs rather than give birth to live young — though you should bear in mind that the platypus and echidna are extremely odd mammals on which we should not base generalizations. Even the bat’s modified forelimb lacks feathers. Even after laying eggs, monotreme mamas will feed their growing young on a milk diet.

Which Birds Species Are Mammals?

A small group of birds known as ratites is sometimes called honorary mammals. One of these five species is the Kiwi. What makes this tiny bird an honorary mammal is that while it lays eggs and does not produce milk, it has bones that are more like mammalian bones than bird bones. In addition, Kiwi has a similar body temperature and whiskers on its face.

Kiwi, the Honorary Mammal


The Kiwi is a flightless bird with emus, ostriches, rheas, and cassowaries. The Kiwi is approximately the same size as a domestic chicken. It has a unique status as an honorary mammal due to its distinct features and non-bird-like characteristics.

The Kiwi’s eggs are the largest in proportion to the body size of any bird on the planet. When kiwi chicks hatch, they are immediately ready to feed themselves and resemble mini-adults rather than chicks. It is highly unusual for birds. In addition, unlike most birds, the plumage of a kiwi resembles hair rather than typical bird feathers. They also have whiskers around their beaks similar to those of mammals.

Which Birds Are Not Mammals?

Birds, as previously stated, are not mammals. However, a few species of birds have striking resemblances to mammals. The ability to produce milk is one of the primary mammalian characteristics that distinguish birds and mammals. Pigeons and doves are two examples of birds that feed their young with milk. But don’t be fooled! There is a difference.

“Milk” from pigeons is a substance composed of fat and protein-rich cells. A Parent’s crop is a pouch in their throat where they keep food until the rest of their digestive system can digest it and produce pigeon milk. It contains proteins, fats, antioxidants, antibodies, and beneficial bacteria, just like milk mammals. Prolactin, the hormone that regulates lactation in mammals, even influences it.

On the other hand, Crop milk is semi-solid rather than liquid, and it isn’t secreted from teats like echidna milk or grooved like platypus milk. Instead, the squab ingests it after the parent has regurgitated it. Flamingos and emperor penguins use crop milk to feed their young. Both female and male birds produce crop milk, which distinguishes them from mammals because only female mammals feed their young ones.

Final Words – Which Birds Are Mammals?

In this article, we answered the question, “Do birds count as mammals?” and discovered that birds are not mammals because they belong to different animals and evolved over 300 years apart.

Birds, unlike mammals, lack fur or hair and instead have feathers, though they may have bristles on their heads or faces that resemble hair. Despite having warm blood, breathing air, and vertebrae, they are not mammals. Even though some species congregate in groups for foraging, hunting, childrearing, and protection, they are not mammals.

You discovered that Aves and Mammalia are both classes of animals within the larger animal kingdom. You now know that an Aves member is a bird, and a Mammalia member is a mammal. So, the next time someone asks you, “Is a bird a mammal?” you’ll be able to respond, “No, a bird is NOT a mammal!”