Are Birds Mammals? Similarities, Differences, and More

Mammals and birds share characteristics that make people wonder if birds are mammals. For one, birds are warm-blooded, just like mammals. But the question is, are birds mammals? If not, what are they? Is there a difference between birds and mammals? 

Read on as we answer these questions and tell you everything you need about birds and mammals.  

So, are birds mammals?      

We have over 10,000 bird species populating our planet, and just like all living things, birds fall under a particular scientific classification in the animal kingdom¹. However, birds are not mammals. Birds belong to their class called Aves, and mammals belong to a class called Mammalia. 

It is believed that mammals and birds evolved from dinosaurs. So, birds and mammals share a common ancestor if we go back. However, after millions of years, the ancestors of birds split from those of mammals. 

Birds share similar characteristics with mammals. For example, mammals and birds breathe air and are warm-blooded animals. A few birds also feed their young with milk, like mammals.

However, even with these mammalian characteristics, birds are not mammals. So what makes a bird a bird?

Read more: Types of Birds.

What are mammals?        

Before we dive into the distinct characteristics of birds, what are mammals? Mammals are a class of animals that have unique traits.

They set themselves apart from other animal species primarily through the feature of hair or fur covering their bodies. Additional distinguishing physical attributes include a heart divided into four chambers, a trio of bones located in the middle ear, a diaphragm, and a singular bone constituting the lower jaw, among others.

Mammals are warm-blooded animals, meaning they regulate their body temperature. Their body temperature can remain relatively stable no matter the season.

Furthermore, mother mammals grow their babies inside their bodies and give birth to live young. Also, mammals feed their young with a special gland called mammary glands.

Related Read: Types of Mammals.

Distinct bird characteristics      

As we mentioned earlier, birds fall under the class of Aves. Aves have unique characteristics that are distinct from other groups of animals. Here are some features that make a bird a bird: 

All birds lay eggs.

All birds lay eggs to reproduce an offspring. So, no bird gives birth to live young. However, while all birds reproduce by laying eggs, these eggs have different incubation times. 

Birds have beaks.

Birds have beaks that function as a bird’s jaw. These beaks are shaped differently. A bird’s beak can even help you know what they feed on. For example, birds with sharp, hooked beaks hunt down other animals as prey. Birds with beaks shaped like a cone use it to crack nuts. 

Related Read: Birds With Big Beaks.

Birds have feathers.

Birds are the only group of animals that have feathers. Birds use their feathers for insulation and flight. Male birds, in particular, often use their vibrant and well-groomed feathers to attract a mate. Most birds can fly, but not all birds fly. Some flightless birds include penguins, kiwis, and ostriches.

Flightless penguins have heavy bones that help them survive the freezing temperature. Ostriches also have heavy bones around their legs to help them run and defend themselves with strong kicks.

Apart from feathers, some other structures found in birds include scaled legs, air sacs, gizzard, cloaca, syrinx, etc. 

Birds have wings.

Every bird possesses wings. This holds even for flightless species such as ostriches and penguins. A bird's wings are constructed of hollow bones, reinforced by structural support, and weaved with air pockets. This design results in lightweight wings which assist the bird in flight.

Wings are not unique to avian creatures despite being universal to all birds. Mammals and insects also exhibit the presence of wings, with bats being the sole mammalian example. 

Birds are closely related to reptiles.

There is a close relationship between birds and reptiles, like lizards, crocodiles, and snakes. This came from a shared ancestry with dinosaurs. Both birds and reptiles are descendants of these prehistoric creatures. 

Dinosaurs are believed to have evolved over millions of years into early reptilian forms, which gradually diminished in size and developed feathers, paving the way for modern birds.

Based on animal classification, birds belong to Diapsida, which includes modern reptiles like turtles and crocodiles. 

Similarities between birds and mammals                     

While birds and mammals have many differences, the two groups share similarities. 

Firstly, birds and mammals are warm-blooded or endothermic. This means that the two groups can regulate their body temperature. So, unlike cold-blooded animals, birds and mammals can adapt to colder temperatures without switching locations. 

Secondly, birds and mammals are vertebrates, meaning they both have a backbone. 

Thirdly, both birds and mammals breathe air. However, the lungs of birds and mammals are different. Birds have a more complex respiratory system with air sacs as lungs. 

Unlike amphibians and most reptiles with three-chambered hearts, birds and mammals also have a four-chambered heart. 

Finally, mammals and birds have a global distribution. Mammals are present in marine and terrestrial environments, enduring the harshest climates, from searing deserts to freezing arctic terrains, lofty mountains, and expansive oceans. Similarly, bird species can be located in virtually every corner of the world, persisting in environments as diverse as scorching deserts, icy seas, and dense forest ecosystems.

Birds vs. Mammals 

We’ve looked at the similarities that birds and mammals share. Now, let’s dive into the key differences between birds and mammals: 

Physical differences   

• Birds are the only animals that have feathers. On the other hand, mammals are the only animals that have hair or fur on their bodies. 
• Both male and female birds have wings, including flightless birds. Most birds use their wings for flights, swimming, balance, and courtship displays. On the other hand, bats are the only mammals that have wings. 
• One notable difference between birds and mammals is their mouth structure. Birds have beaks or bills, which they use to crack nuts, drill holes, and filter food. However, most mammals have jaws instead of beaks. 
• Birds have hollow bones, scaled legs, air sacs, wishbones, and other physical features not found in mammals. 
• Even though birds and mammals have a four-chambered heart, the structure, size, and function are very different. The Mammalian heart is typically bigger than its body size, and the heart of a bird is smaller and beats faster than the bird's body. 
• Another significant difference between birds and mammals is the way they move. All birds can work upright on their legs, and most can fly. Some birds can swim using their legs while propelling themselves with their wings. On the other hand, most mammals walk on four legs and cannot fly. 

Sensory and communication differences

• Regarding sensory abilities, a bird and a mammal differ widely. Mammals have a strong sense of hearing, smell, and vision, ranging from black and white to color. On the other hand, birds have excellent eyesight, but their sense of smell and hearing is less strong than mammals. 
• Birds are known to sing or call. You can hear these vocalizations throughout a forest, backyard, or location. Many people believe that birds sing when they are happy. However, their vocalizations are for a lot more crucial reasons.

Firstly, it is how many species of birds communicate. Birds can vocalize when defending their territory, informing others of a food source, etc. Some birds, like parrots, even have unique vocal organs. 

On the other hand, mammals also use vocalization but less often than birds do. Many mammals rely more on non-vocal ways of communication, like scent marking and body language. 

Diet, adaptation, and social structure differences 

• Mammals have a broad diet range and may be carnivores, herbivores, or omnivores. Carnivores have sharp teeth for tearing animals, herbivores have ridged teeth for chewing and grinding plants, and omnivores may have a combination of both types of teeth.
• On the other hand, birds have no teeth but have beaks instead. Birds can be carnivores, herbivores, omnivores, insectivores, or even feed solely on a specific diet like nectar. However, items like plastic and some trash can stick to their guts and cause a bird’s stomach to expand and even explode.
• Some mammals are solitary and will only come together to mate. In contrast, others, like lions, live in complex social groups involving hierarchies. Meanwhile, many species of birds form large flocks.

Some birds build a communal nest where colonies live together. Some other birds are solitary and form a monogamous pair during breeding season. 

Reproductive differences              

Here are significant differences between the reproductive system of a bird and mammal:

• Birds typically lay eggs, while mammals reproduce live young. The bird and mammal use internal fertilization, but the bird will lay its eggs long before the baby birds take their first breath. 
• Unlike mammals, birds do not carry their children around. After egg laying, birds keep their chicks in a nest until they hatch and can fly. They hide the nests in houses, trees, or underground.
• On the contrary, mammals give birth to their young alive and carry them around. During gestation, the offspring lives inside the mother’s womb until birth.
• Mammals produce milk for their young through the mammary glands. On the other hand, birds do not have milk glands and do not produce milk. 
• When it comes to lifespan, birds can live for as long as a few years to several decades. Some birds, like parrots, may even outlive mammals, including humans.

Meanwhile, small mammals may live for a few years, while larger mammals like elephants may live for several decades. 

Are there bird species or other winged animals that are mammals? 

Birds are not mammals. However, there is one bird considered a mammal. The Kiwi Bird of New Zealand has been called ‘Honorary Mammal’ because of specific characteristics similar to mammals². 

New Zealand had no land mammals except bats for millions of years. But, the kiwi bird is similar to mammals like hedgehogs and badgers worldwide.  

The kiwi bird is a flightless bird with long, thin beaks. They weigh around 7 pounds and have hollow bones, tough leathery skin, and strong legs. Unlike other birds, the kiwi bird’s feathers appear like shaggy hair. They also have whiskers on their beak resembling a cat.

Do birds produce milk? 

Birds and mammals are similar when it comes to parental care. Both birds and mammals take care of and feed their young. However, birds do not have mammary glands and do not create milk for their chicks.  

Birds typically feed their young with food collected using their mouth. However, a few birds feed their young with crop milk. Birds like pigeons and flamingos produce crop milk, similar to mammalian milk. 

These birds produce the milk in the crop (a sac structure for storing food). It is a nutrient-rich fluid that both parents feed their chicks with for the first few days of their lives³. 

What is the smallest bird in the world?

Bee hummingbirds take the trophy for being the smallest birds in the world, weighing between 1.95 grams and 2.6 grams on average for male and female bee hummingbirds, respectively. It weighs less than a dime. 

This small species spends most of its life flying compared to other species. They are agile and have a wingspan of 1.28in. 

This type of hummingbird is a resident of Cuba, an island in the West Indies. Cuba has a subtropical climate, which is conducive for this bird. They prefer to live in areas with the plant Solandria grand flora (their preferred source of nectar). You can also find them in lowlands and highlands in coastal forests, mountains, valleys, gardens, and swampy areas. 

For the bee hummingbird, breeding happens at the close of the wet season and the beginning of the dry season when shrubs and trees are flowering. These birds may mate while hovering in the air or resting on a perch. 

Typically, the female bee hummingbird lays two eggs the size of a pea. The female incubates for 14 to 23 days, and the chicks fledge after 18 to 38 days. They can live for up to seven years in the wild and ten years in captivity.

Read more: World's Smallest Birds and find out more about how birds mate.

Why do birds migrate? 

Perhaps you’ve seen a large flock of birds moving in a V-shape from one location to another. 

Migrating from one location to another can be risky for birds, and some may not survive. So why do birds decide to migrate? 

Majorly, birds migrate from areas with low food and nesting resources to areas with high nutrition and nesting resources for themselves and the families they are raising. 

Birds may also migrate to protect themselves and their young from the onslaught of predators. Regions with high food sources all year may attract more predators. So, migrating birds can avoid predators and increase the chances of their young growing into maturity. Some birds may even migrate to offshore islands or cliffs inaccessible to predators. 

Climatic changes may also contribute to birds migrating from one region to another. Some birds may leave freezing locations for warmer ones, while others may leave hot spots for cooler ones. 

Final thoughts - Are birds mammals?

Mammals and birds share a common ancestor - dinosaurs. However, birds are not mammals. They have a few similarities. However, there are many differences in how they move, reproduce, and function. 

You can go through our article above to find out all you need to know about what makes a bird distinct from mammals.