Are Birds Mammals? The Difference Between Feathers & Fur

Birds and mammals share a few key characteristics that make people believe they belong to the same class. Birds are warm-blooded, just like mammals. But do birds fit into the mammal group?

The short answer is no. Birds are not mammals, and while they share a distant common ancestor, birds are more closely related to reptiles. Feathers are the distinguishing feature of birds, as they’re the only animals with them.

In this article, we’ll discuss what makes a bird a bird, compare them to mammals, and tell you everything you need to know about the animal kingdom class of birds.

A Brief Summary Of Animal Classification

It’s worth discussing how animals are classified scientifically because it can help get rid of a lot of confusion surrounding where animals belong.

Taxonomy is the study of naming, defining, and classifying organisms. Organisms are grouped into taxonomic ranks according to their evolutionary history and traits. While it sounds complicated, you can break it down very simply.

Taxonomic ranks are an upside-down pyramid. The largest groups are at the top, while the smallest are at the bottom. Each level you go down decreases the size of the group.

In short, the ranks go from top to bottom like this:

• Life
• Domain
• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species

At the very least, you’ve probably seen the term “species” before. Species is the term that refers to an organism that is different enough genetically from others to be considered a different thing.

Taxonomic ranks aren’t just for animals. All life on earth fits into these classifications. There are two or three domains, depending upon which model you subscribe to. Archaea, Bacteria, and Eukarya make up the three domains, with eukarya included with archaea in the two-domain system.

To make it simple, the two-domain system is single-celled life without a cell membrane and organisms with a cell membrane.

Below domains are kingdoms. Again, there is more than one system for what groups belong here. Basically, it breaks down into animals, plants, fungi, and two or three types of single-celled organisms.

The animal kingdom is the one we belong to and are most familiar with. Every animal you can think of fits into this category.

Below kingdoms is phylum. Within the animal kingdom are around 31 groups. They’re categorized by shared evolutionary history.

Below phylum is a long list of classes that animals can fit into for each phylum. And on it goes categorizing animals into smaller groups of more closely related life.

So how do the categories we learned in school fit into scientific classification? What about mammals, birds, reptiles, amphibians, insects, and fish?

The reality is that classifying these animals scientifically is more complicated than our early introduction to animals. The ones we know from school are in the class level of taxonomy, but they aren’t the only ones that exist.

Each class has distinct characteristics that differentiate them as animals and in their evolution. Sometimes traits overlap in animals, which leads to some confusion, like whether or not birds are mammals.

When it comes to classes, our normal animal categories have these names:

• Mammals = Mammalia
• Reptiles = Reptilia
• Birds = Avis
• Amphibian = Amphibia
• Fish = Olfactores
• Insect = Hexapoda

Now that this long-winded explanation is done, let’s discuss why some people might think birds are mammals.

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Why Would Someone Think Birds Are Mammals?

As we discussed a bit earlier, birds share some traits with mammals that might be confusing for some people.

Birds breathe air, a trait shared with all mammals. Even mammals that spend most of their time underwater have to go to the surface to breathe air. They have lungs, not gills or pores, to take in oxygen.

Birds are also warm-blooded. All that means is that they have a fairly steady body temperature. The environment being cold or hot doesn’t make them warmer or colder. A bird’s body, like a mammal’s, can heat and cool itself.

Finally, a few birds produce milk to feed their young, just like mammals do. Pigeons are the biggest example, though they don’t have a mammary gland that produces it as mammals do.

So with these shared traits, how come birds aren’t mammals?

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What Makes A Bird A Bird?

All animals in the class Avis are birds. They share a few traits that define them as a class, though a few are shared with other groups of animals.

One of the biggest defining features of birds is their feathers. Birds are the only group of animals that have feathers.

All birds are warm-blooded. It’s a trait they share with mammals and probably why people think birds are mammals. They’re able to burn energy to regulate their body temperature.

All birds also lay eggs. No matter the species, birds are animals that reproduce by laying eggs that contain their offspring. No birds give birth to live babies, though their egg’s incubation times can vary.

Birds also have beaks. Beaks are hard mouth parts that function as the bird’s jaws. Different types of birds have differently shaped beaks. You can look at a bird’s beak to help figure out what they eat.

Hooked, sharp beaks are usually owned by birds of prey that hunt down other animals. Flat bills like the ones ducks species have filter small animals and plants from the water. Short, cone-shaped beaks are usually used for cracking nuts.

Birds have different bones than most other animals. Where our bones are dense and solid all the way through, birds have hollow bones.

Inside their bones are small support structures, like the beams of a house. In between those supports are pockets of air. It helps keep birds light and, when combined with their wingspan, allows them to fly.

There are also a few traits that you might think are unique to birds that aren’t used to classify them in class Avis.

While most birds can fly, not all of them are able to. Penguins, ostriches, emus, and kiwis are all examples of flightless birds. Besides, bats can fly, and they’re mammals. That’s why flight isn’t a defining trait of birds. 

Wings are also not a great way to define birds differently than mammals. Again, it’s because of bats. Bats also have wings that very much function like birds. Insects also have wings in a lot of cases. That makes it not a unique identifier.

Being warm-blooded does help classify birds, but it isn’t unique. Mammals are also warm-blooded.

Birds are more closely related to reptiles than they are to mammals. In actuality, birds are descendants of dinosaurs, just like lizards, snakes, and gators. They simply grew feathers and downsized over millions of years.

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What Are Mammals?

So we went over what birds are, but what are mammals?

Mammals are a different class of animal that have some unique traits of their own.

All mammals are warm-blooded. They can regulate their own body temperature by burning energy. A mammal’s body temperature stays relatively stable throughout the seasons, and its metabolism isn’t reliant upon the weather.

Mammals also give birth to live young. After a female becomes pregnant, she grows and nourishes the baby inside her body. Then, they give birth to living, breathing offspring.

Mammals have a special gland called a mammary gland. It produces nutritious milk that the mother uses to feed their young. It’s a trait unique to mammals.

Mammals tend to have more complex brains than other animals as well. The prefrontal cortex is a brain structure that increases mammals’ brain power and is unique to those animals.

Despite all of these other traits, the defining trait of mammals is the presence of hair or fur. All mammals have some kind of hair on their body, a trait no other animals can claim.

A four-chambered heart, three middle-ear bones, a diaphragm, and a single-boned jaw are also all mammalian traits.

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Mammals vs Birds

Birds

All birds lay eggs. Some can lay eggs that aren’t fertile, such as chickens, but no birds give birth to live young.

Birds are the only animals that have feathers.

The presence of wings on most members.

Hollow bones with support structures.

Beaks instead of jaws.

Warm-blooded metabolism.

Mammals

All mammals give birth to live young. They have a gestation period where their offspring live in their womb until they give birth. Rare exceptions occur, like the duck-billed platypus.

Mammals are the only animals that have hair or fur. Yes, even sleek dolphins and whales have hair.

Four-chambered heart, prefrontal cortex, three middle-ear bones, and diaphragm.

Mammary glands produce milk for their offspring.

Warm-blooded metabolism.

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Common Misconceptions About Birds

There are a LOT of myths about birds, so we’ll just be giving you some of our favorite misconceptions regarding these wonderful creatures.

Birds sing when they’re happy.

Birds sing for a lot of reasons. It’s how many species communicate. It can be for aggression, defending their territory, wooing a lover, or letting others know where the food is.

Birds mate for life.

Some species of birds do pair off with one mate forever. But it isn’t as common as people would normally think. Albatross may stay together forever, but other birds like flamingos never do.

Touching a baby bird will make its mother abandon it.

Birds almost always have a very poor sense of smell. It isn’t likely you’ll leave enough scent on a baby bird to make its mother stay away.

Besides, the parents are probably close by and watching anyway. It’s normal for birds to end up on the ground while learning to fly. You shouldn’t be picking up baby birds anyway, but that’s not because the mother will abandon them.

Birds are dumb.

The term “bird brain” might make you wonder if birds are really just not smart. This is far from the case. Some birds, like crows and ravens, can solve complex puzzles, communicate with people, and use tools.

Even birds that aren’t on that intellectual level can recognize others by sight or sound and remember past events long after the fact.

Bird feeders keep birds from migrating.

That is not remotely true. While some may think leaving food out for birds will keep them around all year, migrating birds will move on even if you put out bird feeders.

Rare cases could easily give this one anecdotal evidence, but overall it’s just a myth.

Rice can expand in a bird’s stomach and make them explode.

Birds don’t have problems digesting any kind of vegetable, including rice. It’s trash and plastic that will stick in their guts and make them expand.

Birds migrate to avoid freezing.

Birds indeed migrate, but it isn’t just to avoid freezing in the winter. Birds migrate for various reasons, mainly for better feeding grounds and places with more resources for their young.

Birds have teeth in their beak to help break up food

Birds don’t have teeth at all. A few might have curves on their beak resembling teeth, but they don’t have true teeth like other animals. Birds usually swallow their food whole or rip it into smaller, swallowable chunks to eat.

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One Bird That Is an Honorary Mammal

Though birds are not mammals and are an entirely different class of vertebrates, one bird is an exception.

The kiwi bird of New Zealand has been given the title of honorary mammal. For millions of years, the island had no land mammals other than bats. They gained their title because they fill a similar ecological niche to badgers and hedgehogs in other parts of the world.

Kiwis are small birds. They only weigh about 7 pounds, though they’re descended from the same ancestors as ostriches and emus. Kiwis have long, thin beaks. But they’re entirely flightless, just like their cousins.

Despite their small size, kiwi eggs are almost as large as an ostrich’s. The egg takes up most of the space in the abdomen, expanding the ribs and rearranging its internal organs.

They do have some somewhat mammalian features, though.

They have whiskers on their beak that function a lot like a cat’s would. Their feathers resemble shaggy hair more than they do actual feathers. Their bones are also heavy and filled with marrow, just like mammal bones.

Kiwis can run fast thanks to their strong legs. It also supposedly helps them fight when they’re in trouble.

Generally, though, populations of wild kiwis can’t survive on islands with populations of mammalian predators. The introduction of cats and dogs to the island, along with people, greatly contributed to their overall decline.

The seven-pound kiwi’s closest ancestor? It’s the extinct 1,000-pound, ten-foot-tall elephant bird of Madagascar.

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