Birds, those beautiful and sometimes terrifying creatures, have been fascinating the human species for thousands of years. Scientists are busy teasing out just how long birds have been on earth. Many learning the classification of animals might wonder, “Are birds mammals?”
Birds are not mammals. Birds are vertebrates, classification, Aves. They walk on two legs, are endothermic (warm-blooded), lay eggs with a hard shell, and are the most recently evolved. Many similarities, such as possessing a skeleton, and differences such as having feathers rather than hair or fur, exist when comparing birds to mammals.
This article will explore what makes a bird a bird and not a mammal by explaining a bird’s anatomy and how it differs from a mammal. I will also explore another question: If birds are not mammals, then are they reptiles?
Differences and Similarities to Mammals
Birds and mammals have many differences with some similarities, some you may know, and others you may not.
The most glaring difference between birds and mammals is how they bring young into the world. Birds lay eggs from which their offspring hatch, while mammals give live birth to their offspring.
Like mammals, though, birds protect their young, beginning right after laying their eggs.
Feeding time is also very different. Mammals feed their young milk produced from the mammary glands. The mother bird goes out and searches for food, such as worms, seeds, and berries.
The mother then eats the food, swallows, and regurgitates or throws up the food into her beak, and feeds it to her young.
Birds have feathers, wings, a tail, a beak, and two stomachs but no vocal cords. Like mammals, they have eyes, ears, legs, feet, bones (although hollow), lungs, a heart, digestive system, blood, muscles, a brain, and a way to communicate.
If Not Mammals, then are Birds Reptiles?
If birds are not mammals, then are they reptiles? This is a reasonable question since paleontologists hold that birds are descendants of the dinosaurs and the first reptiles. They call dinosaurs, for instance, the T-Rex, non-avian dinosaurs (source).
If dinosaurs are reptiles and birds are dinosaurs, then it follows that birds are reptiles. Reptiles are defined as vertebrates that have an amniotic egg — embryo inside the egg — but are cold-blooded. Birds also lay eggs with an embryo inside and a hard outer shell.
Birds belong to the group Diapsida in which other living reptiles also belong. Think crocodilians, turtles, tuataras, and squamates, which are mostly lizards and snakes. While birds are related to reptiles, the closest relation is the crocodile (source).
During the Extinction Event when the asteroid wiped out all dinosaurs, feathered dinosaurs survived (archosaurs). The descendants of archosaurs are both crocodiles and dinosaurs.
Distinguishing Features
The features connecting birds and crocodiles are listed below.
- Scales
- Egg-laying
- Organs
- Skeletal structure
Birds have scales, and their feathers come from tissue similar to those that a reptile’s scales grow from. Birds also have scales on their feet.
Both Birds and reptiles lay eggs with embryos. Their organs are similar, especially their heart and musculature. There are also skeletal similarities, such as a hole in the skull for the passage of air.
While there are similarities, there are also stark differences. The most significant difference between birds and reptiles is that birds are warm-blooded, or endotherms, making them similar to mammals in that regard.
A Bird’s Anatomy: The Outter Body
The best way to explain how a bird differs from mammals is to understand a bird’s anatomy since there are more differences than just egg-laying. Below I will discuss each body part and how it is different and similar to that of mammals.
Let’s begin with the most obvious features of the outer body.
Feathers
There is more to feathers than meets the eye. Feathers are part of the outer layers of the skin (epidermis) of the wing. It is made mostly of keratin, a flexible protein that we find in many animals forming hair and nails.
Feathers are crucial for flight. They are light but strong and provide the surface area required for the bird to be aerodynamic. Feathers also trap pockets of air, effectively serving as insulation against the elements, conserving body heat.
Here, being endothermic (warm-blooded) is a disadvantage for birds, as they must regulate their own body heat.
Birds differ from each other in color, texture, and wing shape. This is important as it is the bird’s method of signaling others of their sex, age, species, and social status. Feather features also serve as camouflage, keeping them safe.
Birds are constantly attending to or preening their feathers by using oil from a gland at their tail base.
This practice eliminates any unwanted parasites, gets out dirt, maintains the feathers’ waterproofing, and keeps the feathers supple. Birds also molt twice a year, ensuring wings are not damaged.
Mammals also primp and preen, but they have sebaceous glands in their skin.
Wings
In contrast to mammals, in particular, land-dwelling mammals, birds have wings. Mammals that live on land have hooves, paws, hands, and feet.
Wings have bones, and the wing’s skeletal structure looks very much like a forearm, although greatly modified from mammals.
Wings are long, short, flat, round, or pointed. The shape influences how the bird flies or its style. The style of flight includes gliding, soaring, or flapping.
Flight muscles are what power the wings and are attached to the chest by large tendons. The muscles are supported by the breastbone, which is in the shape of a boat’s keel.
Tail
All birds have a tail, which they require to steer them during flight and to help them land.
On the tail are two paired flight feathers called rectrices that extend the tail’s length. Covering the tail and sitting on top of the rectrices are small feathers known as coverts.
The coverts’ function is to maintain a smooth airflow over the tail and wings during flight.
Tails are all sizes and shapes, and while birds need their tails for flight and landing, the shape is not as crucial as it is to the wings. Many male birds, like peacocks, use their tail feathers solely to attract females.
Legs
Birds have two legs with the lower section of the leg called the tarsus. They have feet at the end of each leg with toes.
Most birds have four toes, and songbirds have one that points backward named a hallux. Their toes have adapted over the centuries and are used for holding onto perches, swimming, capturing prey, and picking up and carrying food.
Bones
Although all vertebrates have bones, a bird’s bones are hollow as they are filled with air. In contrast to a mammal whose bones are dense and filled with marrow, birds can fly.
Birds are structurally lighter than mammals; after all, mammals were not designed to fly, although humans have found a way around this aspect of nature.
Mouth
Birds do not have heavy, strong jaws or teeth. Instead, their jaws are lightweight and known as beaks or bills.
Birds can move their upper beak without moving their heads and can open them extremely wide, which is very helpful for eating. The size of beaks or bills varies and is largely matched to their diet.
Eyes
Birds have very large eyes that provide them with exceptional vision. The eye is also very well protected by three eyelids. The outer lid looks a lot like a humans lid. There is then a lower lid that closes while the bird is sleeping.
The third lid is a thin membrane that moves from side to side, and its purpose is to moisten, clean, and protect the eye from wind and bright light.
Ears
You may not be able to see the ears, but they are indeed there. The ears are located internally with the openings just behind and a bit below the eyes. Birds need their ears for hearing and balance, which is critical while in flight.
Many birds have auriculars, textured feathers, that work as a screen to protect their ears from anything getting in.
Two birds can fly in the dark and use echolocation like bats to find their way: cave swiftlets and oilbirds.
A Bird’s Anatomy: Organs
The inner workings of birds also differ from mammals and reptiles in several respects.
The Throat
The birds we come into contact with daily have two occupations: one is caring for their young, and two is singing. However, unlike mammals, they do not produce sound with a larynx, but through a syrinx or voice box, an organ comparable to a human voice box.
Birds produce sound when the two membranes of the syrinx vibrate, and the syringes of songbirds are particularly well-developed. A few songbirds, such as the wood thrush, can separately control the membranes, enabling them to sing two songs simultaneously (source).
The Brain
Birds are intelligent as their brain retains instinctive behavior, processes their keen sensory perception, and helps maintain their excellent coordination and balance. The optic lobe of birds is well-developed to control the nerve impulses received from the eyes.
Other highly developed brain elements include the cerebellum, which is the control center of the muscles. Unlike humans, the cerebral cortex in birds, which controls thought, is primitive at best.
However, birds have a forebrain called the hyperstriatum, which the human brain lacks. This is where songbirds learn to sing their songs, and scientists believe it to be a source of intelligence.
Lungs
Birds have lungs, like mammals, but they are very small. A bird’s lungs do not expand and contract like that of a mammal. Instead, the lungs are similar to bellows and blow air into the lungs, filling them with a one-way flow, thereby keeping the lungs full, which they need during flight.
Their lungs also function to keep them cool since they do not have sweat glands like mammals.
Mammal’s lungs expand and contract with each breath to exchange oxygen and carbon dioxide via alveoli — microscopic sacs in a mammal’s lungs. Birds exchange oxygen and carbon dioxide through air capillaries or sacs — walls of microscopic tubules — inside the lungs.
Breath is exchanged in cycles in both animals. The difference is mammals have one respiratory cycle per breath, and birds have two respiratory cycles per breath.
Heart
Like mammals, birds have a four-chambered heart, which functions to pump blood through the bird’s body, supplying much-needed oxygen to muscles. A bird’s heart beats rapidly, especially that of a hummingbird, which beats at 600 beats per minute at rest.
So while a bird’s heart is structurally similar to a mammal’s heart, it is different in how hard it works. It weighs double that of a mammal’s heart, so it can supply the muscles with nutrients while the bird is in flight.
Blood
All birds and mammals have blood that supplies much-needed nutrients and takes away anything toxic. The difference lies in the red blood cells. Birds have nucleated red blood cells, while mammals do not (source).
Digestive System
A bird’s digestive system is similar to mammals but has a few differences. Like a mammal, they have a liver, intestines, and kidneys, but, unlike mammals, a bird also has a crop, gizzard, and cloaca.
The crop, which expands, is what allows a bird to devour a lot of food quickly. It also stores food until the bird is safe and is ready to digest. A bird also has two stomachs.
The gizzard is muscular, and it crushes up the food, while mammals have teeth for this job.
The cloaca is the only opening for the bird to expel feces, reproductive matter, and urine. Mammals with placentas do not have a cloaca.
Final Thoughts
If the only two facts you knew about birds were that they are vertebrates and endothermic, you would immediately classify birds as mammals. Although there are many similarities between birds and mammals’ anatomy, there are just as many differences.
Birds may not be mammals, despite any similarities that exist, but they do not need to be mammals to bring us joy. Listening to a songbird on a beautiful spring day may make you want to sing along.
Their songs are not only beautiful but are distinct to their subspecies. A canary’s song is different than a wood thrush, and their songs are different than a wren.
Bird songs are meant for us to stop and listen even on our busiest day. So do not miss out on the opportunity to sit still and enjoy a free concert right in your own backyard.