Are Birds Mammals Or Reptiles? – The Answer Revealed

Birds are the only animal on the planet with feathers, which sets them apart from any other species. But birds have striking similarities to other animals that used to exist on the Earth which raises questions about their classification. So, are birds just birds or are they mammals…or even reptiles?

Birds are reptiles, not mammals. They share a more recent common ancestor with reptiles than they do with any other animal on Earth. A great deal of fossil evidence shows they evolved from Dinosaurs which were reptiles. Birds are not mammals as they do not share the defining traits of mammals.

three birdwatch world owl logos with their eyes looking down

Read on to discover how birds evolved from dinosaurs and more about what sets them apart from mammals. We will also take a closer look at the similarities birds have with reptiles. Though they look almost entirely different from each other, birds and reptiles have more in common than you might think.

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Birds Vs Reptiles Comparison

The table below shows the common traits of reptiles and how many of them are shared with birds:

Vertebrates (have a backbone)
Ectothermic (mostly)
Dry, scaly skin
Shed their skin/feathers (molt)
Lay eggs (mostly)
Do not have
outer ears
Breathe air via lungs

You can see that it’s amazing just how many traits birds share with reptiles.

Discover how birds make eggs in this article here on my blog.

Traits That Birds Do And Don’t Share With Reptiles

Commonly, a reptile has certain traits that make it a reptile. Having scales all over their bodies, laying eggs, and being cold-blooded are things we generally associate with reptiles. However, not all reptiles have these traits.

a carpet python resting on a log
Diamond Python – image by Barry Callister Photography

Discover how birds hear in this post here on Birdwatch World.

Reptiles Are Cold-Blooded

Firstly, cold-blooded is a misnomer. Reptiles do not have cold blood and in some cases warm their blood to temperatures far hotter than human blood. Our blood is kept at around 98.6°F (37°C). Some lizards will bask in the sun until their blood is around 110-120°F (43.3-48.9°C)?!

Reptiles are what is called Ectothermic, meaning they get their heat from outside sources such as the Sun or warm surfaces such as rocks. We are Endothermic meaning we generate most of our heat internally from the food we eat. Birds are also endothermic.

There are however reptiles that are partially endothermic, such as the Argentine Black and White Tegu.

an Argentine Black and White Tegu lizard
Argentine Black and White Tegu – image by Foto4440 from Getty Images

These large lizards from South America can generate some heat internally either from their food or by secreting certain hormones that cause their tissues to go into overdrive.

Reptiles Lay Eggs

Like birds, reptiles lay eggs. However, there are exceptions to this rule in many reptiles such as Boas.

Every species of Boa Constrictor gives birth to live young as do some Vipers, Garter Snakes, and Skinks. Around 30% of all snakes and 17% of all lizards give birth to live young.

a boa resting on the branch of a tree amonst green leaves
Boa in a tree – image by NirutiStock from Getty Images

There are no birds that give birth to live young.

Dive deeper into the subject of birds giving birth to live young in this article.

Reptiles Have Scales

The most common thing anyone could tell you about reptiles is that they have scales. This is of course something that birds don’t have…or do they?

If you look closely at the legs of many birds, you will see scales. Even feathers are modified scales that gradually changed during the course of evolution. Feathers and scales are even made from the same substance – keratin.

the legs of an Australasian Swamphen clutching a metal railing
The legs of an Australasian Swamphen – image by Barry Callister Photography

Bird legs are very reptilian in appearance.

Birds Vs Mammals Comparison

4 limbs
Feed milk
to young
Have hair
Jaw hinged
to skull
Hear via
bones in
middle ear
Have a diaphragm
Nonnucleated blood

The table above shows how birds do have some things in common with mammals, however, they have more differences.

Traits That Birds Do And Don’t Share With Mammals?

a mother dog with five pups drinking from her
Mother dog feeding her puppies – image by Dariusz Banaszuk from Getty Images

All mammals have the following traits:

  • Warm-blooded (Endothermic)
  • Have 4 limbs (except some aquatic species)
  • Females secrete milk to feed young
  • Have hair
  • Jaws are hinged directly to their skulls
  • Hear via bones in the middle ear
  • Have a diaphragm
  • Nonnucleated (have no nucleus) blood cells

Did Birds Evolve From Dinosaurs?

Many dinosaurs evolved into birds hundreds of millions of years ago and the proof is in the rocks beneath our feet.

a dinosaur in a rocky landscape

In the German state of Bavaria, around a hundred kilometers from Munich (as the dinosaur flies) the countryside is studded with limestone quarries. It was in one of these quarries in 1860 that a worker made an astonishing discovery – a feather.

This feather, perfectly imprinted into a thin limestone slab, looked scarcely different from a feather any of the birds we know today might shed onto the ground.

the fossilized imprint of a feather
von Meyer, Public domain, via Wikimedia Commons

This discovery created more questions than it answered. The answers would come just a year later in another quarry a few miles away. An almost full skeleton of a bird-like creature around the size of a chicken was found, surrounded by detailed impressions of feathers.

a fossil of a bird-like creature
Tylwyth Eldar, CC BY-SA 4.0, via Wikimedia Commons

This was a strange-looking bird. It had a long bony tail, and three separate digits on each foot with curved claws, and later finds revealed the skulls of these animals contained bony jaws with teeth instead of beaks. This animal was clearly a mix of bird and reptile.

This animal was given the name Archaeopteryx. 12 different specimens of this creature have since been found.

An article written for Scientific American in June of 2019 states that the original feather considered to be from Archaeopteryx in fact may have come from a completely different animal. There may have been more than one bird-like dinosaur flying around all those millions of years ago.

How Archaeopteryx Might Have Lived

Archaeopteryx has been the center of a lot of debate among scientists about how it might have lived. It is thought it might have lived in trees and its bones provide a couple of clues to this.

The first clue is the clawed fingers on its wings.

a close-up of the fossil of a bird-like creature showing clawed fingers on it's wings
Tylwyth Eldar, CC BY-SA 4.0, via Wikimedia Commons (modified from original)

There are birds alive today that have the same clawed fingers on their wings. One such bird is the Hoatzin which lives in South America. Young hoatzin chicks have two claws on each wing which they use to cling to thin branches of trees.

The rear toes of archaeopteryx are pointed backward as they do in many modern birds that use them to grasp perches.

a close up of the feet of a Snowy Egret perched on a stick
The feet of a Snowy Egret with backward-facing toes – image by Genfirstlight from Getty Images

One further clue which hinted towards archaeopteryx being a tree-dweller was the well-preserved tails of the specimens. If these creatures had been ground dwellers, their tails would have become disheveled from scraping along the ground.

But if this ancient cross between a bird and a dinosaur lived in the trees and also had wings, could it fly? Or did it merely use its wings to glide from one tree to the next?

Did This Ancient Bird Fly?

One of the mechanisms birds have that enables them to fly is that their wings muscles are anchored to their large breastbones.

a diagram of the sternum and wing bones of a bird
L. Shyamal, CC BY-SA 2.5, via Wikimedia Commons

The first specimens of archaeopteryx had no evidence of muscle structure like this which lead scientists to surmise that this creature must have simply glided from tree to tree.

This changed with the discovery of the seventh archaeopteryx fossil in 1992. This specimen was smaller than others and contained other differences which lead scientists to believe it to be a different species. It was therefore given the name archaeopteryx bavarica.

This fossil had a large bony breastbone which was more than sufficient to anchor wing muscles.

an archaeopteryx fossil
Ghedoghedo, CC BY-SA 3.0, via Wikimedia Commons

This was all the proof needed to believe that these amazing creatures must have flown. Just imagine them gliding and flapping through lush ancient forests.

a digital rendering of Archaeopteryx
Artist’s digital rendering of Archaeopteryx – image by dottedhippo from Getty Images

The Transformation From Dinosaur To Bird

If birds did evolve from dinosaurs, how did that evolution take place? What natural occurrences or forces pushed a species to develop feathered wings and take flight?

Archaeopteryx was so similar to small dinosaurs that one specimen spent decades in a museum classified as a dinosaur until faint impressions of feathers were discovered around its forelimbs.

These small dinosaurs were possibly predators that chased after their prey. Such activity requires a great deal of energy and to produce such energy, an animal’s body must be warm.

a digital rendering of Velociraptors in a forest
Velociraptor rendering – image by Foto-front from Getty Images

Learn how birds fly in this post here on my blog.

Some believe that Velociraptors were endothermic, like mammals. This would have been an advantage to them as it would have allowed them to be active in the early morning when their prey was still cold and sluggish.

If this is the case, then acquiring an insulating coat would have been invaluable. It is not a stretch of the imagination to believe that the scales of such animals could have, over many generations, become longer and fibrous to provide this insulation.

a digital rendering of Utahraptor
Utahraptor computer rendering – image from Science Photo Library

If these small dinosaurs ate large insects which is quite possible, then it is also possible they may have had to rise up onto their hind legs to catch these insects. This would leave their forelegs free.

It makes sense then to have these forelegs covered with fibrous scales which would then enable the creature to rise into the air to catch a flying insect just beyond reach.

From here it is a logical progression, in evolutionary terms, to the development of more and more feathers and a body more suited to flight for longer periods.

a digital rendering of a flying Archaeopteryx
Archaeopteryx flying – rendering by dottedhippo from Getty Images

The transformation from dinosaur to bird is complete…

The Closest Living Relative Of Birds

Are you sitting down? If not, you may want to before you read this…

Birds are most closely related to Crocodiles. Yes…crocodiles?!

a wren on the left and a crocodile on the right with blue arrows pointing between them and the text "cousins" above

Now, to explain this, I would have to type another 3000 words or so, so instead, I will leave it up to this video from LMSD Science on YouTube to explain it to you:

Is this such a stretch of the imagination? If you go back far enough in the evolutionary tree, WE are related to crocodiles too! Heck, if you go back to the beginning of life on Earth, we all originated from the same organism.

I will leave you with that thought…


Barry Callister

Barry is a bird photographer and bird watcher with over 7 years of experience. He runs his own YouTube channel about photography and promotes his nature photography on his personal website

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