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Are Birds Mammals?

“Are birds mammals?! Pfft, obviously they’re not mammals!” 

I can almost hear the scoff of disbelief.

And, yes, you’d be correct. Birds are not mammals. 

But wait. 

Birds and mammals are more similar than they first appear. 

Evolution is a curious beast. Throughout milenia, we have been graced with egg-laying mammals and flightless birds – both of which we wouldn’t expect of either group.

So, in actuality, it’s a valid question. 

A Red-capped Manakin, a bird with the coolest moves.
Image by Digital Plume Hunter via Flickr.

In this article, we’ll be exploring how birds are classified, as well as the similarities between birds and mammals that got us asking are birds mammals in the first place. 

Classification Tells Us Birds Aren’t Mammals

So what makes a bird a bird and a mammal a mammal? 

Why not just group all living things together?

Well, with an estimated 8.7 million living species (there could be more, there could be less – we’re still trying to get to an exact figure), putting everything together in one group can become a real mess for scientists. 

Instead, we put species into groups that share similar features. Features that are more or less unique to that particular group. 

Take birds, for example. 

Most birds have feathers, produce hard-shelled eggs and have hollow bones. These traits are shared amongst all birds, and so they can be grouped together in one class. 

Classification of Birds and Mammals

Birds and mammals both belong in Kingdom Animalia (fancy way of saying Animal Kingdom).

They also both belong in the phylum Chordata – a group they share with approximately 80,000 other species (representing just 3% of life on Earth!). 

But that’s where the family tree stops. 

From here, birds can then be separated into the class Aves. 

Then, scientists classify birds (and any other animal) in further groups, based on traits and observable characteristics. These groups are: 

  • Family
  • Genus
  • Species

The closer to species, which is the lowest taxonomic rank, the more closely related animals are. 

What Are Birds? 

If birds aren’t mammals, what are they? 

The more you think about it, the more complicated the answer can become. 

A Scissor-tailed Flycatcher
Image by Wesley Barr via Flickr.

You can’t really define birds as exclusive flying animals. Afterall, an array of different animal groups can fly – from mammals to insects. 

Besides, there are some birds, such as ostriches and penguins, that can’t fly. In fact, around 0.5% of the world’s bird species are flightless, which is about 60 species. 

But, you can blame humans for that. Before we came into the picture, there were many more flightless bird species – I’m talking the likes of the dodo, the elephant bird and great auks. But we’ll save that for another article. 

You also can’t refer to them as warm-blooded vertebrates as, again, that term can also be given to mammals. 

They aren’t even the only animals to lay eggs. 

However, there is one key characteristic that is unique to birds: feathers

No other animal group, at least that we know of, has feathers. 

Whilst different birds use their feathers for different reasons, the three main functions of feathers are:

  • Flight
  • Temperature control 
  • Mate attraction 

Other functions of feathers include waterproofing, camouflage and protection. 

All birds have feathers – even the flightless birds. This is one of the most important differences which separates birds and mammals. 

How Are Birds Similar to Mammals?

Right, this is where we get into the nitty gritty. 

Over the years, scientists and naturalists alike pondered over the very existence of birds. Where did they come from? Are they mammals? What are their closest relatives?

And for many years, the scientists were stumped. 

To us modern day scientists, it’s clear a bird is a bird and a mammal is a mammal. We know that now. 

But, it was a long journey to get there. As it turns out, birds and mammals share many similar features. Here’s just a few:

The Cloaca 

A cloaca is a multifunctional opening for the digestive, reproductive, and urinary tracts of many vertebrate animals – including birds and mammals. 

Unlike mammals, all birds have cloacas like this Wagtail.
Image by Tim Melling via Flickr.

Whilst all birds have cloacas, only a few mammals have this rear orifice. These mammals, most notably monotremes, tenrecs, golden moles, and marsupial moles use the cloaca in much the same way a bird would. 

These mammals, especially monotremes, are the oldest branch of the mammalian tree and still share similar characteristics to their earlier bird ancestors. 

Egg Laying

If you thought egg-laying was unique for birds, think again. 

Many animals – from fish to amphibians – lay eggs. As do some mammals. Sorry, birds, you’re not unique. 

Egg-laying mammals, or monotremes, share key aspects of their reproductive biology with birds. Like birds, eggs are passed through the cloaca and incubated externally. 

However, there is a difference in the texture of the eggs. Unlike avian eggs, which are hard, mammal eggs are soft and leathery, much like the eggs of reptiles. 

Advanced Brain

No disrespect to other animal groups, but birds and mammals have some of the most advanced brains in the animal kingdom. 

Up until recently, it was thought that just mammals possessed the power to yield tools to obtain food. 

However, at least 33 different bird families have been observed using tools to access food. 

Corvids – crows, ravens, magpies and the likes – are perhaps the most cited bird group for tool usage. 

An American Crow - Highly Intelligent Corvidae
Image by David B. Adams via Flickr.

Their ingenious solutions range from fashioning hooks out of twigs to snag insects (as seen in New Caldonian crows), to using cars to help crush hard-shelled nuts (as seen in Japanese carrion crows). 

However, tools aren’t exclusively used by corvids. 

The black kite, a species of raptor found in Australia, has been observed carrying burning sticks and deliberately starting fires to flush out prey species. 

Striated Herons use bait – either in the form of breadcrumbs or vegetation – to lure fish into their strike zone.

Physiological Features 

Both mammals and birds are endothermic vertebrates with four-chambered hearts. 

That was a lot – let’s break it down. 

Endothermic is a fancy way of saying warm-blooded animals. Both birds and mammals can regulate their body temperature internally, regardless of the external environment. Unlike reptiles, which need to bask in the sun to gain energy, birds and mammals can remain active in varying temperatures. 

Vertebrate is a fancy way of saying a backbone. The backbone, or spinal column, supports their body and protects the spinal cord, allowing for greater mobility and complex movements.

Lastly, both birds and mammals have a four-chambered heart. This allows them to separate oxygen-rich blood from oxygen-poor blood, maximizing energy efficiency for energy demanding actions, such as flight or running. 

So, there. 

It’s easy to dismiss that birds are mammals. 

But when you really get down to it, there are many similarities between the two animal groups. 

If an alien came to our planet, would they be able to conclusively say birds and mammals differ? Or would they, like scientists in the past, question whether birds are mammals?

How Are Birds Different to Mammals?

Some birds eat the same food as mammals. Some move in the same way. Others even give birth in the same way. So, it’s looking likely that birds are mammals. Right?

This picture shows birds are different to mammals comparing a deer with an egret.
Image by Gary Seloff via Flickr.

But, as we’ve already discussed, there is a big difference: feathers. 

But are there other differences?

Surprisingly, there aren’t as many differences as you might expect. 

However, the other big difference that makes us certain that birds are not mammals is their reproductive and feeding strategies. 

Cloacal Kiss 

This rather romantic phrase refers to the copulation, or mating, practices of most birds. 

Males mount the females back, and positions himself to align his cloaca with hers (that’s right, both sexes have the same cloaca). Once both cloacas are pressed together genetic material is exchanged and internal fertilization can occur. 

The act itself takes mere seconds. Enough time to get the deed done. 

Feeding Strategies 

Birds do not have teeth. 

Instead, they have specialized beaks that have evolved to a specific diet. 

Some birds have pointed beaks, which they use to tear flesh. Others have long, thin beaks which they use to reach nectar in tubular flowers. 

A Broad-billed Hummingbird
A Broad-billed Hummingbird. Image by Matthew Studebaker via Flickr and website.

And what about what happens to all this food once it’s been consumed?

Unlike mammals, herbivorous and omnivorous birds have a digestive system that features a crop and gizzard. 

The crop acts as food storage, possibly to be used to feed to young, as well as the first step to the digestion process. 

Some bird species, such as pigeons and doves, produce crop milk. This highly nutritional substance is produced by both males and females and is vital for chick development. 

Mammals, however, have mammary glands which are used to produce milk in which offspring suckle. Only females can produce milk.

Did Evolution Get Confused?

Sure, birds and mammals did once share a common ancestor. 

But we’re talking many years ago – some 310–330 million years ago

Along the way, however, it seems that evolution couldn’t quite decide which route it wanted to take. 

So it took both. 

For these evolutionary oddities, we need to head to Australisia and discover one of nature’s biggest mysteries: the duck-billed platypus. 

Found in freshwater of eastern and southeastern Australia and Tasmania, the platypus puzzled early explorers. 

What was this animal that had a bill like a duck and laid eggs like literally every bird ever, but also had fur like so many mammals?

Was it a bird or was it a mammal?

Despite having so many bird-like attributes (granted, flightless birds), scientists deduced that this Frankenstien of an animal really was a mammal. 

A Duck-billed Platypus, a mammal which confused scientists with its bird-like characteristics.
Image by Danny McCreadie via Flickr.

Although it laid eggs and had no teeth, the platypus produced milk and was covered in fur. They do not possess a crop nor did they have feathers. Further studies revealed the bones of a platypus were not the hollow, honeycomb structured bones of a bird. 

The platypus really was a mammal. 

Are Birds Mammals? Final Thoughts

So, what are your thoughts now?

Are birds mammals?

The similarities between the two groups are closer than we once thought.

But, despite sharing some anatomical features (such as cloacas in some mammals), and physiological features (such as both being endothermic vertebrates with four-chambered hearts), birds are not mammals. 

Birds are more closely related to reptiles than they are mammals. 

A Singing Cardinal.
Image by Dr. Phil Wheat via Flickr.
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