What Is A Bird? [dinosaur Scientific Sciences Of Science]

What is a bird?

Birds are the only surviving clade of theropod dinosaurs that are characterized by...
uh...
Hold on.

What... IS a bird?
I know what you are thinking.
"Birds are the things with feathers, wings, and hollow bones!"
But by your logic, this-

(Velociraptor mongoliensis, otherwise known as Pointy Birb, by ryuukibart.tumblr.com / liberaldinosaurfan.tumblr.com )
is more of a bird than this-

(Dromaius novaehollandiae, otherwise known as Emu. It even has a single finger with a claw on it!)

So, our classical definition of "bird" is wrecked because the term "bird" is a leftover from Linnean taxonomy, which ignores fossil evidence only taking into account living organisms. Now science has moved to Cladistics- a method of classification by defining things by their evolutionary relationships (everything that descends from organism A or every organism more closely related to B than C)

The most common "first bird" is Archaeopteryx. But- Archaeopteryx couldn't fly, had fingers with claws, a long tail, and a beakless snout. So... that's not very birdlike.
So, the most conservative way of defining bird is Neornithes- a clade consisting of all living birds and their common ancestors. I mean, there is an argument to be made that ratites like emus and ostriches are not birds but that's stupid.
However, a problem arises when we have things like Hesperornis;

(by Carl Buell)
which is essentially a bird. Granted, it was also flightless, but it definitely had a flying ancestor. Both are outside Neornithes.

(By cladocircles.tumblr.com) (DO NOT SEARCH AVIALAE ON GOOGLE I KNOW YOU WANT TO NOW BUT NO PLEASE DON'T DEAR GOD WHY ARE THERE SHIRTLESS PEOPLE THIS MAKES ME FEEL AWKWARD HELP)
So. Do we move it back to Ornithurae, our group containing both Hesperornis and our trusty Kiwi birds?
NOPE. We have Songlingornis!

Try and not call this a bird. I dare you.
So... uh...
Our birds are Ornithuromorphs... right?
Nah.

yeah
is this not a bird?
So Archaeorhynchus moves our birds back to Euornithes...

right?
I think you can see a trend here.
So, all dinosaurs that could fly and their common ancestor.
That would be Ornithoraces and...
wait.
Penguins can't fly.
Hesperornis can't fly.
Emus can't fly.
Flight is not a defining feature of a bird.

How about all feathers?
Well then. We move our birds back to OH GOD IT'S ALL OF THEM

(Because Kulindadromeus proves that all dinosaurs were ancestrally feathered)

But wait
Pterosaurs have pycnofibres!
Which are basically feathers!
So that makes it Ornithodira... right?

OH BUT WAIT! There's more!
Crocodiles have scales that are strikingly similar in appearance and composition to avian scales (Feathers reverted to scales commonly seen on bird feet). So that puts our bird arrow at Archosauria.

AND WHILE WE'RE AT IT TURTLES WHICH ARE CLOSE TO ARCHOSAURS HAVE BEAKS SO...
uh...
Actually, where am I going?
Who ever calls crocodiles birds?
Like, ever?


Huh.
Maybe Coelurosauria?
I mean, but would you call tyrannosaurus a bird?
honestly I would

But for most people it would probably be around Avialae.

Truth is, the term "bird" sucks.
Basically, when deciding what to call "bird", it boils down to-

 

a bird is something that have wings
bye

 

because this is a bird

 

Yes

 

GONNA GO SEARCH.

edit: OH MY GOSH YOU WEREN'T KIDDING

KIWIS WOOO

oh.
oh.

Spoiler: maybe this will brighten your day

Birds with arms. What a time to be alive.

 

I WARNED YOU!
BUT DID YOU LISTEN?
NO.

 

A bird is...

is....

 

Mad props for Jerma clip

oh yeah wasn't there something about birds i guess that was neat