What ARE birds?

We've been playing this on IRC for the past half hour or so - you may ask me "Are x birds?" to determine what manner of beast, exactly, birds are.

I will answer either that:
-"x are birds"
-"x are not birds", or
-"not all x'es", indicating that some but not all x'es are birds.

Using only this information, the first person to correctly explain what are birds (with the phrase "birds are things which ***") wins, and gets to redefine what birds are for the next round.

I'll give you a hint to start with!
-Not all birds are birds.
-Rhinos are not birds.


Game History
Schazer - Birds are bright green animals (guessed by Sai, 44 posts)
Mirdini - Birds are animals that are faster than a running human (guessed by Akumu, 143 posts)
a52 - Birds are things which Pokemon are based off of (guessed by Kieros, 17 posts)
Akumu - Birds are animals which are Threatened (guessed by Ix, 37 posts)
Kieros - Birds are animals found in GTA5. (guessed by btp, 44 posts)
earthexe - Birds are things which make distinct noises (guessed by btp, 17 posts)
btp - Birds are species which have had their genome mapped. (guessed by Gatr, 20 posts)
- Birds are animals with novelty songs written about them (guessed by Ix, 2 posts)
Slorange - Birds are animals with common names containing x, q, or z (guessed by Schazer, 23 posts)
Schazer - Birds are animals whose common names incorporate the name of another animal (guessed by Akumu, 65 posts)
a52 - Birds are animals which are bigger than a breadbox (guessed by Kieros, 18 posts)
Akumu - Birds are things that Megaman Mavericks are based on (guessed by btp, 112 posts)
Gatr - Birds are animals that can be found on multiple continents (guessed by btp, 66 posts)
ShiningKatana - Birds are things that have organic optical receptors, aka eyes (conceded @ 30 posts)

Are men birds?

Homo sapiens are not birds.

Birds are animals, for ease of definition.

Are pandas birds?

are gatrs birds?

are crocs birds?

caimans? gharials? false gharials?

Pandas are not birds!

alligators, crocodiles, caimans, gharials, and false gharials are not birds.

Are fish birds?

Not all fish.

Are bees birds?

are sea cucumbers birds

not all bees, nor all sea cucumbers.

Are penguins birds?

Penguins are not birds.

Are seagulls (fact-stating or otherwise) birds?

seagulls are not birds.

Every five or so posts, I'll give you an extra hint.

Basilisks are birds.

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sharks are not birds!

is medusa a bird

are phoenixes birds?

dragons?

Are snakes birds?

not all medusa (free-living stage of jellyfish) are birds.

Phoenixes and dragons are fictional creatures.

Not all snakes are birds.

we just don't know

With that out of the way, are trout birds?