What ARE birds?

Okay time for some lists:

_____ are birds:

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• Tree wasps
  
• Rattlesnakes
  
• Naked Mole Rats
  
• Bullet Ants
  
• Golden orb weave spiders
  
• Howler Monkeys
  
• True crocodiles
  
• Caimans
  
• Fennec Foxes
  
• Arctic Foxes
  
• Whelks
  
• Keel Billed Toucans
  
• Hourglass dolphins
  

_____ are not birds:

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• Anacondas
  
• Honeybees
  
• Chickens
  
• Yellow Fever Mosquitos
  
• Alligators
  
• Gharials
  
• Dogs
  
• Platypuses
  
• Ptarmigans
  
• Toco toucans
  
• Any oceanic dolphin except the hourglass dolphin
  

Not all ______ are birds.

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• Not all mosquitos are birds
  
• Not all pufferfish are birds
  
• Not all bears are birds
  
• Not all spiders are birds
  
• Not all species that existed prior to homo sapiens are birds.
  
• Not all birds are birds.
  
• Not all organisms that do not fit in the kingdom Animalia are birds.
  
• Not all plants are birds.
  
• Not all animals that stay fixed to one location are birds
  
• Not all unicellular organisms or prokaryotes are birds (probably)
  
• Not all river dolphins are birds

So it's a category that is selective across kingdoms, but ostensibly limited to living things. It doesn't have to do with interactions with people. Bluarg! This is a good one. I'm having a really hard time seeing a distinct pattern here (and lots of little patterns that just don't fit!)

Okay lets check out some plants!

Are angiosperms birds?
Are gymnosperms birds?
Are ferns birds?
Are mosses birds?
Are carnivorous plants birds?

Then...

Are pandas birds?
Are orcas birds?
Are dimetrodons birds?

Are any of the archaea responsible for the coloration in the grand prismatic spring birds? (or is it possible to tell without extensive research?)