What ARE birds?

MASTERPOST

I've removed things from the 'not all' category when there's examples of the larger group in both the Yes and No answers. (Reasonably) larger groups in straight Yes or No are bolded.

Yes: Antarctic Knobbled Octopuses, Arctic Foxes, Bandicoots, Bearded Dragons, Boxer Octopuses, Boxfish, Bullet Ants, Caimans, Chickens, Common Whelks, Elephants, Fennec Foxes, Giant Antarctic Octopuses, Giant Anteaters, Giant Pandas, Glass Octopuses, Golden Orb Weaver Spiders, Hourglass Dolphins, Howler Monkeys, Keelbilled Toucans, Naked Mole Rats, Pink Fairy Armadillos, Pyramid Butterflyfish, Rattlesnakes, Red Foxes, Screaming Hairy Armadillos, Silky Anteaters, Tree Wasps, True crocodiles, Venus Fly Traps

Kind Of: Four toed Hedgehogs

Probably Not: Indominus rex

Not All: Angiosperms, Bears, Carnivorous Plants, Dinosaurs, Gymnosperms, Mosquitos, Mosses, Pitcher Plants, Pufferfish, RiverDolphins, Robins, Sessile Animals, Spiders, Sundew Plants

No: 9-Banded Armadillos, Alligators, American Robins, Anacondas, Anomalocaridids, Blue-ringed Octopuses, Cats, Common Octopuses, Dimetrodons, Dogs, European Robins, Ewoks, False Gharials, Four toed Earless Skinks, Four toed Elephant Shrews, Gharials, Gungans, Hippopotami, Honeybees, Humans, Japanese Robins, Jerboas, Komodo Dragons, Maned Sloths, Manta Rays, Metroids, Oceanic Dolphins (Except Hourglass), Orbfish, Platypuses, Ptarmigans, Red Pandas, Rhinoceri, Sarlaccs, Stegosauruses, Tamandua, Toco Toucans, Tribbles, Two-toed Sloths, Yoshis

• Birdness is determined taxonomically, so "birds are species which _______" is a good way to think about it, yes.
  
• A hint: birdness is not predicated on scientific names. All species have a scientific name.
  
• Not all unicellular organisms are birds, nor are all prokaryotes. Probably. There's a chance that either of the above are not birds, and establishing as such is really goddamn time-consuming.
  
• I cannot without extensive research confirm whether or not all ferns are not birds.
  
• No species of anomalocaridid have the requisite feature to qualify as birds.
  
• As-yet undiscovered species lack the required feature to qualify as birds.
  
• Four toed hedgehogs are kind of birds? I'd feel bad calling it one way or another.
  

• Birds are not animals with sharp teeth or pincers.
  
• Birds are not the lesser extant species of their given subfamily.
  
• Birds are not organisms which have had their taxonomic classifications changed.
  
• Birds are not creatures that remind us of our limited time on earth.
  
• Birds are not species which hide for some time then reappear later.
  

QUESTIONS

You said 'Orcas are not whales.' - are Orcas birds?
Are Felines birds?