What ARE birds?

All <x> are birds:

• All birds
  • Owls
    
  • Zebra Finches
    
  • Peacocks
    
  • Moltres (does that count?)
    
  • Skylarks
• All mammals
  • Dogs
    
  • Reyweld (presumably)
    
  • Cats
    
  • Dolphins
• All insects
  • Katydids
    
  • Bees
• Some fish:
  • Trout
    
  • Eels
    
  • Goldfish
    
  • Mudskippers
    
  • Angelfish
    
  • Clownfish
    
  • Lionfish
    
  • Stingrays
    
  • Sharks
    
  • Dead stingray
    
  • Betta fish
• All reptiles
  
• Moltres
  
• Squids
  
• Lobsters
  
• Velociraptors

No <x> are birds:

• Mountains
  
• The sky
  
• Windmills
  
• Plants
  
• Fungi
  
• Bacteria
  
• Archaea
  
• Worms
  
• Tables
  
• Bones
  
• Words

Not all <x> are birds

• Fish:
  • Birds:
    • Trout
      
    • Eels
      
    • Goldfish
      
    • Mudskippers
      
    • Angelfish
      
    • Clownfish
      
    • Lionfish
      
    • Stingrays
      
    • Sharks
      
    • Dead stingray
      
    • Betta fish
  • Not birds:
• Non-insect invertebrates:
  • Birds:
    • Squids
      
    • Lobsters
  • Not birds:
    • Worms