What ARE birds?

Snails are birds!
Some fictional creatures are birds.
Canines that are not wolves are not birds.
Felines that are not lions or tigers are not birds.
Extinct vertebrates besides mammoths and tyrannosauruses are not birds, probably.
Plants that are not angiosperms are not birds (are there non-angiosperm plants that are not gymnosperms?)
Rodents are not birds.
Bears beside grizzly bears and giant pandas are not birds.
Wasps that are not hornets are not birds.
Great white sharks are not birds.
Tiger sharks are not birds.
Some beetles with horns or pincers are birds.
Some extant lizards are birds.

Creatures that have not represented groups or individuals in a tournament bracket are birds.

Phew, I think I got all that right!