What ARE birds?

ok it's been far too long so it's time for the big birdreveal;

Birds are organisms which possess distinct visual bilateral symmetry

ie; species with asymmetrical organs didn't count, it had to be something you can see somewhat

Asymmetry like that is pretty rare in Bilateria, since all Bilateria originally were symmetrical, with asymmetry being an evolved trait for various reasons (like snail shells or owl ears)