What ARE birds?

Is it ok for a hint? Is birdness based on a physical attribute?

it sure sounds like it could be

you have to look carefully to notice

it might be more useful to think about not birds than birds

Is a barn owl with the horns of the horned owl a bird?

A barn owl is not a bird, with or without such 'horns'

are frogs birds?
Are whistling tree frogs birds?

frogs 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)

Birds are characterized by their trait of when you show them to either my cat or my dog (separated since 2002), they will get upset.