What ARE birds?

schazer:
in order for me establish whether something is a bird
it must first have a particular attribute that not all species have
there's no point counting how many toes an animal has if it doesn't have legs
similarly, there's no way I can determine an organism's bird-ness if it doesn't have ____________

btp:
Oh my god!
Birds are creatures that don't have a maned sloth!
BRILLIANT

schazer:
what have you found out about the maned sloth?
a final hint, for now, cuz i should sleep
you may, based on the way I tend to structure my answers, be tempted to order birds and not-birds taxonomically when trying to figure out the commonalities
this however is unhelpful

Woah Mirdini that is a super helpful post!
Orcas are dolphins which means they are not birds.

Are birds creatures that have something particular on their wikipedia page?