What ARE birds?

Animals that cannot survive on land are not birds.
Animals that only breathe with gills are not birds.
Extinct species are not birds.

These are not the conditions I am looking for, however.

American crocodiles are birds.
Saltwater crocodiles are birds.
It can be debatable whether New Guinea crocodiles are birds, but I will say no, they are not.

Every other crocodile species are not birds.

Birds are not immortal space entities.