What ARE birds?

I am beginning to believe I made a poor choice of birdefinition.

Members of genus Himantura are not birds.
Makararaja chindwinensis are not birds.
Maskrays are not birds.
Cowtail stingrays are not birds.
Pelagic stingrays are not birds.
Ribbontail stingrays are not birds.
Porcupine rays are not birds.
None of spiny, Australian, California, smooth, backwater, nor longtail butterfly rays are birds.
Zonetail butterfly rays are birds.
None of common, spotted, nor longheaded eagle rays are birds.
Neither bull nor cownose rays are birds.
Manta rays still not birds.
Mobula are not birds.