bittern vs heron

bittern and heron both are nouns.

Word	Noun	Adjective	Verb	Adverb
bittern	Yes	No	No	No
heron	Yes	No	No	No

As nouns, heron is a hypernym of bittern; that is, heron is a word with a broader meaning than bittern:

bittern: relatively small compact tawny-brown heron with nocturnal habits and a booming cry; found in marshes
heron: grey or white wading bird with long neck and long legs and (usually) long bill

bittern (noun)	heron (noun)
relatively small compact tawny-brown heron with nocturnal habits and a booming cry; found in marshes	grey or white wading bird with long neck and long legs and (usually) long bill