water bird vs aquatic bird

water bird and aquatic bird both are nouns.

Word	Noun	Adjective	Verb	Adverb
water bird	Yes	No	No	No
aquatic bird	Yes	No	No	No

As nouns, aquatic bird is a hypernym of water bird; that is, aquatic bird is a word with a broader meaning than water bird:

water bird: freshwater aquatic bird
aquatic bird: wading and swimming and diving birds of either fresh or salt water

water bird (noun)	aquatic bird (noun)
freshwater aquatic bird	wading and swimming and diving birds of either fresh or salt water