bird watcher vs ornithologist

bird watcher and ornithologist both are nouns.

Word	Noun	Adjective	Verb	Adverb
bird watcher	Yes	No	No	No
ornithologist	Yes	No	No	No

As nouns, bird watcher and ornithologist are synonyms defined as:

bird watcher (noun)	ornithologist (noun)
a person who identifies and studies birds in their natural habitats	a zoologist who studies birds
a zoologist who studies birds