birder vs bird watcher

birder and bird watcher both are nouns.

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

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

birder and bird watcher: a person who identifies and studies birds in their natural habitats

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