bird watcher vs birder

bird watcher and birder both are nouns.

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

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

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

bird watcher (noun)	birder (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