amateur vs birder

amateur and birder both are nouns.

Word	Noun	Adjective	Verb	Adverb
amateur	Yes	Yes	No	No
birder	Yes	No	No	No

As nouns, birder is a hyponym of amateur; that is, birder is a word with a more specific, narrower meaning than amateur:

Other hyponyms of amateur include bird watcher, dabbler, dilettante, sciolist, hobbyist.

amateur (noun)	birder (noun)
someone who pursues a study or sport as a pastime	a person who identifies and studies birds in their natural habitats
an athlete who does not play for pay

amateur (adjective)	birder (adjective)
engaged in as a pastime
lacking professional skill or expertise