early bird vs riser

early bird and riser both are nouns.

Word	Noun	Adjective	Verb	Adverb
early bird	Yes	No	No	No
riser	Yes	No	No	No

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

early bird (noun)	riser (noun)
a person who gets up very early in the morning	structural member consisting of the vertical part of a stair or step
a person who arrives early before others do	a vertical pipe in a building
	a person who rises (especially from bed)