riser vs early bird

riser and early bird both are nouns.

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

As nouns, early bird is a hyponym of riser; that is, early bird is a word with a more specific, narrower meaning than riser:

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