birdsong vs song

birdsong and song both are nouns.

Word	Noun	Adjective	Verb	Adverb
birdsong	Yes	No	No	No
song	Yes	No	No	No

As nouns, birdsong and song are synonyms defined as:

Other synonyms of birdsong include birdcall, call.

birdsong (noun)	song (noun)
the characteristic sound produced by a bird	the act of singing
	a short musical composition with words
	the characteristic sound produced by a bird
	a distinctive or characteristic sound
	a very small sum