birdnest vs bird-nest

birdnest is a noun but bird-nest is not a noun.

Word	Noun	Adjective	Verb	Adverb
birdnest	Yes	No	Yes	No
bird-nest	No	No	Yes	No

As verbs, birdnest and bird-nest are synonyms defined as:

birdnest (noun)	bird-nest (noun)
nest where birds lay their eggs and hatch their young

birdnest (verb)	bird-nest (verb)
gather birdnests	gather birdnests