field mouse vs fieldmouse

field mouse and fieldmouse both are nouns.

Word	Noun	Adjective	Verb	Adverb
field mouse	Yes	No	No	No
fieldmouse	Yes	No	No	No

As nouns, field mouse and fieldmouse are synonyms defined as:

field mouse and fieldmouse: any nocturnal Old World mouse of the genus Apodemus inhabiting woods and fields and gardens

field mouse (noun)	fieldmouse (noun)
any of various small mouselike rodents of the family Cricetidae (especially of genus Microtus) having a stout short-tailed body and inconspicuous ears and inhabiting fields or meadows	any nocturnal Old World mouse of the genus Apodemus inhabiting woods and fields and gardens
any nocturnal Old World mouse of the genus Apodemus inhabiting woods and fields and gardens