fieldmouse vs field mouse

fieldmouse and field mouse both are nouns.

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

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

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

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