titmouse (n.)
small, active bird, early 14c., titmose, from tit (n.2), expressing something small, + Old English mase "titmouse," from Proto-Germanic *maison (cognates: Dutch mees, German meise), from adj. *maisa- "little, tiny." Spelling influenced 16c. by unrelated mouse, "when mose had long been obsolete as an independent word" [OED]. The proper plural is titmouses.