Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
mōtăcilla, ae, f., the white water-wagtail: motacilla, quod semper movet caudam, Varr. L. L. 5, § 76 Müll.; Plin. 37, 10, 56, § 156; Arn. 7, 223.