Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
grŭo, ĕre, v. n. [grus], the note of the crane, to crunk, crunkle: gruere dicuntur grues, ut sues grunnire, Paul. ex Fest. p. 97: grus gruit, Auct. Carm. de Philom. 23.