Nocturnal creature
'nocturnal creature' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for owl that I've seen before include "Nightbird" , "Traditionally wise bird" , "Bird of prey with front-facing eyes" , "Wise old bird" , "Athena's bird" .)
(Another definition for fieldmouse that I've seen is " rodent".)