I think the sentence is ambiguous and could mean the stated translation or the one given by the other commenters.
But isn't Atroksī in the accusative? Then the owls are not eating, they are being eaten.
I might very well be wrong, but maybe atroksia is a "Dāria subtype" noun? If so, then atroksī would be nominative as well as accusative. If not, well, then my theory goes up in smoke.
Oh yes i'm sry. You are totally right. It can be nominative and accusative. I will change my comment above...
First Lunar declension, Dāria subtype:
nom. sing. atroksia
nom. plur. atroksī
acc. sing. atroksie
acc. plur. atroksī
So it's plural nominative or accusative.