grim-beaked baron: the little owl of the tree.
keep: an older name for castle; sometimes for the dungeon.
Page 20
for him to call the summer rain: alluding to his evening and his cloudy-day call as a sign of coming rain.
Page 22
castings: the disgorged lumps of hair and bones of the small animals eaten by the owls.
Page 24
Altair and Arcturus: prominent stars in the northern hemisphere.
CHAPTER IV
TO THE TEACHER