The Collaborative International Dictionary of English v.0.48:

Tu-whit \Tu-whit"\, Tu-whoo \Tu-whoo"\, n. & interj.
   Words imitative of the notes of the owl.
   [1913 Webster]

         Thy tu-whits are lulled, I wot,
         Thy tu-whoos of yesternight.             --Tennyson.
   [1913 Webster]