F.C. Craighead and J.J. Craighead combined their master's theses into 
the book 'Hawks, Owls and Wildlife' (1956, Dover).  They did surveys
of raptor populations over the year.  They showed that Great Horned
Owls were extremely effective at cleaning out the nests of nearby 
hawks.
                                      Jim

Jim Felley
Smithsonian Institution