Owls : ambassadors for the protection of nature : in their changing landscapes : proceedings of the fourth world owl conference, 31 October - 4 November 2007, Groningen, The Netherlands / ed. by David H. Johnson, Dries van Nieuwenhuyse and James R. DuncanOwls, ambassadors for the protection of nature, in their changing landscapes : proceedings of the Fourth World Owl Conference, 31 October-4 November 2007, Groningen, the Netherlands / edited by David H. Johnson, Dries van Nieuwenhuyse and James R. Duncan