Letter from the Dutch ornithologist F. Haverschmidt to Tinbergen, 5 May 1930; letter from the Director of the Amsterdam Zoo A.F.J. Portielje to Tinbergen, 4 September 1931; draft of letter from Tinbergen and Luuk Tinbergen (brother) to the German ornithologist Uttendörfer 19 October 1931; letter from Jos [van Room, later Baerends] 20 July 1935; letter from G P Baerends, 28 July [1935].