Five letters (in Dutch) from Tinbergen to his family from Rossitten, East Prussia, 27 August, 5, 19, 22, and 30 September 1925, and one letter from Tinbergen to his parents from Nidden, East Prussia, 25 September 1925.

The letters are accompanied by a note by Mrs. E.A. Tinbergen (in English):



A friend of his parents, Prof. Ehrenfest, advised his parents to send Niko for some months to the "Rossitten Vogelwarte" in what was then East Prussia, and to get acquainted with other trends in biology from those he was bored with.

Niko was then 18 and came back after 3 months completely convinced that that was what he wanted to do.