This is the presentation of two old texts of the first part of the XX century. One of them was written by Viktor Zebe:
• Viktor Zebe, Oktober 1936 – Zur biologie des Schlangenadlers [Biology of the Short-toed Eagle] – Berichte des Vereins Schlesicher Ornithologen /map/. Just read the translation of it (Fr) from German made by one of the greatest adepts in Short-toed Eagles in France – Yves Boudoint.
The second is Nikolay Gavrilenko‘s work, that is mentioned in the first one:
• Гавриленко Н.И., 1929 – Птицы Полтавщины. Змееяд [The birds of the Government of Poltava. Short-toed Eagle] // Полтава: Издание Полтавского союза охотников: 28-30 (Ru) /map/.
Just download a scan of the part dedicated to all birds of prey on the Ukrainian Birds of Prey Research Centre website.