Autumn tradidionally starts with the Migrating birds of the Volga delta tour. Two join-in groups in the mid of September and in the begining of October visit Asytrakhnsky nature reserve. Tour program is here:
http://www.ecotravel.ru/eng/tours/1/2/.
First autumn moths are also the best time of the year in Russian Far East starting from Kamchatka and Commanders moving south to Sakhalin and Primorie. The autumn colours of the region is hard to describe in words, its a must see. Some of the pictures one can admire here::
http://www.ecotravel.ru/eng/tours/1/58/.