This video says about itself:
TENGMALM´S OWL, Finland
From British Birds:
Moult, ageing and sexing of Finnish owls
Published on 05 January 2015 in Book reviews
By Heimo Mikkola and Jouni Lamminmäki
The Ornithological Society of Suomenselkä, Saarijärvi, 2014; pbk, 96pp; photographs and line-drawings
ISBN 978-951-98263-1-8 Subbuteo code M24340 £17.50
Until now, accurate data on the moult of European owls has been somewhat fragmented. This handbook brings together all current knowledge available on the moult, age and sex determination of the 13 species of European owl. The book begins with a useful glossary of terms and abbreviations used in the tables and throughout the text. It is followed by a general overview of moult and its use in ageing and sexing, and aspects such as the process and sequences of moult, how moult varies between species due to influences such as habitat, reversed sexual dimorphism (RSD), breeding cycle, migratory or sedentary behaviour, and the various investigative methods used in determining age and sex. The remainder of the book consists of detailed species chapters comprising three sections: moult (with detailed information on the moult strategy); ageing (analysis of feather shape, colour and pattern during each moult cycle); and sexing (analysis of weights and measurements with corresponding tables).
I found this book to be highly detailed, informative and easy to use in locating specific data quickly. It is published in both Finnish and English, but the English translation is shorter and may not reflect all the details given in the original Finnish text; although I did not get the impression that I was ‘missing’ something, there is surely scope for a future ‘English’ edition. My only criticism would be in the layout of the dual language keys for the tables, as they are not easily readable at a glance. This is an ambitious work and, while not perhaps definitive (the authors admit that further research needs to be carried out on some species), it will surely prove to be an extremely valuable resource.