Owls of the World - Helms Identification Guides.
Claus Konig, Friedhelm Wieck
Owls are enduringly popular birds, but due to their nocturnal habits most species are difficult to see well. The plumages of many species are cryptic and difficult to separate by plumage alone. This problem is compounded by the different morphs that many adopt. This book fully describes every known species and subspecies of owl, as well as presenting the latest evidence on owl taxonomy, based on DNA work and vocalisations. Because voice is vital in owl identification, much emphasis is placed on it in the book and sonograms are provided for many species.