In 1997 Peter Pyle published an excellent summary of age determination in North American owls. This document was originally published by the American Birding Association. The document is now out of print and difficult to obtain. Peter has graciously given us permission to post a portable document file (pdf) version on the Project Owlnet web site. We are very grateful to Peter for this courtesy.
Flight-feather Molt Patterns and Age in North American Owls (Pyle 1997b) can be downloaded by clicking HERE.