As a life-long bird watcher and university graduate in avian sciences, I have learned a lot about various species of birds.  However, no one person can know everything or keep up with all of the latest information.  As a result, I used the following sources to add to my knowledge when it was lacking:

Eastern Birds 1980 (R.T. Peterson) Houghton Mifflin Company, Boston

A Field Guide to Identification: Birds of North America 1983 (C.S. Robbins, et. al.) Golden Press, New York

Wikipedia