These drawings are not derived from any existing photos but rather are a imaginary description of my encounter with that particular bird. If I needed reference material for things like markings etc I would refer to one of my own photographs.
There is, and rightly so, much emphasis on bird identification in the birding world. In my case the experience of being in the field is as important as the birds themselves. Their environment, the individual personalities and what that particular bird is doing at that moment are as important to me as identifying the bird itself.
Enjoy!