When possible let your bird have eye contact with the viewer rather than looking forward or to the side. This will give your painting interest and establish more of a connection with the viewer.
Just like in portrait painting the eyes need to be believable. In the eye above notice two different values of grey around the highlight. I usually find a few greys or grey blues around the highlight which adds more depth to the highlight. Sometimes you will find an entire outdoor scene in the birds eye. Study the reflections and use as many colours as you can to describe the form.
Use the computer to enlarge and lighten the eye and see the shape of the pupil and the other reflections going on. Often in a photograph the birds eye will be black with no pupil detail. But in nature, even with the blackest birds eyes there will be a colour difference between the pupil and iris, its a matter of how much light hits the eye. I like to Identify the difference between the iris and pupil and let it show even when my reference doesn't.
Rose Tanner is an award winning artist who loves the outdoors and studying birds. She is dedicated to portraying her subjects using traditional oil painting techniques, travels widely for her subjects and is active in supporting endangered birds and their habitat.