in answer to Birdpotter.....
Dont feel at all foolish, its a good question, a little hard to answer properly...
I think it is fair to say that the majority of bird artists use photo references, personally I will use them at times to back up field sketches, which may be incomplete due to restricted time before a bird dissapeared, or often it can be immensly helpful for background detail and tones. Most of the time though I will work mainly from field sketches as they provide me with much more...
I think when you field sketch a lot your eye becomes quicker at taking in details of shape and appearance. Also your eye is moving over the subject as it turns and moves and you gain an appreciation of the bird in a fully dimensional way, its shape, volume, mannerisms, movements, its whole jizz. To answer your question, the answer is yes, given time a few glances at a subject will be enough to get its outline dowm on paper, but this takes a lot of practise. Photos just dont convey this and that is why if you have not spent time sketching or at least watching a species in the field you will never be able to convey the birds essential vitality on paper or canvas.
I think field sketching combined with a few photos is enormously helpful. Finished work can also be stunning from field sketches alone. Though it is possible to work from photos alone if you are familiar with the species I think that working on an unfamiliar species from a photo is doomed to a degree of failure. As an exercise it may help in areas like mixing paint etc. but I do feel that copying a photo directly and as accurately as possible is a little futile, given that the photo already exists.
I field sketch a lot, but this means I had to change one aspect of my birding. I generally bird alone now, as it is hard to sketch when in company. I stop whereever I see something of interest to me and others are often interested in pressing on for more species. I think to sketch properly this is required. I do occassionally bird with others, but always get little done. Twitching rarities is an exeption as you can usually sit relatively undisturbed and sketch away.
Hope this helps! (and that I have not meanderered off topic