One of the best ways to train yourself to rapidly draw birds in the field is to practice at home with clear sharp photographs. Set a timer and give yourself one minute for each sketch. Do not worry about details but concentrate on getting the basic shape or silhouette. Give it a try. Stop what you are doing now and give it ten minutes. Try doing ten more each day for a week and watch your speed and ability to capture the shape of a bird improve. My favorite site for quality bird photos is Robert Royce's Bird Photography Pages.
I start by capturing the posture of the bird (axis of the body). Then add an oval for the body and a proportionate circle for the head. I stop and double check the proportions before I go on. Then I add lines showing the angle of the beak and tail. Now focus on the edges of the bird and carve in the angles around the head and tail. It may help to focus on the negative shapes you see beyond the bird rather than the bird itself. Think "posture, proportions, and angles".