Watching birds in flight is thrilling. The wings of small birds flap too quickly for the eye to catch. If all you see is a blur, draw the blur. Large soaring birds such as raptors, gulls and ravens hold their wings still and often return to similar positions, making it possible to draw them in flight. Just as in drawing perched birds, there are three elements to block in before adding any details: posture proportions, and angles. If you want to take these lessons further, print out a bird model and practice sketching angles in flight with bent wings.
Start by drawing the axis of the body and the angle at which the wings cross it. If the bird is directly overhead, these may form a right angle. If the bird is coming or going, these lines will cross at an angle.
Build a flight frame over the posture noting the width of the wings, thickness of the body, length of tail, and proportions of primary to secondary feathers.
Cut in the negative space angles around the head, wingtips, and tail-wing angle. These angles are critical. Carve them in to sculpt the bird before adding any detail.
These three steps will help you block in the necessary shapes to accurately draw birds in flight. Lock in the overall shape before adding any details. Once details are down on the paper you are unlikely to let yourself go back and modify your basic proportions.
If you live in the Bay Area, try heading out to Hawk Hill to sketch the raptor Migration.