The overall shapes and proportions of birds provide important clues to their identity. In fact, as birders gain experience, they rely on shape more and more. The subtle clues form the basis of subconscious recognition that allows us to identify a species at a glance.
As we gain experience, we also come to learn that such clues can be misleading. Shape varies from individual to individual, as well as from moment to moment, as a bird’s attitude or behavior changes and as our angle of view changes.