David Sibley on bird ID: How shape changes with the seasons 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 … Read More “David Sibley on bird ID: How shape changes with the seasons” To access this content, you must purchase BirdWatching Membership. Common Grackles, two males and a female, fly in the nonbreeding season. The males are holding their tails tightly closed. Art by David Allen Sibley.