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David Sibley on bird ID: How shape changes with the seasons

Bird ID and shape: Common Grackles fly in the nonbreeding season.
Bird ID and shape: 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.

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.

Another source of variation is less well known: that a bird’s shape can change with the seasons. The illustrations on this page show seasonal changes in the typical shapes of Common Grackles in flight. The image above shows three grackles, two males and a female, as they would appear in the nonbreeding season, about September to February. The image below shows the three birds as they would appear in the breeding season, about March to July.

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David Sibley

David Sibley

David Sibley writes the column “ID Toolkit” in every issue of BirdWatching. He published the Sibley Guide to Birds in 2000, the Sibley Guide to Bird Life and Behavior in 2001, and Sibley’s Birding Basics in 2002. He is also the author of the Sibley Guide to Trees (2009), the Sibley Guide to Birds-Second Edition (2014), and guides to birds of eastern and western North America (2016). He is the recipient of the American Birding Association’s Roger Tory Peterson Award for lifetime achievement in promoting the cause of birding and a recognition award from the National Wildlife Refuge System for his support of bird conservation.

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