David Sibley on Bird Identification: How Shape Changes with the Seasons

Bird ID and shape: Common Grackles fly in the nonbreeding season.
Bird Identification and Form: The Common Grackle flies outside of the breeding season. Bird Identification and Form: Grackles, two males and one female, fly outside of the breeding season. Males hold their tail tightly closed. Illustration by David Allen Sibley.

The general shapes and proportions of birds provide important clues to their identity. In fact, as birders become more experienced, they rely more and more on form. Subtle cues form the basis of subconscious recognition that allows us to identify a species at a glance.

As we gain experience, we also learn that such cues 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 our viewing angle changes.

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

Common Grackle, two males and one female, observed during the breeding season. Males fly with a characteristic keel-shaped tail. Illustration by David Allen Sibley.

In this case, the feathers don’t change, but the way the grackles hold them does. Outside of the breeding season, males hold their tails tightly closed and streamlined in flight. However, during the breeding season, they fan out their tails, creating a unique keel shape. The tail forms a large triangle.

The problems arise from the selective way in which we birders form mental impressions of shape. We tend to remember only the most obvious and distinctive shapes. If we do not realize that its shape changes with the seasons, our impression of the “typical” Grackle will be rooted in its appearance during courtship flights during the breeding season, when the tail is fanned out in a broad triangle. Then, when October rolls around and we see a grackle flying with its tail closed, the identification might give us pause.

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This is just one striking example; there are many others. The head shape of male ducks, for example, can change when they are in a courting mood. Usually, as in the case of the grackle’s tail, the result is a more extreme and obvious expression of the characteristics that were there all along. Hawks, swallows, and other birds have distinctive wing flapping styles and wing postures that they use when courting.


Observing differences in shape early in the courtship and nesting season can help you appreciate the subtle changes and also provide insight into the birds’ life cycle.

This ID Toolkit article by David Sibley appeared in the January/February 2017 issue of BirdWatching.

See reader photos of the Grackle