David Sibley explains why a bird’s shape can be deceiving

David Sibley explains why a bird's shape can be deceiving
Three views of an American Goldfinch, with smooth, typical, fluffy feathers. The general shape is very different, but the beak, tail and main projection remain the same. Artwork by David Sibley

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Experienced birders often emphasize the importance of shape and proportions for identification. There’s no doubt that shape is one of the most valuable identifying clues, but it takes a bit of practice and experience to be able to notice and interpret the differences. A bird feeder is a great opportunity to practice.

It is important to remember that this is an impression and not a measurement. The differences in proportions cannot really be measured. This is because we judge the size of one part of the body relative to other parts of the body. An American robin’s tail measures much longer than that of a chickadee, but a chickadee’s tail appears longer in relation to its body size.

Another challenge is that a bird’s overall shape can vary greatly depending on its posture and the arrangement of feathers. That’s why when we talk about the shape of a bird, we tend to focus on a few particular aspects of the structure that the bird cannot change: the length of the tail, the shape of the beak, the primary projection and leg length.

The shape of the head and that of the body vary greatly depending on how the feathers are held (inflated or pressed), but this variation remains within fairly strict limits and is determined by the relative lengths of the feathers. For example, a bird with a crest (longer feathers on the crown) may appear crestless when the feathers are sunken, but when the crown feathers are puffed out, the species will still appear with a crest. In contrast, a species without extra long feathers on the crown will never appear crested, no matter how much it puffs out its crown.


Other features to know:

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• The shape of the beak always remains the same, but the apparent size of the beak may change slightly, appearing relatively small when the head feathers are puffed out or relatively large when the head feathers are sunken;

• The primary projection always remains the same. The long wing feathers are anchored to the wing bones and their arrangement cannot be changed;

• The length of the tail always remains the same, but the tail may appear shorter when the body feathers are puffed out and longer when they are sunken;

• Leg length still remains the same but is less useful because the legs can be hidden (appearing short) when the belly feathers are puffed out or exposed (appearing long) when the belly feathers are pressed against the body.


To familiarize yourself with the range of variations in shape and proportions, look at common birds, note their characteristics, and compare different species.

This article from David Sibley’s ID Toolkit appeared in the January/February 2018 issue of BirdWatching.