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Experienced birders often stress the importance of shape and proportions for identification, and there is no doubt that shape is one of the most valuable identification clues, but it takes some practice and experience to be able to notice and interpret the differences. A bird feeder is a great opportunity to practice.
It’s important to remember that this is an impression and not a measurement. Differences in proportions can’t really be measured. This is because we judge the size of one body part relative to other body parts. The tail of an American Robin measures much longer than a chickadee’s, but the chickadee’s tail appears longer relative to the size of its body.
Another challenge is that the overall shape of a bird can vary a great deal depending on its posture and how the feathers are arranged. This is why, when we talk about the shape of a bird, we tend to focus on a few particular aspects of structure that the bird can’t change: tail length, bill shape, primary projection, and leg length.
Head shape and body shape vary a lot depending on how the feathers are held (fluffed out or pressed down), but this variation stays within some fairly strict limits and is determined by the relative lengths of feathers. For example, a bird with a crest (longer feathers on the crown) can appear uncrested when the feathers are pressed down, but when the crown feathers are fluffed up, the species will always appear crested. In contrast, a species without extra-long feathers on the crown will never appear crested, no matter how much it fluffs up its crown.
Other characteristics to be aware of:
• Bill shape always remains the same, but the apparent bill size can change slightly, looking relatively small when head feathers are fluffed up or relatively large when head feathers are pressed down;
• Primary projection always remains the same. The long wing feathers are anchored to the wing bones, and their arrangement cannot be modified;
• Tail length always remains the same, but the tail might seem shorter when body feathers are fluffed out and longer when they’re pressed down;
• Leg length always remains the same but is less helpful because legs can be hidden (appearing short) when belly feathers are fluffed out or exposed (appearing long) when belly feathers are pressed against the body.
To become familiar with the range of variation in shape and proportions, watch common birds, take note of the features, and compare different species.
This article from David Sibley’s “ID Toolkit” column appeared in the January/February 2018 issue of BirdWatching.