When considering impressions of shape and size as in Optical illusions and bird identification or More on illusions of size, one important aspect that I’ve mentioned only in passing is relative size.
Consider the two birds in the sketches below, which one has the longer bill?
Tricolored Heron (left) and Reddish Egret (right) approximately to scale. Pencil drawing copyright David Sibley.
I suspect that viewers will say either “They’re about the same length” or “The Tricolored is longer-billed.”
The actual measurement of bill length (from the forehead feathering) is about the same in these two images, as it is in real life, and many people will probably say that is how they appear. If you consider relative bill size, however, you will say that the Tricolored Heron has a longer bill, because the rest of the bird is obviously smaller than the Reddish Egret, while the bill is about the same length.
I wonder if experienced birders are more likely to ignore the absolute bill size, knowing that judging the actual size of a thing is fraught with error, and instead to focus on the proportions of the bill relative to head, eye, and neck. In that sense Tricolored Heron is clearly longer-billed than Reddish Egret.
Depending on your approach, both answers are correct.
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