This is a follow-up to my recent post about Optical illusions and bird identification.
While working on some heron studies the other day, I found a good example of an illusory bill size comparison. Compare the bill in the two images of Reddish Egret shown below.
To my eye, at least, the bird on the left appears to have a smaller bill than the one on the right, but the two are actually the same size. Any apparent difference is an illusion created by the difference in the head shape, resulting from the position of the head feathers.
In most studies of live birds in the field, our impressions of size would be an “average” of many different views as the bird moves around, so we would automatically correct for this kind of illusion. It can be more of an issue in assessing photos, when we only get to see the bird in a few isolated moments. It can be an issue in the field if a bird is persistently holding its feathers differently than the birds around it, in which case we might misinterpret fluffed head feathers as a smaller bill.
It can also be an issue if we have a preconceived notion about a bird. In that case, we might selectively pluck the “large-billed” impressions from the stream of information as we watch, and come away with the conviction that one bird has a larger bill, simply because we prefer that result.