Birds shake shelled peanuts to figure out the quality and quantity of "nut" inside. This is important because birds don't want to waste time and energy opening up a nut only to discover a small, or rotten, nut inside.

Birds prefer heavier nuts, and form expectations about how much a nut should weigh given the shell size. So, if a large-looking nut and small-looking nut weigh the same, the jays will choose the small nut (presumably, the researchers argue, because they realize that "something is wrong" with the large nut).

However, humans are subject to a cognitive bias whereby object size distorts our perception of weight: if a large box and small box are the same weight, we judge the large box to be lighter. This optical illusion remarkably holds even if the test subject only sees the boxes before using a pulley to lift them, or if the test subject is blind but using echolocation to assess size.

Could the birds be subject to a similar cognitive bias?