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?
A new study published in Journal of Ornithology suggests that some birds can also use similar tricks in choosing the peanuts from the feeder. Their study was carried out in Arizona by an international research team from Poland and Korea and revealed that the Mexican Jays (Aphelocoma wollweberi) may be able to "weigh" peanuts and maybe even "listen to" peanuts while handling them in their beaks. Drs. Sang-im Lee, Piotr Jablonski, Maciej and Elzbieta Fuszara, the leading researchers in this study, together with their students and helpers, spent many hours delicately opening shells of hundreds of peanuts, changing the contents and then presenting them to the jays in order to see if the birds can figure out the differences in the content of identically looking peanut pods (peanuts in shell).