In yet another example of avian ingenuity, researchers from Poland and Korea have discovered that birds have the ability to use weight and sound to distinguish more desirable nuts from their less desirable counterparts.
The study was conducted on Mexican Jays (Aphelocoma wollweberi) and published in the Journal of Ornithology. An international team of researchers observed the birds in their natural habitat in Arizona and found that the clever jays actually "weighed" and "listened to" the nuts by shaking them in their beaks.
"We presented the jays with ten empty and ten full identically looking pods," said Dr. Sang-im Lee of Seoul National University. "We noticed that after picking them up, the birds rejected the empty ones and accepted the full peanuts, without opening them."
The researchers also offered the jays identical nuts that differed slightly in their weight. After grasping the nuts in their beaks, the clever jays consistently chose the nuts that were 1 g heavier. To observe the subtleties of the birds' selection process, the teams used slow-motion video to record the activity.
"We found out that birds shake the nuts in their beaks," co-author Dr. Piotr Jablonski says. "We think that these movements may provide them with the information generally similar to our feeling of 'heaviness' when we handle an object in our hands."
In another round of experiments, the scientists opened the shells of peanuts containing three nuts, removed two out of three of the nuts, and then closed the shell. They offered these to the birds, along with smaller shells that normally contain only one nut, to see how the birds chose among nuts of similar content and mass, but of different size.
"The jays figured out that the larger pods did not weigh as much as they should and the birds preferred the smaller pods, which weighed as expected for their size," said Dr. Elzbieta Fuszara.
The video records the birds shaking the nuts, and opening and closing their beaks against the shells. The scientists believe the birds are using the sounds produced, combined with the weight of the peanuts, to make their selection.
"Our next goal is to disentangle the role of sound relative to the perception of 'heaviness' and to determine if jays use the same sensory cues for acorns - their natural food."
