Using slow-motion video-recording and experiments, an international team of ornithologists has found that the Mexican Jays are able to ‘weigh’ peanuts while handling them in their beaks.
The Mexican Jay. Image credit: M. Fuszara.
The scientists, led by Dr Sang-im Lee of Seoul National University, 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).
“When we presented the jays with ten empty and ten full identically looking pods (pods without or with three nuts inside), we noticed that after picking them up the birds rejected the empty ones and accepted the full peanuts, without opening them,” said Dr Lee, who is a co-author of the paper published in the Journal of Ornithology.
A series of similar experiments with identically looking normal nuts and nuts that were 1g heavier (pods with some clay added) confirmed that jays always were able to distinguish and preferred the heavier nuts.
“We found out that birds shake the nuts in their beaks. 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,” said study lead author Dr Piotr Jablonski of Seoul National University in Korea and the Museum and Institute of Ecology in Poland.
In another experiment the team prepared one type of peanut pods by opening the shell, removing two out of the three nuts and closing the shell again. The second type of pod was prepared by opening a small pod, which normally contains only one nut, and closing it. Thus, the birds were to choose between 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. They behaved as if they knew that something is wrong with the larger nuts,” said co-author Dr Maciej Fuszara of Cardinal Stefan Wyszynski University in Warsaw, Poland.
When the birds shake the nuts in their beaks, they produce sounds by opening and closing their beaks around the peanut shell for brief moments. Dr Lee and co-authors think that the jays also take this sound into account.
“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,” the scientists said.
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Piotr G. Jablonski et al. Proximate mechanisms of detecting nut properties in a wild population of Mexican Jays (Aphelocoma ultramarina). Journal of Ornithology, published online March 12, 2015; doi: 10.1007/s10336-015-1193-6
