Scientists discovered that the Mexican Jay is not just taking any peanut it finds. The bird is comparing peanuts to find the heavier ones and takes those away.
Nuts and seeds are tricky food as there is no easy way to know what is inside. I have often found that the hazelnuts laying on the ground under the bush are empty when openend. My explanation is that the squirrels know that they are empty and drop them and pick the next. Now scientists discovered that birds are weighing peanuts to chose the heavier ones to pick and carry away.
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An international research team from Poland and Korea discovered in field research in Arizona 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).
"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." says Dr. Sang-im Lee of Seoul National University -- the corresponding author of the paper. 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. How did they know which were empty without opening them? The researchers used slow motion videos to see what happens when the bird is deciding whether to drop or take away the peanut pod. "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", says Dr. Jablonski.
In another experiment the researchers 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 jays 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", comments Dr. Fuszara. They behaved as if they knew that "something is wrong" with the larger nuts.
So how do they know it? When they shake the nuts in their beaks, the birds produce sounds by opening and closing their beaks around the peanut shell for brief moments. The researchers 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", conclude Dr. Lee and Dr Jablonski.
A video showing the Mexican Jays weighing peanuts can be viewed here. It definitely looks like the birds are weighing peanuts. They obviously do not pick the first one and "shop around."
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The full details of these findings have been published in the paper: Jablonski PG, Lee SI, Fuszara E, Fuszara M, Jeong C., Lee WY. 2005. Proximate mechanisms of detecting nut properties in a wild population of Mexican Jays (Aphelocoma ultramarina). Journal of Ornithology, [DOI 10.1007/s10336-015-1193-6]