Birds 'Weigh' Their Options When Choosing Nuts

When it comes to birds, they apparently "weigh" their options when choosing nuts and searching for the heaviest ones, new research says.

Many animals besides birds feed on seeds, acorns or nuts. But the common conundrum they all face is that there's no direct way of knowing what's hidden inside their shells - that is, how much and what quality of food they contain. And while simply breaking open the shell seems like the obvious solution, unfortunately, that takes time and effort that birds and other species cannot spare - especially if their prize is not worth all that work.

But according to a new study published in the Journal of Ornithology, Mexican Jays (Aphelocoma wollweberi) have figured out a way around this problem. Like humans, they can feel and weigh their food to detect which one is heavier.

During the study, researchers 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.

"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." Dr. Sang-im Lee of Seoul National University, the paper's corresponding author, said in a press release.

A series of similar experiments with identically looking normal nuts and nuts that were one gram 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 each 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," explained researcher Piotr Jablonski.

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 take this sound into account when trying to choose the best (heavier) nut.

"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 research team concluded.

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