Finding a host of fallen nuts can be to a bird like Christmas morning for a child. Just as deciding which present to open first can be a challenge for a child, birds often need to work out which nut, inside its shell wrapping, most deserves their attention. Their methods have been revealed to be quite sophisticated.
Breaking shells open can be an effort, so animals don't want to waste energy on nuts that are rotten or undersized. Moreover, opening to check isn't an option for a species that puts nuts away for winter. So, their challenge is far greater than the child on Christmas Day; it's not just a matter of getting the best present first, choose wrongly and you could end up going hungry.
Dr Sang-im Lee of Seoul National University wanted to see how birds make their decisions, and used transvolcanic jays (Aphelocoma ultramarina) as test species, and peanuts as the food. However, one problem was the possibility that some unknown clue on the outside of the peanut might tell the Jays which shells had the best nuts inside.
Consequently, some long-suffering researchers, research assistants and even students went through the no doubt painfully boring process of opening hundreds of peanut shells and swapping the nuts into other shells. Once they had made sure that any relationship between shell and nut was entirely coincidental, the team sealed the shells back up and gave them to the jays.
"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 Lee.
As revealed in the Journal of Ornithology, Lee and colleagues filmed the birds in the wild, to avoid any distorting effects of birds raised in captivity, The authors also tried adding bits of clay with similar weight to peanuts to shells to see what difference that made.
The jays “evaluated each peanut by performing fast movements of the head combined with additional fast movements of the beak, which may open and subsequently close, producing sound at the moment of hitting the shell,” the paper reports. The authors concluded that the jays were able to assess the weight of a shell to an accuracy of a gram (0.035 ounces), consistently taking the heavier nuts, even when the shell was larger.
The birds also treated oversized nuts with the same weight as smaller ones with suspicion, which the authors argue “indicates that birds may have a concept of how much a nut of given size should weigh.”
On the other hand, the effort involved in opening and closing their beaks around the nut may give the birds an idea of how hollow the nut is through its sound.
The authors state, "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."