I just listened to a neat podcast from Scientific American's Karen Hopkin in which she described a new study published in the Journal of Ornithology that suggests Mexican Jays (Aphelocoma ultramarina) pick which peanut to eat only after literally weighing their options. The researchers modified some peanuts to remove the contents. The Mexican Jays were then offered unmodified nuts along with the empty shells. Perhaps not surprisingly, the birds snubbed the empty shells. Rather, they only chose shells that contained nuts. In fact, when the researchers added clay to some shells to increase their mass by 1 gram, the birds chose the heavier nut. The birds made their selections only after picking up several nuts and shaking them. These observations suggest that the birds may be judging the relative weights of the nuts or at minimum just listening for the rattle of the contents inside.
Piotr G. Jablonski et al, Proximate mechanisms of detecting nut properties in a wild population of Mexican Jays (Aphelocoma ultramarina) Journal of Ornithology
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Very educative story..............thanks.