In a study recently published in the Journal of Ornithology, researchers from Korea and Poland travelled to Arizona to conduct experiments that showed that Mexican Jays (Aphelocoma wollweberi) have a method of choosing the best peanuts. Although much research has been done on foraging habits of birds, a lot of research on how the birds determine what seeds or nuts are the best has not been conducted.
Shake and Rattle
Researchers Drs. Piotr Jablonski, Dr. Elzbieta Fuszara, Maciej Fuszara, Choongwon Jeong, Won Young Lee and Sang-im Lee, presented Mexican jays with varying types of peanuts still in the shell to see if they could determine the best nuts to take and what the birds would do to make the determination. What they found is that the birds shake and rattle the peanuts to learn what is on the inside; much like a human will thump a melon to test its ripeness and handle it to feel the weight compared to size. A melon that feels heavy for it’s size should be ripe and full of juice.
Mexican Jays Weren’t Fooled
In the first phase of the research, the scientists presented the Mexican Jays with 20 identical looking peanut pods, 10 were empty and 10 were full. The birds picked up the nuts and consistently rejected the ones that were empty, without ever opening the pods.
In another experiment, researchers took pods containing three nuts and removed two of them, closing the pod back up. They then opened one-nut pods and closed them. They then presented the Mexican Jays with the different sized pods. This was to see what the birds would do when presented with pods of different size, but similar content. The birds were able to figure out that the larger pods didn’t hold any more than the smaller ones and consistently chose the smaller pods because they weighed what was expected.
Birds Weigh the Difference
In another phase of the research, the Mexican Jays were presented with pods that appeared identical, yet some were one gram heavier than the others were. The birds would pick up the pods and shake them and would travel away the heavier pods.
Recording in slow-motion video, the researchers studied the actions of the birds to determine what they were doing to find the differences in the nuts. What they found was that the birds would shake the nut, as well as quickly open and close their beak on the shell. It is presumed that the quick clicking behavior gave off noise that the bird used to determine the value of the nut. Picking up the nut allowed the Mexican Jays to weigh it and compare the weight against what was expected.