New research suggests that some birds evaluate their food in similar ways humans do.
Humans detect which food items are heavier by moving the items up and down several times and focusing on the "feeling of heaviness" we perceive. They can also detect the quality of a water melon by knocking on it. Researchers found that Mexican Jays can also use similar tricks in choosing the peanuts from the feeder.
For 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 (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." Dr. Sang-im Lee of Seoul National University said in a statement.
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.
To determine how the birds knew which were empty without opening them, 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," Jablonski said.
The findings are detailed in the Journal of Ornithology.
