The New Zealand robins recognized when they had been "cheated" by researchers, who put two mealworms into a box that would then show the robin only one. These results imply that the robins can measure values—a skill that might have developed along with the species' propensity for stealing stored food from their mates.
These Birds Can Count
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Key Facts In This Video
See how a "magic" box is used to hide mealworms and prove that birds can count: 00:34
New Zealand robins appear to count the number of mealworms that a researcher places into a box, and will search if they know one is missing. 00:59
The counting ability of New Zealand robins may stem from the fact that they commonly steal from one another. 01:20