Evidence has been mounting for a while that birds and other animals can count, particularly when the things being counted are items of food. But most of the research is done under controlled conditions.

In a recent experiment with New Zealand robins, Alexis Garland and Jason Low at Victoria University of Wellington tested the birds in a natural setting, giving them no training and no rewards, and showed that they knew perfectly well when a scientist had showed them two mealworms in a box, but then delivered only one. The researchers reported the work this fall in the journal Behavioural Processes.

The experiment is intriguing to watch, partly because it looks like a child’s magic trick. The apparatus used is a wooden box that has a sliding drawer. After clearly showing a robin that she was dropping two mealworms in a circular well in the box, Dr. Garland would slide in the drawer. It covered the two worms with an identical-looking circular well containing only one worm.

When the researcher moved away and the robin flew down and lifted off a cover, it would find only one worm. The robins pecked intensely at the box, behavior they didn’t show if they found the two worms they were expecting.