[thethirdone]

The study is testing a very specific type of "recognizing shapes"; which the title of the article calls "geometric regularity". The "background stimuli" are shapes that crows would be expected to be able to distinguish, and are used to train the crows on the task. Whereas the "probe stimuli" are the actual experiment.

As a sibling indicated baboons can not distinguish these shapes easily. Additionally, rather than a binary "crows can recognize shapes" the study shows how well crows process the shapes. One of the graphs in the paper, but not the article shows that two different crows have a similarly hard time with the rhombus.

In other studies, this same test was applied to humans to find that it is a fairly innate skill rather than developed by doing geometry in school.