Every image was shaped like a square decorated with a design on each corner. Designs included starbursts, blotchy blobs, spirals and stars. Groups were defined by just two of the four designs.
So, if an image contained a starburst and a blob on any two of the four corners, the overall image belonged to group A. The other two designs varied and could overlap with designs that appeared in images belonging to other groups.
After a period of training with food rewards, pigeons learned to select with about 85 percent accuracy whether an image belonged to category A or B, which they indicated by first pecking multiple times on the image and then selecting one of two boxes to indicate the correct category.
When the researchers then analyzed pecking behavior during trials, they discovered that the birds were pointing right at one of the two designs that defined the object's category, the researchers reported in the journal Experimental Psychology: Animal Learning and Cognition.
That suggests that the birds were using the same kind of selective attention that people use to zero in on features that matter most when determining how to put objects into groups.
"This is another example of a really long line of innovative and groundbreaking work coming out of this lab," said Peter Urcuioli, who studies animal cognition and categorization at Purdue University in West Lafayette, Indiana.
In a simple and elegant fashion, Urcuioli said, the study manages to demonstrate what pigeons are looking at when they categorize. And the results suggest that, even without language, pigeons manage to do what people do: learn to group objects by focusing on what's relevant and ignoring what isn't.
"It's a great thrust in the field of animal cognition to show that the mechanisms by which we are able to accomplish the extraordinary things we do are shared by other species," he said. "It's identifying very fundamental things that allow not only us but also other animals to solve problems we need to solve."