While communicating using the eyes is something that primates and humans often do, this type of behavior has never been documented in other types of animals. However, a new study shows that certain birds can also and do use their eyes to communicate.

A team of researchers studied the behaviors of Western jackdaws, dark colored birds often found in Africa and parts of Europe and Asia. These birds, which are closely related to crows, are known to have eyes that are similar in appearance to human eyes. According to their findings, these birds often use intimidating glares to scare off other jackdaws.

The research team led by Gabrielle L. Davidson from the University of Cambridge published their paper in the online journal Biology Letters. The research team also chose to study jackdaws due to the conspicuous nature of their eyes. Jackdaws have very dark pupils ringed by white irises, which are especially visible even from a short distance away.

"Because jackdaws compete for nest sites, their conspicuous eyes may act as a warning signal to indicate that a nest is occupied and deter intrusions by conspecifics," said the team in their paper.

To study the behavior of the jackdaws, the team used a set of circles with different characteristics. The two of the four circles contained the front view of a jackdaw's face. However, each circle had a different design for the eyes. The first circle had completely white eyes and the second had completely black eyes. The third circle didn't contain a jackdaw face. Instead it only had a pair of white eyes. The last circle was used as a control and was painted black throughout.

"We tested whether jackdaws' pale irides serve as a deterrent to prospecting conspecifics by comparing prospectors' behaviour towards nest-boxes displaying images with bright eyes (BEs) only, a jackdaw face with natural BEs, or a jackdaw face with dark eyes," said the team. "The jackdaw face with BEs was most effective in deterring birds from making contact with nest-boxes, whereas both BE conditions reduced the amount of time jackdaws spent in proximity to the image."

Due to the fact that different birds from different species have a wide variety of eye colors, it is difficult to draw more general conclusions from the study. In the case of jackdaws, however, it seems they really are thinking of doing something bad when they stare at people and other birds with an intimidating glare.