The first proof that at least one non-primate species can use its eye to communicate with other members its own species was presented by Gabrielle Davidson of the University of Cambridge in the United Kingdom in the Feb. 4, 2014, edition of the journal Biology Letters.
Jackdaws are a variety of crow that nests in naturally occurring holes in trees. The birds cannot excavate the holes but compete for the prime nesting spots. Jackdaws are noted for their bright irises that contrast distinctively with the bird’s natural dark color.
Davidson randomly placed photographs of a black background, a pair of jackdaw eyes, a pair of jackdaw eyes in a jackdaw's face, or a jackdaw's face with a pair of black rook eyes in 100 nesting spots and found that the jackdaws avoided the nesting spots with the picture of a jackdaw with its bright eyes.
This is the first evidence that any other animal species uses its eyes for communication other than primates. The researchers contend that jackdaws use their eyes as a means to protect their nests.
The behavior may be dependent on eye or iris color but future research may indicate that communication through a species eyes is not a primate only adaptation as was previously thought.