Jackdaws, crow-like birds, use their eyes to communicate with each other, the first time the ability has been observed in non-primates, British researchers say. Researchers at the universities of Cambridge and Essex, writing in the journal Biology Letters, report jackdaws use their eyes as a warning signal to keep competitors from coming near their nest holes in trees. "Jackdaw eyes are very unusual. Unlike their close relatives, the rooks and crows -- which have very dark eyes -- jackdaw eyes are almost white and their striking pale irises are very conspicuous against their dark feathers," Cambridge researcher Gabrielle Davidson told NewScientist.com At the start of breeding season last year, Davidson installed one of four different pictures in 100 jackdaw nest boxes on the outskirts of Cambridge -- either black (the control), 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. "Jackdaws are unique among the crow family in that they nest in cavities in trees," she said. "These hollows are natural -- the birds cannot excavate their own nest cavities as some woodpeckers do -- so they have to compete for a limited resource." After analyzing videos of jackdaws peeking into each other's nest boxes, Davidson found those containing picture of a jackdaw with its bright eyes was much more likely to deter the birds from landing on it, and that the birds spent less time near such a nest box. Davidson's study is the first to show eyes being used as a means of communication between members of the same species outside primates, a Cambridge release said Tuesday.