Humans use their eyes constantly while communicating with others. Eye movements can be gestures, so that when we see someone glance to the side, we look in the same direction. Eyes can also be a warning to others: when pictures of eyes are on display, it encourages generosity and has been said to discourage antisocial behaviour such as theft. The thought of someone watching us is enough to change our behaviour, and as social creatures we are constantly aware of our actions in relation to others. Jackdaws are also social animals and our research suggests that they too communicate with their eyes.
Jackdaws, a member of the corvid family that includes crows, rooks, ravens and jays, forage, roost and nest in groups. Interacting with one another is an essential part of their daily lives, but it also encourages competition. Because jackdaws rely on natural cavities for breeding, they fight to secure this limited resource. Even when a jackdaw has claimed a nest cavity, other jackdaws will approach and try to take over. How can a breeding pair prevent jackdaw competitors from taking over their nests? It seems that their eyes act as an early warning signal to put off any would-be usurpers.
Jackdaws are best known for their incredible cognitive abilities. Their use of tools, memory skills and social behaviour rival those of the great apes, but what makes jackdaws stand out amongst their corvid relatives is their ability to track and respond to human eye movement and follow the path of another’s gaze – something even primates fail to comprehend. They have bright blue eyes that can be seen staring out from within tree-holes, chimneys or nest boxes.
Lots of animals have been known to respond fearfully to eyes because the eyes resemble predators. So what makes jackdaw responses to human eyes so special? Jackdaws respond to very subtle movements of human eyes and can even follow them to locate hidden food. Like humans, jackdaws have very conspicuous eyes, whereas most of their corvid relatives (and indeed 90% of perching birds) have dark eyes. Perhaps sharing this feature with humans makes jackdaws particularly responsive to their eyes. But, until now, there has been no evidence to show that jackdaws use their bright eyes to communicate with each other.
The question of whether jackdaw eyes are important for nest defence began at Cambridge University’s Comparative Cognition Lab. When aviary-housed jackdaws were inside their nest boxes, their eyes were very visible against the dark background, and nest box owners stared out of their box most when competitors were nearby. To test whether the bright eyes were an effective deterrent against nest competitors, it was essential to understand the behaviour of would-be intruders when faced with these bright eyes.
If having bright eyes helps ward off competitors, jackdaws scouting out potential nest sites would be more cautious of nest boxes displaying bright eyes than nest boxes displaying a jackdaw with dark eyes instead of bright eyes.
The eyes have it
Using jackdaw nest boxes of the Cambridge and Falmouth Jackdaw Projects, the role of eye colour and brightness in nest defence was investigated. During the pre-breeding season when jackdaws prospect potential nest sites, one of four circular images was suspended at the entrance hole within the nest boxes to give the appearance of something inside.
The first image was a jackdaw face with normal bright eyes, the second image was a jackdaw face, but with dark eyes. The third image was jackdaw eyes only (to test whether eyes alone were enough to frighten away intruders), and the fourth was a black circle to serve as a control. A hidden camera filmed for 2-3 hours to capture the reactions jackdaws had to the images when investigating nest sites.
When the jackdaw face with dark eyes or the image with eyes only was on display, jackdaws approached as though there was nothing there. But when the jackdaw face with bright eyes was in the nest box, jackdaws approached much less. They also spent less time on a perch closest to the entrance hole when either image with bright eyes was on show. This demonstrated that bright eyes in jackdaws have an important function: to warn others that a nest is occupied and to stay away. This is the first demonstration that members of the same species outside of the primate lineage can communicate with their eyes.
This exciting finding in jackdaws prompts further interest into the function of coloured eyes in birds. Jackdaws’ ability to communicate in this way is the first evidence that bright eyes could act as signals in the bird world. It is unknown whether all birds that nest in cavities tend to have bright eyes more than dark eyes. An even more intriguing question is whether jackdaws, or indeed other birds, use their eyes to communicate in ways other than as warning signals. This is a question that warrants further exploration.