Sun Lane & Beyond #5:The Eyes Have It
The cache, a small screw-capped, camo-taped jar, is the 5th of a 10-cache series circuiting through some fine countryside to the west of Burley-in-Wharfedale.
For background info on the trail including a Map (also available in this cache's Gallery) showing caches and parking spots, and waypoints for parking | trail access, see GC7E48T SLB#1: What a Tip!.
As I was finding a suitable hide location a small flock of these noisy corvids was foraging in the nearby field . . . jackdaws!
Researchers in Cambridge and Exeter have discovered that jackdaws use their eyes to communicate with each other – the first time this has been shown in non-primates.
Unlike their close relatives, rooks and crows, jackdaw eyes are almost white and their striking pale irises are very conspicuous against their dark feathers.
While what humans do with their eyes has been well studied, we know almost nothing about whether birds communicate with members of the same species with their eyes.
The study, published in Biology Letters, shows that jackdaw eyes are used as a warning signal to successfully deter competitors from coming near their nest boxes.
The leader of the study, Gabrielle Davidson said: '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.'
While most birds have black or dark brown eyes, bright eyes are not unknown in the avian world, and 
around 10% of passerines (perching birds) have coloured irises. The question for the study was - do jackdaws use their bright eyes to communicate with fellow jackdaws?
Just before the spring breeding season arrived in 2013, one of four different pictures was installed in each of 100 jackdaw nest boxes on the outskirts of Cambridge. The pictures were 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. The effect the different pictures had on the birds’ behaviour was then filmed.
Jackdaws are unique amongst corvids (crow family) in that they nest in tree cavities. 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. Because jackdaws nest close to each other, they fight a lot to gain the best nesting sites. What often initiates these fights are jackdaws approaching nest boxes that are not their own.
After analysing 40 videos of jackdaws peeking into each other’s nest boxes, she found that compared with the other nest boxes, those that contained the 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.
This is the first study to show eyes being used as a means of communication between members of the same species outside primates.
Before now very little was known about why some birds have brightly coloured eyes. In jackdaws, the pale 
eyes may function to improve their ability to defend their nest and chicks from competitors. It also raises the question of whether this is unique to jackdaws, or if other cavity nesting birds also use their eyes in a similar way.
To see the research in action, view video here.