Birds’ adaptation an eye-opener

JACKDAWS have greyish white eyes. They make it easy to identify the bird — the other crows have dark ones.

But how did this change in colour come about? What possible advantage could it be to a jackdaw to have white eyes? Cambridge PhD student Gabrielle Davidson has tried to answer these questions. At the start of the breeding season, she placed colour photographs in a hundred jackdaw nest-boxes. Some pictures were blank. Others showed jackdaw eyes against a black background. A third group featured a jackdaw’s eyes and head while a fourth had a jackdaw’s face with the dark eyes of a rook. Davidson recorded the responses of jackdaws to the photos as they visited the nest-boxes. She found that birds tended to shun boxes where they encountered pictures of white eyes.

Jackdaws, unlike most other crows, nest in holes in trees, old buildings and chimneys. Such locations are dark, making it difficult for a prospecting bird to check if a box is already occupied. White eyes are easy to see against black plumage and a dark background. Davidson concluded that a nesting bird shows its face to a would-be intruder, the eyes providing the avian equivalent of an auctioneer’s ‘Sold’ notice. Jackdaw eyes, therefore, are not just organs of sight. They have a communications role as well.

Nor are jackdaws unique in using eye colour as a defence. The dipper is a dark brown bird, slightly smaller than a starling. Resembling a fussy little waiter, with a cocked-up tail, it lives along babbling brooks and fast running streams, bobbing up and down while standing on rocks in mid water. Extraordinarily for a songbird, the dipper swims and walks along the bottoms of rivers catching creepy crawlies and the occasional fish. I joined Pat Smiddy and Barry O’Mahony, recently, for a nocturnal dipper ringing session on the Glashaboy River. The gleaming white flashes of dipper eye-lids were conspicuous in the darkness. Dippers roost and nest under bridges and rocky banks, also dark locations. Do they blink their white eyelids to discourage other dippers from intruding?

The jackdaw uses its eyes for truthful communication, but the dipper may be out to mislead. Perched on a stone in midstream, a bird is out in the open with nowhere to hide. Peering down into the water, watching for prey, is a high risk activity; a sparrowhawk may be on the prowl. By constantly bobbing up and down, however, the gleaming white bib mimics the white flecks on the tumbling water. Flashing a white eyelid adds to the effect. This camouflage-in-motion makes the dipper notoriously difficult, even for experienced bird-watchers, to spot.

A sleeping drake mallard uses a similar trick to fool its enemies. His closed white eyelid stands out against the dark colour of the head. A would-be predator thinks that the bird is awake and that there is little chance of catching it. Deception goes even further in some butterflies. Their wings carry impressions of the eyes of owls which they flash at their enemies.

Dogs see a person’s fixed stare as a threat; no creature in the animal kingdom communicates with its eyes to the extent we do. Maintaining full eye contact during a row is our equivalent of the jackdaw’s nest defence. A walker in a dimly lit street keeps eyes averted to avoid attention, the opposite of the dipper’s eye-flashing behaviour. Lovers gaze into each others’ eyes, those ‘windows of the soul’.

As in jackdaws and dippers, whiteness is crucial in human eye communication. No other mammal has such conspicuous eyes. The irises are the only visible part of most animal eyes but, in us, the ‘sclera’ can be seen. They provide gleaming white backgrounds, against which the beautifully coloured irises stand out and move, transmitting and receiving a constant stream of signals.

* Salient eyes deter conspecific nest intruders in wild jackdaws, Davidson et al. Biology Letters. 2014.