I was lucky enough to be on one of the berms at the Celery Fields at dusk the other day when a Wilson’s Snipe flew in and fed out in the open just below me. I’m not a photographer but I carry a camera for moments such as these and when I later looked at the photos I was impressed by two things. One was, once again, the remarkable camouflage of this bird. From a distance I wouldn’t have been able to see it because of its entirely cryptic coloring against the mass of broken, dead and battered reeds. It is of course typical of our encounters with this bird to see it when flushed zig-zagging swiftly up into the air because it is so difficult to see on the ground.
The other thing to impress, and intrigue, me was the position of its eyes. In the photograph the bird appeared to be looking straight ahead but at first I wasn’t quite sure whether it was facing towards me or away from me and it struck me that this bird has just about 360 degree vision. But given that its eyes are located high up on the sides of the head, does it see one or two images?
Tim Birkhead in ‘Bird Sense’ (reviewed last month) describes the three broad categories of visual field in birds. Type 1, he says, is what typical birds, such as blackbirds, robins and warblers, see: some forward view, excellent lateral vision, but (like us) no rear vision. Incidentally these birds are not able to see the end of their beak and any photographer will note that when taking a picture of a bird looking directly at the camera, they look slightly ‘cross-eyed’.
Type 2 includes birds like ducks, woodcocks and snipe. They don’t have a great forward view but they do have panoramic vision, above and behind – helping them detect potential predators, even when their bill, as in snipe and woodcock, is fully buried in mud. He points out that as the view from each eye barely overlap, they probably see two separate images.
Type 3 birds are those, such as owls, with forward-facing eyes like our, which have no vision behind. Obviously, they compensate for this by their flexible necks whereby they are able to move their heads around to almost 180 degrees.
But back to the Wilson’s Snipe (type 2 vision) for a moment. If this bird doesn’t have great forward vision how on earth does it manage to manipulate that improbably long bill to locate and seize its prey? Well, if you’re a long-billed shorebird such as a godwit, curlew, snipe, knot or dunlin, vision has little to do with feeding other than glancing sideways to check if other birds near you have found a productive spot. These waders have high set eyes perfect for spotting predators, rather than looking down their beaks. Instead they use a key feeding adaptation, used by all wading birds, a tiny sensor called a Herbst corpuscle. The snipe’s bill tip is pocked with pits to accommodate masses of these miniature sensors. This enables them to detect the movements of prey without actually touching them because they can feel the tiny waves of pressure caused by worms and grubs as they move in soil, wet mud or sand. When feeding, waders steadily move forward, probing the mud with a stitching action as they feel for the next pressure wave. It’s a bit like echolocating, only in soft ooze rather than air.
They also make use of a remarkable adaptation known as ‘distal rhynchokinesis’ which means that the final section of the upper part of their bill can flex upwards independently of the rest of the bill. This action allows the bird to open and close the tip of the bill wide enough to seize prey even when buried in deep mud or sand. Smaller prey items are swallowed with the bill still probed, larger prey items are removed from the soil for easier manipulating and swallowing.
Once again, birds never cease to amaze us, do they?