The morning after the night’s frost, the field basks in sunshine. Waders are foraging among the grazing sheep and my gaze falls on the bobbing head of a redshank as it probes a tuft of grass. A golden plover dashes past and my view switches. At speed, the plover’s head is steady; but when it slows, the head starts to bob; and then the bird pecks at the ground.
Head-bobbing is found in about half of all avian species. It happens for similar reasons that my mammalian eyes make involuntary flickers as they look at something; that is, to build up a picture of the scene, picking out the important bits from the unimportant. As I look back at the redshank, although my gaze feels perfectly steady, I know my eyes are minutely moving and pausing their focus. I can also assess where the different birds are in relation to each other because of my forward-facing eyes. My brain combines the slightly different images produced on each of my retinas, which is what gives frontal vision its sense of perspective.
