Ballykinlar, County Down: Down on the shore, plover and redshank are going about their business, bobbing as they go to get a clearer view of their surroundings

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.