A fascinating talk at the Cheltenham Bird Club by Graham Martin, whose speciality is bird vision. Having given us the elegant idea that “a bird is a wing guided by an eye“, he then proceeded to argue against it.
He suggested that the evolution of birds’ eyes is not driven by the long-distance vision required by flying, but by the requirements of close-up work, specifically that of feeding. If you can’t fly accurately, you can usually get away with a clumsy landing; but if you can’t peck accurately or grab from your parent’s bill, you starve quite rapidly. The vast array of different bill-shapes is complemented by a wide variation in birds’ visual capability (although this is much less obvious) which allows each species to use its particular bill to good effect.
Graham’s work on birds’ eyes and blind spots has practical implications for collisions with man-made structures like pylons and wind turbines. Some birds have evolved to scan the ground while flying, and so their blind spots – the top of the head and the back of the neck – face the way they’re moving. This can be dangerous today, but then there hasn’t often been anything in their line of flight until the last fifty years.