Not Small, Possibly Far Away

Head movements and range-to-target...

Both osprey youngsters are almost fully grown and taking a keen interest in the various things to be seen around their nest. This is not idle curiosity – in a few days (weather permitting) they will be obliged to avoid some of these structures while in flight, and perhaps land and perch on others.

They need to get a good appreciation of the size and position of all such things, and how to apply Dougal's Rule to them (“These are small, but the ones out there are far away”), otherwise known as perspective.

You will often see ospreys moving their heads from side to side when they are studying an object at medium range. They do this in order to get a better estimate of distance and size. Like almost all birds of prey, ospreys have binocular vision (two eyes working in unison for forward vision) but the difference in perceived position of objects (parallax) is not very great – the effective range for parallax alone is only about two metres, whereas we can use the same method out to almost three metres because our eyes are further apart.

Something else is needed.

Doing the head-wiggle gives additional scope to this method of judging distance, known technically as “qualitative stereopsis.” It includes the picking up of many different visual cues, including shadows and colour-based edge detection.

To land on a perch, or to avoid objects at speed, birds need to integrate all this visual information very rapidly. It has recently been discovered that structures in the optic nerves of birds contribute to this “image processing”, without all the work having to be done in the visual centres of their brains.