'Colour vision makes hawks successful hunters'

Harris's hawk has the best colour vision of all the animals investigated to date, helping them to successfully detect, pursue and capture prey, a study suggests.

The findings by researchers at Lund University in Sweden may help protect threatened birds of prey against hazards such as wind turbines and power lines.

"I did not think that colour vision would be of such significance, rather that birds of prey simply have better visual acuity than humans and that was the reason they detect objects so early and at a great distance. However, colour is of considerable importance," said Almut Kelber, a biologist at Lund University.

Normally, the size of the eyes determines optical resolution and thus what people or animals can see. The bigger the eyes, the higher the resolution.

The size of the eyes in turn is usually linked to body size. Large body, large eyes; small body, small eyes, according to the study published in the journal Proceedings of the Royal Society B: Biological Sciences.

Particular to birds is a poor ability in general to see contrasts between different objects. Their contrast vision is almost ten times lower than ours, researchers said.

However, there are exceptions, and the Harris's hawk (Parabuteo unicinctus) is one of them, they said.

The study shows that if an object is not distinguishable from the background and the colour is approximately the same, it is more difficult for a bird of prey than a human to detect it.

If, on the other hand, the object has a different colour than the background, the Harris's hawk can detect it at twice the distance compared to human vision.

"It's exciting! The hawk weighs less than one kilo and has small eyes. Nonetheless, it can see many times better than us, even though it is so small and light," said Simon Potier, who is a falconer.

The study shows that colours are important for enabling birds of prey to detect quarry at a great distance.

Good colour vision is also particularly important in environments such as forests, where shadows for example can confuse visual impressions.

The findings are based on studies of Harris's hawks in France.The hawks could fly to one of two perches within a bird enclosure.

A uniform colour was projected on a large screen behind one of the perches, and a multi-coloured grid pattern was displayed behind the other perch.

If the hawk chose the screen with a uniform colour it received a reward, whereas the other choice offered no reward.

Once the hawks learned that a uniform colour without a grid pattern meant a reward, the researchers gradually changed the grid pattern's contrast and closed the distance between the grid lines until the hawk could no longer see the difference between the two screen images.

"The more fine-meshed patterns an animal can see, the sharper their visual acuity," said Potier.