If the hawk chose the screen with a uniform colour it received a reward, whereas the other choice offered no reward. A uniform colour was projected on a large screen behind one of the perches, and a multicoloured grid pattern was displayed behind the other perch. The hawks could fly to one of two perches within a bird enclosure. When hunting, eye vergence allows their binocular overlap to increase up to 75°. The location was chosen because falconry (displays and hunting using birds of prey) is forbidden in Sweden. Moreover, Sepia cuttlefish require a precise distance estimation strategy when they strike using two tentacles to apprehend prey by gripping them with suckers before subduing them through the use of biting and injecting toxins. The findings are based on studies of tame Harris's hawks in France. Good colour vision is also particularly important in environments such as forests, where shadows for example can confuse visual impressions. Simon Potier and his colleague Almut Kelber show in the study that colours are important for enabling birds of prey to detect quarry at a great distance. Researchers have considered that the colour vision of animals has been most important at relatively close range and thus for quite large objects. Up to now, research has not focused on the significance of colour for the hunting success of birds of prey. Nonetheless, it can see many times better than us, even though it is so small and light," says Simon Potier. "It's exciting! The hawk weighs less than one kilo and has small eyes. 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. The study by the Lund biologists 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. However, there are exceptions, and the Harris's hawk (Parabuteo unicinctus) is one of them. Their contrast vision is almost ten times lower than ours. Particular to birds is a poor ability in general to see contrasts between different objects. Large body, large eyes small body, small eyes. The size of the eyes in turn is usually linked to body size. The bigger the eyes, the higher the resolution. Normally, the size of the eyes determines optical resolution and thus what people or animals can see. However, colour is of considerable importance," says Almut Kelber, biologist at Lund University. 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.
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