In the combined image above, the photograph on the left shows leaves in the colours that a human observer would see them. The photograph on the right, taken by a special camera, shows the same leaves in the colours that a bird would see them.
The camera was designed by scientists from Lund University in Sweden, who have published their findings in Nature Communications (https://www.nature.com/articles/s41467-018-08142-5).
As the images show, birds see a very different reality compared to what humans see. Human colour vision is based on what is known as the RGB combination — red, green and blue.
Birds’ colour vision is based on four colours — the RGB combination and also ultraviolet. Among other things, birds see contrasts in dense forest foliage, whereas people only see a wall of green.
For birds, the upper sides of leaves appear much lighter in ultraviolet. From below, the leaves are very dark. In this way the three-dimensional structure of dense foliage is obvious to birds.
This in turn makes it easy for them to move, find food and navigate. People, on the other hand, see the foliage in green.
Source: Lund University