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As many of you know, the sight of owls is particularly developed. The development of their vision system, however, is different from the diurnal birds of prey. In the latter, the view is developed towards a higher definition of images, thanks to a high concentration of cones in their fovea and the presence of two fovee. In owls, instead, the vision system is developed to a particularly efficient capability of night vision; in fact these birds of prey eyeballs are huge (in an Eagle Owl are as large as the human ones!). This required a special modification of the skull, furthermore their retina also has a very high density of rod cells (the sensory cells in the eye able to perceive light).
The eye of a nocturnal bird of prey can intensify the very modest ambient light to allow the bird of prey to see in the dark. Precisely for this circumstance, a nocturnal bird of prey can see at night but is not able to have a perfect view of the environment, especially in the darkest nights where it is totally lacking any minimum ambient light. The view is therefore used only for moving but is not sufficient to ensure the owl a perfect identification of a small prey as a little rodent tens meters away: for this purpose, Strigiformes use, in fact, their extraordinary hearing.
Photography: European Eagle Owl (Bubo Bubo, Linnaeus 1758).