How does an owl see clearly at night?

Have you ever seen an owl at daytime? An owl is a nocturnal bird of prey; no wonder we rarely see it during the daytime. In fact, owls ‘come to life’ at night and are an apt example of ‘nocturnal’ birds. Birds which can see better at night than during the day are known as nocturnal birds. On the other hand, birds that can see better during the day than in the night, are called ‘diurnal’. About 130 species of owls are found around the world. In some countries, they are treated as auspicious birds, while in others they are regarded as symbols of wealth and wisdom. An owl has a large head and equally large eyes, short neck, broad wings and sharp claws. What is peculiar about owls is that they can see more clearly during the night than during the day and have an exceptionally good listening power. You must be very curious to know how nocturnal birds, especially owls in particular, can see so clearly in the night. To understand this, it is essential to know how human beings see things. Light scattered by an object is focused on to the retina of our eyes by the lenses inside the eyes. An inverted image of the object is formed on the retina, which is carried to the brain by optic nerves. The brain inverts this upsidedown image, and thus we see the object as true object. The owl’s eyes are large, forward- pointing and have four special features due to which it can see more clearly at night. Firstly, the distance between the lens of its eye and the retina is more than the distance in case of the human eyes. Due to this, the image formed on the retina is bigger in size. Secondly, the number of ‘rods’ and ‘cones’ (special cells which help in clarity of vision) in the owl’s retina is very large —almost 10,000 per square millimetre compared to 2,000 per square millimetre in our eyes. Thus, the owl can see five times more than us. Thirdly, its eyes have a red-coloured material, chemically a protein, which makes its eyes more sensitive to light. Fourthly, the pupils of its eyes can dilate more, thus allowing even the smallest amount of light to enter the eyes. Because of these four factors, an owl can see more clearly in darkness. Due to the extreme sensitivity of its eyes to light, objects appear too bright and uncomfortable in the daylight. Surprisingly enough, an owl can rotate its head by 180°, i.e., it can turn its head to see what is at the back of its head.