View to a kill
BUILDING KNOWLEDGE
THE most important sense for a bird is vision and most species devote a large portion of their brain to it. There is a wide difference in vision between the different bird families, and this is largely to do with food type and feeding methods.
Owls catching mice, terns catching fish, swallows catching insects and sparrows finding seeds all have different visual requirements to be successful. Light-sensitive receptors are cones, which are used in daylight and provide colour vision, and rods, which are used in dim light, resulting in mainly black-and-white images. The density of these receptors differs. In pigeons it is 328,000 cones per sq mm, for House Sparrow it is 400,000 per sq mm and for Common Buzzard it can reach 1,000,000 per sq mm (in humans it is 100,000300,000 per sq mm).
The highest density of receptors is in the fovea, a small pit in the retina which gives the sharpest resolution. Most birds have a single fovea in each eye. Some, such as chickens, quail and guineafowl, do not have fovea, but there are many birds which have two. The main one is called the central fovea and is situated close to where the optic nerve joins the retina. Birds with only one fovea, such as owls and some waders, will bob their heads to see better. The second is called the temporal fovea, towards the side of the eye, and this helps with judging speed and distance. Birds which are bifoveal include raptors, shrikes, swallows and martins, swifts, terns and kingfishers, all of which actively pursue prey. The fovea found in most birds have receptors concentrated in a circular area.
Hirundines have an unusual visual system for songbirds. Each of their eyes has two fovea and their eyes are long, with narrow binocular vision and their two fovea seeing four different areas, which do not overlap, forming an arc around the beak which enables them to track aerial prey in front of them at high speed. Seabirds, such as Northern Fulmar, have their receptors also concentrated to the sides of the fovea, forming a horizontal patch often called a linear fovea. Yellow-legged Gull has a deep central fovea and a shallow fovea to the side. This gives the species two monocular visual fields to the sides, and a small central binocular field.
The fovea usually forms a depression, which can be quite a deep pit, and it has been suggested that this may serve to magnify an image which would help detect movement. The number of photoreceptors that attach to a nerve ganglion results in a higher visual acuity. In pigeons there are 2.6 to 1 while in Eurasian Sparrowhawk it is 11 to 1.