Birdwatching: The Five Senses
This is of prime importance as it allows them to find their food, to avoid their enemies and to recognize their mates. Some birds have a very highly developed sense of sight which is not surprising when one remembers how fast a bird has to react to so many different stimulae. Some birds have an eyesight which is eight times keener than man’s. Birds have the ability to change focus rapidly and have a comparatively large eyeball. Relatively speaking a Starling’s eyeball is fifteen times heavier than a man’s; an eagle’s eyeball is actually larger than a man’s. They are generally so large that there is little room for movement and as a result birds have very flexible necks and most species can rotate their heads through 180 . Birds can also see through a much wider angle as a result of the position of the eye in the head. An owl can see through about 100”, up to 70 of which will be binocular vision while a pigeon can see through 340 , only a small percentage of which will be binocular. Birds see through 3600 which is very necessary when its beak is probing deeply in the ground. Raptors have binocular vision which allows them to judge distances more accurately.
Colour in essentially the same way as human beings but they also have an adaptation which may help to improve their vision in hazy weather. In addition to the normal eyelid birds have a nictitating membrane which acts as a third transparent eyelid and is drawn horizontally across the eye from the nasal-side backwards. It can clean or moisten the surface of the eye without shutting out the light. In waterbirds such as ducks, divers, and waders which use their eyes under water this clear window-like lens is so refractive that it bends the light rays even under water.
Birds depend on their sense of hearing for keeping in touch with other members of the flock or their family, for hearing alarm calls or in some cases even hearing their prey. Apparently some of the plovers can actually listen to earthworms; the Barn Owl can apparently locate its prey by sound alone. Birds have a similar range of hearing to human beings – from 20 to 20,000 cycles per second. They can hear sounds that are too rapid for us to hear. An examination of a sonagram suggests that birds can extract more information than a human listener. They can automatically locate sounds by assessing the time-lag between its arrival at either side of the head. It is also easier to locate the origin of brief sounds rather than long drawn out ones which is probably why the alarm calls for many species are long drawn-out whistles or other sounds.
Smell, taste and touch
These senses are of little value to birds since their eyes are so strongly developed. Some species may be able to smell food while it is in their mouth and this ability can cause some birds to spit out food which is tainted or obnoxious, such as ants. Taste and touch are generally poorly developed as well – indeed it is not well known what birds can taste. The sense of touch is much the same as man’s. However, birds do have special nerve-endings which appear to be particularly sensitive to vibrations such as the shaking of their perch. Waders have sensitive tips to their beaks which allows them to detect their unseen prey. It is also thought that the rictal bristles around the mouth of flycatchers and Nightjars, which are modified feathers, may not only help the bird to catch food but may also be sensitive to vibrations caused by their prey.
- The Anatomy of Birds (birdwatchtips.info)