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The Special Senses:
Robin Roscoe, DVM, Dipl.ABVP (Avian)
Birds are an amazing group of creatures. They all have one thing in common the ability to fly (or at least their ancestors did, in the case of emus and ostriches). The amount of variation between species and specialization of features is incredible. I'd like to discuss the "special" senses vision, hearing, smell, and taste and a "6th sense", the sense of direction used by homing pigeons and by migrating birds such as Canada Geese.
For most birds, the ability to see is the most important sense. This is reflected in the facts that the optic lobe in the brain is larger and better developed and that the size of the eyes in relation to the size of the head is greater than in mammals. In fact, in many birds the weight of the two eyes is greater than the weight of the brain! Good eyesight is necessary for survival: for hunting and to avoid being someone else's dinner. This is accomplished in several ways the position of the eyes in the head, the shape of the eyeball, accomodation, light regulation, and special variations in the retina.
In predators such as hawks and owls, the eyes are on the front of the head. This creates a smaller field of vision, but improves binocular vision (the coordinated movement of both eyes gives depth of field). In non-hunters or prey species, the eyes are on the side of the head. This extends their field of vision, giving them virtually "eyes in the back of their head". For example, pigeons have a 300 degree field of vision. With a slight movement of the head they easily extend that to 360 degrees.
There are three basic shapes of avian eyeballs: flat, globular, and tubular. The longer the bulbar axis, the greater the image on the retina and hence the better the visual acuity. Birds that are grain-eaters (e.g. chickens) don't need to see long distances or in great detail to find their food, and hence have flat eyeballs. Diurnal birds (birds that hunt during the day) such as crows or hawks have globular eyeballs. Nocturnal birds such as owls need the greatest visual acuity and have a tubular shaped eyeball.
Accomodation is the ability to focus on an object. Birds can focus faster and better than human beings. Focussing is accomplished by changing the shape of the lens and/or the cornea. The lens is softer in birds than in mammals and this allows accomodation to occur more quickly. Ciliary muscles (within the eye) control the shape of the lens and are best developed in hawks, who need the most rapid accomodation to focus on moving prey. The angle of the light changes as it moves through the cornea (referred to as the refractive index). Muscles within the eye can change the angle of the cornea. Diving birds must focus under water as well as in the air. Since the refractive index of water and the cornea are similar (remember our bodies are mostly water), these birds deal with the problem differently. Terns become "long-sighted" in the water. They may see a fish from the air, but then miss it in the water. Penguins see accurately under water, but are "short-sighted" on land. In cormorants and diving ducks the lens is especially soft and the internal muscles very well developed, so they are able to focus well in either air or water.
The amount of light reaching the retina, which is controlled by the size of the pupil and a photochemical reaction in the retina, also affects what is seen. The pupil is under voluntary muscular control; the larger it gets the more light the retina receives. Birds have a thicker retina than mammals. It does not contain blood vessels so more light and and a more accurate image can be obtained. If too much light is reaching the retina, a photomechanical reaction occurs in the epithelial cells to increase the amount of retinal pigment.
The retina receives images and transfers them to the brain via rods and cones. Cones are responsible for visual acuity and colour vision. The avian retina has all round acuity (vs focal acuity in mammals). This means that a bird can accurately see objects at the edge of its field of vision whereas our eye must turn and focus on the object. Birds do have colour vision, but the colours they actually see vary amongst species. Most birds see at least 3 and possibly 4 colours. Rods are sensitive to the intensity of light. As you might suspect, diurnal birds have more cones than rods while nocturnal birds have more rods than cones.
The retina has areas where the cones are more concentrated (the central and temporal areas). A fovea is a "dip" or "divet" in the central or temporal area that has even greater acuity. Most species (e.g. parrots) have a round central area and fovea. Birds that chase fast-moving prey or feed as they fly (e.g. falcons, swallows) need accurate perception of distance and relative speed. For this thay have both a central and temporal areas & foveas. Kingfishers have two foveas one for use in the air and a second for use in the water. Owls, having more rods with which to see at night, have only a temporal area and fovea. Water birds have a horizontal central area a band of tissue that allows them to fix the horizon as a reference.
One last special adaptation in birds is the third eyelid. This is present in some mammals as well to assist in spreading tears over the cornea. The difference is that in birds it is clear and hence protects the cornea from drying out during flight, and generally protects the cornea when under water.
Hearing is important for different reasons in various species. For example, it aids owls in hunting, and songbirds in imitation of complex songs for communication.
The ear consists of three parts. The external ear collects sound waves and conducts them to the middle ear. The external ear is usually covered by specialized feathers. On the leading edge the feathers reduce the drag caused by turbulence in flight and therefore reduce turbulence-induced masking of sounds. On the posterior or trailing edge the feathers may form a funnel to collect sound. Owls have an operculum (a vertical skin flap) covered with a row of feathers that aids in sound location.
The middle ear is an air-filled cavity containing small bones (ossicles). It amplifies sounds and conducts them to the inner ear. The tympanic membrane (eardrum) between the external and middle ear is especially large in owls.
The inner ear contains fluid and is responsible for hearing and balance. Mechanical vibrations in the air are transferred to the fluid. The vibrations in the fluid are picked up by cilia (fine hairs) attached to receptor cells which send information to the brain.
Birds hear differently than mammals. They can discriminate pitch, but in a narrower band than mammals. Birds have superior temporal resolution. A complex song would have to be slowed down 10 times for us to pick out the details that a bird hears. Directional analysis is probably the most interesting feature. It is achieved by noting the difference in the time of arrival of sound at each ear, and the difference in intensity. It is easier to locate a sound if it is repeated or if there is a melody. Diurnal birds (e.g. parrots) have the same ability as man. Nocturnal birds are better at this, in fact the barn owl is the best. It can hunt by sight or by sound. The operculum (flap of skin) allows it to pick up finer details. The ears are asymmetric. This also fine tunes location of sound. Better developed tracts in the brains of owls reflects the importance of hearing to them.
Echo location is only used by a very few species (e.g. birds that live in caves cave swiftlets). The sounds are barely audible to man and this ability is not as well developed as in bats. Penguins use echo location to hunt underwater.
As the large optic lobes in the brain indicate the importance of vision, the reduced size of the olfactory lobes indicate the lesser importance of the sense of smell. The olfactory lobe is smallest in passerines and parrots, intermediate in pigeons and gulls, and largest in water birds (e.g. ducks and loons) and the Brown Kiwi. In fact the Brown Kiwi is the only bird with nostrils at the tip of its beak. This bird is nocturnal and has very poor eyesight. It finds buried food by sniffing for it. Vultures will congregate at the smell of carrion. Pigeons have a good sense of smell and use it for navigation over long distances. Some ocean birds (e.g. petrels) navigate back to their islands and nests by sense of smell.
Birds don't always share human tastes. Some things we enjoy they may find offensive, and vice versa. Birds can distinguish tastes, but with less acuity than mammals. This can be explained by the number of tastebuds present. A parrot may have 300-400, a kitten 473, a rabbit 17000, a human 9000, and a snake zero.
Nectar- and fruit-eating birds are more likely to prefer sugars than insect-or grain-eating birds. Hence some of our parrots with a "sweet beak" (avian version of a sweet tooth). Most birds will avoid high levels of salt which is toxic. Only birds with nasal salt glands (e.g. seabirds) can process higher levels of salt. A bitter taste may prevent birds from eating toxic plants and insects.
Sense of Direction
Have you ever wondered how a homing pigeon finds its way home? Or how Canada Geese know where and when to migrate? There are several known factors and several possible additional factors. The sun and the stars can provide orientation, but obviously birds are still able to migrate in cloudy weather.
Odours are important for homing pigeons over long distances (up to 500 km) but are not a factor within 10 km of their loft. Birds can use barometric pressure to assess their altitude. Homing pigeons can recognize a change in altitude of as little as 10 meters.
The two factors that I find most fascinating are infrasound and magnetic fields. Infrasound is very low frequencies (as opposed to ultrasound very high frequencies). These are background noises in the environment, inaudible to man. These sound waves travel hundreds of kilometers without dying down. Birds can detect them while moving using directional analysis. Magnetic fields also help birds find their way. Deposits of iron-containing material have been found in a small piece of tissue between the dura mater and the skull in pigeons. The earth's magnetic field runs north-south and alters birds' behaviour. How many species do you know that migrate east-west?
Other possible factors are gravity, and UV or polarized light. Bees use light to navigate, but it remains to be shown whether birds do too.
Birds have adapted their senses to meet their particular species' needs. Whether it is the retinal changes in hawks that allow them to see all the mice in a field at once (and assess how fast they are travelling), or the adaptations in the ears of the barn owl that allow him to hunt "blind", I hope you have gained an appreciation for some of the wonders of our avian friends.
1. King, A.S. & McLelland, J. Birds: Their Structure and Function
2. Sturkie, P.D. Avian Physiology
3. Altman, R.B., Clubb, S.L. et. al.
Avian Medicine and Surgery