Bird Senses

Birds are fascinating aren't they? Many of us wonder what it would be like to be in the form of these feathered flying creatures. How well would we see, hear, taste and smell? How sensitive would we be to touch?

Since it is imperative for flight, birds rely heavily on sight for survival. In birds, the eyes are the largest organs in size relative to their bodies of all animals. Our eyes take up about 1% of the total weight of our heads. Most of the avian eye lies hidden by their skull and eyelids. Ostriches have eyes 2 inches in diameter, the largest eyes of any animal that lives on land. The eyes of large owls are about the size of ours, but account for one third of the weight of their heads. Starlings' eyes make up 15% of their head weight. In many birds, the eyes weigh more than the brain!

SEE

As in our eyes, birds have a sensitive retina in the back of their eyes that absorbs incoming light, senses it, integrates the information in it, and sends this information on to their brain. An avian retina is much thicker than ours and contains more rods for seeing black /white and in dim light and more cones for daylight and color vision. Logically enough, the eyes of nocturnal birds like owls have more rods than cones. Barn owls can see a mouse at 2 meters with an illumination of .00000073 foot-candles - the equivalent of us seeing the mouse by the light of a match a mile away. Diurnal birds have move cones than rods to see in sharp color the insects and seeds on which they feed during daylight hours. Hummingbirds, unique as they are in other respects, see ultraviolet light like some insects can.

On our retina, we have an area of denser concentration of receptor cells called a fovea. When we focus on an object, the image falls on our fovea, giving us a sharp perception of the object, but weak peripheral vision. Some birds are like humans and have one fovea per eye. Those birds that need to be good judges of distance and speed, hawks, owls, eagles, terns, parrots, hummingbirds, swallows and doves, have two fovea per eye. One centrally located where the optic nerve enters the eye provides sharp monocular and lateral views for searching for objects on the ground or in foliage. The other is found in the temporal area of the eye towards the rear margin of the chamber, functions in looking forward with both eyes, and is a pursuit fovea used to track flying insects or other moving prey. A few terns and swallows have three fovea. Owls and sandpipers bob their heads to gain perspective vision as they have only one poorly developed fovea. Hawks and eagles have a total of four fovea which gives them the ability to hunt moving prey at a great distance.

While the acuity of avian vision is a debatable topic among ornithologists, birds do see better than we do. In man's fovea, there are about 200,000 sensitive cells per square millimeter. The house sparrow has about 400,000. The European buzzard, a buteo similar to our Red-tailed hawk, has about 1,000,000 visual cells to each square millimeter of its fovea. This gives them their incredible resolving power to see clear and distinct images of very small prey.

HEAR

Next to vision, birds find hearing most important. In ancient history, the sacred geese of Juno's temple are credited with cackling and waking the sleeping guards in time to save Rome from a nocturnal attack of the Gauls. Long kept as watchdogs, geese feed a lot at night, and make quite a commotion over any unusual sound. Parrots were kept on the Eiffel Tower during World War 1 to give warning of planes too far away to be seen or heard by humans.

Birds don't have external outer ears like we do. Instead, most have openings in the sides of their heads that are covered with soft auricular feathers. These feathers have no barbules and do not interfere with hearing, but they do protect the ear from damage during flight by minimizing air turbulence. Old World vultures and ostriches have no auricular feathers. Owls come closest having an outer ear like ours to help catch sound waves. They have a movable skin flap most prominent along the front edge of the ear opening to help reflect and concentrate sounds coming from behind. Great horned owls, barn owls, and other owls that hunt by night, have incredibly sharp hearing. They can find prey in total darkness with an error of only 1° horizontally and vertically. A Barn owl needs only hear, not see, a mouse rustling about on the ground to locate and strike. It actually judges the direction in which the mouse is moving just before it strikes.

Humans rule when it comes to the range of frequencies that we can hear. Our range extends from a low of 20 cycles per second to a high of 20,000 cycles per second. The range of frequencies heard varies among birds according to species. The Hairy woodpecker hears from a high of 18,400 to a low of 34 cycles per second, giving rise to the concept that this woodpecker can hear wood boring insects as they move along underneath the bark and wood of trees. Starlings hear nearly as well as we do at high to mid range (15,000 cycles per second), but their hearing fails at 650 on the low end. Man can hear about four octaves lower than the pigeon and five octaves lower than the starling or house sparrow. While they may not hear as wide a range as we can, birds are about ten times as sensitive to rapid changes in pitch and intensity. They can differentiate between frequencies less than 1% apart and separated by only .6 to 2.5 milliseconds.

SMELL

On to smelling! In general, it is believed that birds have a poor sense of smell. However, exceptions do occur. Being subjective in nature, olfaction is difficult to measure, and it may be that birds can smell better than we can measure. Audubon and Darwin both found that vultures did not find carcasses if they were covered from view. But Stager collected dead animals that Turkey vultures would normally feed upon. He placed the carcasses inside closed units during the night to avoid the vultures seeing what was going on. He used a fan and a blower to waft the odors from the ground up to the sky. When large numbers of vultures gathered in following days, he felt he had proof that Turkey vultures did hunt by smell.

TASTE

Birds have taste buds on the base of their tongues, but not nearly as many as man and other mammals. Rabbits have about 17,000 taste buds in their mouths. Humans have about 9,000. A domestic pigeon has 27-59. The tastes perceptible to us are sweet, sour, bitter and salt. Only birds that feed on fruit or nectar, like parrots and hummingbirds, show any interest in sweet tastes. Sour is widely tolerated by birds. Salt tolerance varies. Seabirds can excrete excess salt through their nasal glands, but will prefer fresh water to salt water if given a choice. Bitter, a taste often associated with poisons gets a variety of responses. Blue jays were fed Monarch butterflies that were fed on cabbage. No ill effects occurred. Then they were fed Monarchs that had been fed on milkweed, which contains cardiac glycosides that are fatal if eaten in large enough doses. The jays ate the monarchs raised on milkweed. Twelve minutes later they vomited but recovered in about half an hour. From then on, the Blue jays rejected all Monarchs, no matter how they were raised. If starved, the jays first tasted the butterflies before eating them, and then only ate the cabbage raised Monarchs.

TOUCH

Touch is another vital sense for birds, and all animals. Birds are very sensitive to the lightest touch of their feathers. We have touch receptors in our skin as do birds at the bases the feathers and elsewhere. Some birds have facial or rictal bristles around the mouth that probably serve as touch receptors. Many species have a group of nerve endings on the tongue and palate called Grandry's corpuscles known only to birds. They allow the wood stork to get its prey by "swallowing anything that touches its partly opened bill as it walks through shallow muddy water." (Kahl, 1964). Woodpeckers have groups of nerve endings called Herbst's corpuscles on the tips of their long sticky tongues to sense not touch, but vibrations. Snipes, kiwis, ducks, flamingos, swallow, and sandpipers also have Herbst's corpuscles. The well-known sensitivity of birds of explosions and earthquakes may be the result of sensing vibrations via these structures.

Birds can see better, respond to sounds and touch well, but are not as discriminating to taste and odor as we are. Interesting, now, if we could only learn to fly…