Bird Calendars
NOTES FROM NATURE
By Jerry Toll
At the November meeting of the Nature Study, George Allen posed a question (paraphrased):
When I ride a motorcycle, my eyes water. Do birds' eyes water when they fly?
As it turns out, they don't. Through an adaptation, their vision is unimpaired during flight.
Here's how it works. Birds have a nictitating membrane, the so-called inner eyelid that dogs, cats, and Vulcans show when they sleep. Its function is to protect and clean the external surface of the eye.
It sweeps transversely across the eye from the nasal passage area towards the ear. As it does, it releases lacrimal fluid—tears—on to the surface of the eye through a duct located between the nictitating membrane and the surface of the eye.
In this manner, the lacrimal fluid lubricates and protects the surface of the eye by maintaining a constant thin layer of fluid between the two surfaces where friction would occur without wasting excess fluid.
Additionally, the leading edge of the nictitating membrane as it sweeps across the cornea acts as a squeegee polishing the cornea, limiting the amount of lacrimal fluid left on the eye, and sweeps foreign objects off the eye before it.
Mammals possess eyes that rotate in the socket. The entire eye needs to be lubricated, requiring the production of larger amounts of lacrimal fluid.
The eyes of reptiles, amphibians, and birds are fixed in the skull and have very limited movement in the socket. Only those surfaces exposed to the elements require lubrication.
Humans possess tear ducts on the edge of the eyelid. Their production of tears is distributed across the exposed surface of the eye.
Thus when the exposed surface is dried out, as in riding a motorcycle, excess tears are produced to protect the eye.
The resource used doesn't elaborate on which, if either, system of lacrimal distribution is pervasive in the higher orders of the animal world. However, if memory serves, the ability kof humans to express emotion through the production of tears is unique to our species.
Other interesting factoids about avian eyes: In all species except owls and dippers, the nictitating membrane is transparent or slightly opaque. They blink the membrane rapidly and frequently.
The nictitating membrane is something I don't recall seeing except in owls. It's something to look for next time we get a chance to study a bird closely.
It has been suggested that the nictitating membrane is used by aquatic birds to compensate for the differences in refraction of water. This has not been adequately studied; however, the evidence suggests this is not the case.
Vision is a bird's primary connection to its environment. The ability of birds of prey to see objects at great distance and in excellent detail is well documented. Indeed, the physiology of all birds' heads is primarily designed to accommodate large eyes in a small skeletal structure.
For that reason, the eyes are fixed in their orbits, do not need to be spherical, and have evolved to best accommodate the genus. To accommodate fixed eyes, a bird's neck is very slender and flexible, allowing a wide range of movement of the head. A bird can rotate its head as rapidly as a mammal can rotate its eyes.
The shape of a bird's eye fits into three broad categories: disc-shaped, globose, and bell-shaped. The shape is determined by needs of the specific Family. For instance, a sparrow would need a disc-shaped eye to see details at close range but have a broad field of view to watch for predators. A hawk would need a bell-shaped eye that can identify a vole a thousand feet below.
I have observed peregrines hawking moths and small birds at the Woodmen Tower, who are attracted to the strong lights illuminating the tower at night from below. I have seen them accomplish this by closing their lower eyelids, effectively shading their eyes from the glaring lights below.
These are just some of the ways birds' eyes have evolved. It in no way reflects the many subtle, complex and varied adaptations characteristic of each species.
For the record or if you wish further reading on this subject, I consulted "Form and Function in Birds" Volume 3, Chapter 7. Edited by King & McLelland and written by Graham R. Martin.
Which bird has the largest eyes of any living creature? Why is it a bird and not, let's say, a mammal?
Which bird has the smallest eye in relation to body size of any bird?
Answers next month.
01/24/08