On this week’s Earth and Beyond program, host and producer Robin Song learned about how birds’ eyes and metabolism are uniquely suited to their lives. Audio runs about 7 minutes. Text follows. Photos by Robin Song.
This is the time of year when I put up my bird feeders and help the winter birds make it through the tough times ahead. Watching the chickadees, red polls, woodpeckers, gray jays, magpies and ravens coming to the food I put out each day, I ponder the details of these amazing creatures. I went to my collection of birding magazines and books. The wealth of information known and still being discovered about birds is extensive. I decided to briefly explore two subjects: eyesight and metabolism.
I referenced an article about avian eyesight by Margaret A. Wissman, DVD.:
Birds see in color. Diurnal birds-birds active during the day-have the best color vision, and nocturnal birds have the best night vision. Kingfishers and other diving birds have eyes adapted to aerial and aquatic vision due to some unique adaptations to the deeper structures of the eye. Loons, Cormorants, and other diving species have red irises which help them see underwater, enabling them to hunt for and pursue fish. Water birds and birds that live on open plains have a specialized area in the eye that allows them to fix the horizon accurately as a reference point.
Birds have brightly colored eye droplets within the eye that are involved with interpretation of color vision. It is thought that the different colored oil droplets enhance contrast by acting as in-the-eye light filters. For example, the yellow oil droplets would remove much of the blue color from the background, which would increase the contrast between an object and the trees. The enhanced contrast would considerably increase visual acuity.
Birds are masters at detecting and following movement. While a bird and a person may both be able to see a mouse from the height of 250 feet, a person can only do so if his attention was accurately directed to it, but the bird can see the mouse without even looking directly at it. Moreover, the bird can see all the mice in a field at a single glance, but a person could do that only by scanning the area meticulously.
Referenced from the book: A Guide To Birding:
Eyesight is extremely important to birds, and their eyes have evolved in response to this, and are amongst the most sophisticated sensory organ in the animal kingdom. They are large, for one thing. Examination of any bird’s skull shows the dominance of the eye sockets. An ostrich’s eyes are the same size as an elephant’s. If a human’s eyes were proportional to a Great Horned Owl’s, they would be larger than oranges and weigh three pounds each.
The visual acuity of birds is due not only to the large size of their eyes but also to the dense concentration of visual cells on the retina. A Buteo hawk may have a million visual cone cells per a thousandth of a square inch on the most sensitive part of the retina- the fovea. Nature has bestowed a lot of megapixels on birds.
Most bird’s eyes are virtually fixed in their sockets, but a bird can easily turn its head 180 degrees or more. The visual field of most birds is far greater than a human’s- up to 360 degrees. Most birds can see light into the near ultraviolet range, and some birds can see in the ultraviolet range. Bird irises are remarkably variable in color, from blackish to bright white, yellow, red, green, or blue. Even pupil shape can vary, being round in most birds, but vertical in Skimmers. The iris color can vary by sex- such as yellow in female adult Bushtits, but dark brown in males- and it can change color with age. Immature Bald Eagle’s eyes are dark brown from hatching until maturity at age four or five, when they turn pale yellow.
And now, on to bird metabolism. The following is referenced from an article by Eldon Greij, Birder’s World, February 1997:
Birds possess a super-charged metabolism, which is critical for them to function as they do. Their almost constant activity requires a frenzy of physiological activity on the inside. Their activities far exceed those of other vertebrates.
Birds are perfect flying machines, incredibly adapted for life on the wing. With hollow bones giving them reduced body weight, a rigid skeletal frame, feathers, and wings which give both lift and thrust, they have become masters of the air. Anchored to the keel of the breast are large flight muscles, which make up from thirty to forty percent of a bird’s weight. To function at high efficiency, a bird’s body requires a higher temperature. This increases the speed of most body functions, such as muscle contraction, nerve impulse conduction, and all chemical reactions. The body temperature of birds ranges from 106 to 113 degrees.
The high-powered avian engine requires a high metabolism. Even at rest, birds have a high metabolic rate, which requires a good supply of blood glucose and an abundance of oxygen to process it. The blood glucose level in birds is about twice that in humans, and avian lungs are more efficient as well, providing a larger supply of oxygen. A high body temperature and metabolism allows a bird to almost always be ready for quick muscular activity required for fast movement or for taking flight.
Birds have a heart proportionately five times the size of a human’s heart. This large heart pushes the blood rapidly through arteries and into the small capillaries of muscles and organs. The circulatory system transports glucose from the liver and oxygen from the lungs to all parts of the body. Birds average blood pressure at fifty percent higher than a human’s. The heart rate is astounding as well. In large birds, it beats at 300 times per minute, and in small birds, such as hummingbirds, it commonly exceeds 600 beats a minute.
With high temperature, heart rate, metabolism, and blood pressure helping to convert the massive flight muscles of birds into precision high-compression engines, birds are feathered bundles of energy. Standing at the kitchen window, cup of tea in hand, watching the birds coming and going at my feeders, I now think about what I have learned about these wondrous creatures in a few hours of research and reading and I am filled with even more admiration. There is a lot more to learn and share about them, too- a person could spend a lifetime studying these feathered enigmas.
KTNA NatObs 120212-Those Amazing Birds