Eggs and poultry make up a significant part of peoples' diets. Similarly, products made from feathers such as pillows, comforters, or down-lined coats are widely used. There is an increasing awareness and heightened passion for our feathered friends sweeping the nation. The popularity of feeding wild birds has actually changed the ranges of several common songbirds. Additionally, because of the low fat content of turkey and chicken meat and America's fascination with losing weight, the poultry industry is booming. And what other animal has a day dedicated to it like the tradition of roast turkey at Thanksgiving? Clearly, birds are a relevant and special part of our lives.
But what makes them so unique? How are they different from animals in the other major classes of organisms? And most importantly, how are they different from us?
Position in the Animal Kingdom
Birds make up the class Aves of the subphylum Vertebrata, phylum Chordata. Class Aves contains 28 orders, 163 families, 1,975 genera, and nearly 10,000 species. Their distribution is worldwide, including open oceans. While the majority of the world's bird species are known to science, a handful of new birds are still discovered each year. Most of these rare birds are found in remote regions of the world.
It is widely believed that birds descended from two-footed, lizard-like reptiles that lived in the Jurassic period some 208 million years ago. Birds still have many resemblances to reptiles, such as their habit of laying eggs, the possession of scales on their beaks and legs, and the arrangement of many internal structures.
The three highest classes of vertebrates—reptiles, birds, and mammals—have adapted their reproduction to terrestrial life, largely through the evolution of an egg whose embryo is enveloped in a protective membrane called the amnion . Hence these three classes are grouped under the term "amniota," or " amniotes . " Among all animals, only birds and mammals have evolved the high, constant temperature or homeothermism that makes energetic activity possible in all habitats and at all seasons. This, more than any other advance, is what makes these two classes the dominant vertebrates.
Birds have numerous characteristics that make them distinct from all other classes of organisms. While not all birds fly, a large number of these characteristics complement their amazing adaptation for flight.
All birds have feathers, which no other animals living or extinct are known to have had. The number of feathers is relatively constant within a species, although birds tend to have more feathers in the winter than in the summer. Smaller birds tend to have more feathers per square surface inch than larger birds, although fewer total feathers. For example, a ruby-throated hummingbird, with a relatively small surface area, has approximately 940 feathers, while a Canada goose, with a much larger surface area, has 33,000.
Feathers serve many purposes, including warmth, protection, flight, attractive adornment for courtship, and sex recognition. The heat-insulating value of feathers is so extraordinarily effective that it permits birds to live in parts of the Antarctic too cold for any other animal.
For their weight, it is estimated that feathers are as strong as the best human-made materials used in the aerospace industry today. Their flexibility allows the broad-trailing edge of each large wing feather to bend upward with each downstroke of the wing. This produces the equivalent of pitch in a propeller blade, so that each wingbeat provides both lift and forward propulsion.
There are well-documented stories of birds deliberately swallowing their feathers. Grebes, for example, consume feathers by the hundreds. Fifty percent of the stomach contents of horned or pied-billed grebes may be feathers. This odd behavior seems to have a purpose. Scientists believe the action of the gizzard in these primarily fish-eating birds is insufficient to crush the bones that are swallowed. The feathers are thought to protect the stomach by padding the sharp fish bones and slowing down the process of digestion so that the bones dissolve rather than pass into the intestine. This belief is supported by the observation that the least grebe, which, of all the grebes consumes the fewest fish, also accumulates the smallest amount of feathers in its stomach. Additional studies are needed to test this hypothesis.
Fusion and Reduction of Bones
Bird bones are extensively fused and thus reduced in number. Birds have no teeth or heavy jaws. Unlike mammals, which have a single bone, the lower jaw of birds is made up of five small fused bones. Additionally, the bones in the pectoral , pelvic girdle, and spinal column are fused, which serves as a rigid framework for flight muscles, limbs, and major flight feathers of the wing and tail. Birds have no tail vertebrae. The upper limbs show extensive fusion in the carpal and metacarpal bones. The finger bones are reduced in both size and number; two of them are completely missing and two of the other three are fused together. The ankle and foot bones in birds have also been fused and reduced in number.
Hollow, Thin Bones
The major bones of most birds' bodies are thin and hollow, while most other animals possess denser, more solid bones. The skeleton of the pigeon, for example, accounts for a mere 4 percent of its total body weight, while the skeleton of a mammal of comparable size, such as a rat, amounts to almost 6 percent of its total body weight.
Although the bird skeleton is thin and lightweight, it is also very strong and elastic. This is very helpful, since most birds' skeletons are subject to great and sudden stresses of aerial acrobatics. Interestingly, the wing, leg, and skull bones in some large, soaring birds have internal, trusslike reinforcements much like the struts inside airplane wings.
Not all birds have such hollow bones. To decrease their buoyancy and make diving easier, some diving birds, such as loons and auklets, have relatively solid bones.
In addition to lungs, birds possess an accessory system of air sacs connected with the lungs. These air sacs often branch throughout their bodies, frequently entering the larger bones of the body to occupy their hollow interiors. While this system of air sacs certainly contributes to weight reduction, it is believed that they have a more important contribution. The air sac system appears to supplement the lungs as a supercharger, increasing the utilization of oxygen.
In addition, air sacs provide buoyancy for aquatic birds. Swimming species have particularly large abdominal and thoracic air sacs whose volume can be controlled for swimming or diving.
Air sacs also serve as a cooling system for the bird's speedy, hot metabolism. It has been estimated, for example, that a flying pigeon uses one-fourth of its air intake for breathing and three-fourths for cooling.
Nervous System and Sense Organs
Birds have a very high metabolism. They may consume thirty times the amount of energy as reptiles of similar size. Several factors contribute to their metabolism level. Of all the million or so animals on Earth, birds have evolved the highest operating temperatures. Their average body temperatures range between 104°F and 110°F (42°-43.5°C). Birds live intense lives and their metabolic "engine" is always warm and ready for action.
Behind the high temperature in birds lie some interesting anatomical and physiological refinements. Besides eating an energy-rich diet, birds possess digestive equipment that processes their food rapidly, efficiently, and in large amounts. Fruit fed to young cedar waxwings passes through their digestive tracts in as little as sixteen minutes. Other perching birds may take from one-half to two hours to pass food through their bodies.
The excretory system of birds is also extremely efficient and fast. Their kidneys are roughly twice as large as those of comparable mammals. Except in ostriches, there is no urinary bladder. Its absence further assists flight by reducing weight, since there is no stored urine. Birds do not have a urethra to discharge urine.
Avian cardiovascular systems are extremely efficient, enabling birds to withstand cardiopulmonary stresses far beyond what mammals can tolerate. Like mammals, birds have a four-chambered heart. Relative to their size, however, it is large, powerful, and very rapid in beat. The world altitude record for birds is held by a Rüppell's griffin, which was pulled into the jet engine of an airliner at nearly 11,000 meters (36,000 feet). Although the vulture was undoubtedly soaring passively, no mammal of equivalent size could breathe enough air even to remain conscious at that altitude.
The respiratory system of birds is a complex network of lungs and specialized air sacs. This unique system acts as a supercharger for their fast metabolism by supplying large amounts of oxygen. While the lungs of humans constitute about 5 percent of body volume, the respiratory system of a duck makes up about 20 percent of its body volume (2% lungs and 18% air sacs).
Birds have no sweat glands and lose heat through their respiratory system and exposed skin. To cool off, most birds pant, which is an important form of heat loss. Additionally, many if not all birds flutter the throat area during heat exposure, resulting in heat loss from the mucous membranes of the throat. This throat flutter may account for 35 percent of heat loss in chickens, for example.
Finally, birds have a highly developed central nervous system and rapid nerve impulses. Birds are highly visual animals; they must be in order to fly. The importance of birds' eyes is implied by their size; of all animals, theirs are the largest relative to the body. Some hawks and owls have eyes as large as human eyes. In some owls, the eyes comprise up to one-third of the total weight of the head. In starlings the eyes comprise 15 percent of the head weight; in humans it is only one percent. In most aspects, the avian eye structure resembles that of mammals. The eyes of birds are able to adjust to light about two times as well as those of a twenty-year-old person.
There has been a lot of debate regarding the acuteness of avian vision. Generally, it appears that it is better than human vision, but there are exceptions. A vulture sees about as sharply as humans, whereas a chicken appears to see only about one twenty-fifth as well as humans. Hawks and songbirds see about two and one-half times as sharply. Birds also appear to see in dim light better than humans because of the density of receptor cells in the retina . Barn owls can see an object at two meters with an illumination of 0.00000073 foot candles. This is the equivalent to a person seeing an object by the light of a match a mile away.
The brain of a small perching bird weighs about ten times that of a lizard of the same body weight. The cerebral hemispheres in birds are large and well developed, as in mammals, but the location of the complex behavior in the cerebrum is different in the two. The brain of a mammal is dominated by the top layer of the cerebral hemispheres, which have a high capacity for learning. The bird brain is dominated by the middle of the cerebral hemisphere, which lacks learning capacity. So mammals, in general, learn behavior, while bird behavior tends to be instinctive and stereotyped. This is probably the basis for the well-known phrase "bird brain."
Laying of Eggs
All birds lay shelled eggs and incubate them outside of their body. Eggs range in size from 25 centimeters (10 inches) long for ostriches to only 8.5 millimeters (0.3 inch) long for hummingbirds. Smaller birds lay eggs that weigh more in proportion to body weight than do the eggs of larger birds. Hummingbirds lay eggs that are 15 percent of their body weight, while ostriches lay eggs that are only 2 percent of their body weight. Flightless kiwis lay only one huge egg that can be 25 percent of their body weight.
see also Phylogenetic Relationships of Major Groups.
Stephanie A. Lanoue
Ehrlich, Paul R., David S. Dobkin, and Darryl Wheye. The Birder's Handbook: A Field Guide to the Natural History of North American Birds. New York: Simon and Schuster, 1988.
Faaborg, Janus. Ornithology: An Ecological Approach. Upper Saddle River, NJ: Prentice Hall, 1988.
Forshaw, Joseph, ed. The Encyclopedia of Birds, 2nd ed. New York: Facts on File, Inc., 1998.
Gill, Franklin B. Ornithology, 2nd ed. New York: W. H. Freeman and Company, 1995.
Kaufman, Ken. Lives of North American Birds. New York: Houghton Mifflin, 1996.
Koeppel, Dan. "Eureka! An Antpitta!" Audubon 100 (1998): 96-101.
Natural History of North American Birds. New York: Simon and Schuster, 1998.
Van Tyne, Josselyn, and Andrew J. Berger. Fundamentals of Ornithology. New York: John Wiley and Sons, 1976.
Welty, Joel Carl, and Louis Baptista. The Life of Birds, 4th ed. Orlando, FL: Harcourt Brace Jovanovich, 1988.
Many birds show a high degree of social behaviour in forming large flocks and pair bonding for nesting, egg incubation, and rearing young. Fertilization is internal and the female lays hard-shelled eggs. See also Ratitae.