Jason Frishman


Birds belong to the Kingdom of Animalia, which means that it is a eukaryotic organism; Phyla of Chordata, signifying the organism has a spinal cord; Class of Aves, the class of birds. To get to this in depth classification, birds first underwent an enduring evolutionary process. Birds ultimately evolved from dinosaurs and are closely related to reptiles, in fact, many scientists believe birds belong in the same classification as reptiles. To be more specific, birds are closely related to theropods, a group of relatively small, bipedal, carnivorous dinosaurs. It is thought that a feathered theropod may have sparked the evolution of birds, yet this is not confirmed. Another piece of evidence that connects birds to reptles is that scientists believe bird feathers as we know them today developed from reptile scales (Walker K.).

Diagnostic Characteristics
Anatomy of a Bird
Anatomy of a Bird

Birds are unique because they have the ability of flight. This advantage has enabled birds to hunt more efficiently, flee from danger quicker, and utilize techniques like migration which yields benefits such as more diverse mating partners and new food and resources. Much of the characteristics and anatomy of birds are directly related to flight, as birds are light in weight and have keen senses. One of the most important characteristics to a bird is its bone structure. As pictured to the right, the bone structure of birds are honeycombed, which makes them strong but light to enhances the ability to fly. Aside from flight, modern birds also have characteristics that are related to active metabolism, which is the sum of all the chemical reactions occuring in a cell and within the organism. These traits include a beak, no teeth, a muscular stomach, feathers, a firm skeleton, and hard-shelled eggs. Parent birds invest a lot of parental care. (AK) Birds are either prococial or altricial. A precocial bird species is born with feathers and learns to feed and fly on it's own. An altricial bird species is born without feathers and requires high parental investment for food and protection. (APS)
[In this picture, a parent is feeding their young, caring for them. (DG)]
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The anatomy of a bird centers around its wings and feathers. The wing is an evolved forelimb from tetrapods, four legged animals with a vertebrate, and are the key to birds existence. Using their strong pectoral muscles, birds are able to generate enough power to take off by flapping their wings. Feathers, made of keratin, consists of a shaft, barb, barbule, and hook. Feathers act as the vane of the bird, helping the organism stay aerodynamic, provide insulation, and maintain balance and coordination.
Here, the male peacock exemplifies how feathers are not only used for locomotion and insulation, but also for attracting mates. KS

There are birds that do not fly, however, called ratites. Among these are animals such as penguins, ostriches, and emus. These birds Do not have quite as strong of a chest muscle as those who fly would have, but they are able to make use of their anatomy in other ways. For instance, penguins do not fly, but that use their strong chest muscles to swim.

Acquiring and Digesting Food

Most birds acquire their food through hunting. Enhanced by flight, birds are able to hunt in wide areas and are fast enough to catch many organisms. Some birds, such as eagles, are carnivorous and hunt smaller animals; some are detrivores and feed on dead animals; some are herbivores and feed on plants; and still others hunt marine life such as fish. Depending upon which kind of feeder a given bird is, it may look very different. Seed eaters tend to be smaller while carnivores are larger and have sharper beaks (AS).

bald-eagle_255.jpgRabya S.

Although the specific digestion process varies from species to species, in general birds digestion process consists of the mouth, esophagus, crop, stomach, gizzard, intestine, and anus, as pictured below. The food enters the bird through the mouth, however there is not physical digestion here in the form of chewing as there is in many other classes of animals. Instead, the physical digestion takes place in the gizzard, which acts as a grinder to mush up the food and make it easier for the stomach to digest the food chemically. The crop is a fascinating part that some birds do not have. The crop acts as a side plate, collecting all excess food that cannot fit in the stomach and waiting for those foods to digest before releasing it back into the digestive track. In this way, birds are able to consume more food than their stomachs would normally allow because they can store the excess until the stomach is ready for more. Then, once the food has been properly digested by the stomach and gizzard, the remains move through the intestines where it is further digested chemically and the nutrients are absorbed. Eventually the waste is excreted though the anus.

Sensing the Environment

Birds have very keen sense which allow them to fly. With good vision they are able to spot their prey from afar and above. Birds also have good sense of smell and good auditory ability. Some birds, like night-hunting owls rely almost entirely on hearing to find their prey. Some birds use their sense of smell to find prey, like the Kiwis in New Zealand, and others use it to find their homes, like the storm-petrel in the Bay of Fundy. Smell is not a constant though, and it is much more evolved in certain species than others (JL).
external image GENERAL_AVIAN_HEARING_SYSTEM.jpginner ear anatomy of pigeons(MSR)


The main mode of transportation for organisms of the aves class is obviously flight. Birds have essentially evolved for the purpose of flying: their strong heart, respiratory system, and efficient waste removal are all key to the ability to fly. Despite this, there are a few birds that are not able to fly, called ratites. Penguins is one example of this and they are fantastic swimmers. Other ratites simply walk or run.
Ratites are often larger than birds that can fly, such as an ostrich, which is the largest living bird standing 9 feet tall and weighing 330 pounds. In addition, some birds are very well adapted for climbing, such as woodpeckers and tree creepers. These birds cling with their strong feet and brace themselves with stiffened tail feathers, allowing them to climb veritcally upward. (SW)

These are all examples of Ratites including the ostrich which can run almost 40 mph!(JS)
These are all examples of Ratites including the ostrich which can run almost 40 mph!(JS)

Flight is not unique in the animal kingdom- bats, the extinct pterosaurs, and some insects also fly. However, this behavior is not indicative of a common ancestor. The bone structure and flight pattern of their wings all vary. Insect wings are an extension of their exoskeletons, pterosaurs had additional bones to suit their wing's function (called the pteroid bone,this was by their wrist bones and pointed to the dinosaur's shoulder), and bats' wings are formed by the fingers of their hands, which are homologuous to ours).(DRM)


The respiratory system in birds consists of two locations for air to collect in, the anterior air sac and the posterior air sac. When a bird inhales, fresh air moves through the trachea, or air canal, and into the posterior air sacs. In the figure to the right fresh air is represent by light blue while stale air is show by gray. Then, as the bird exhales, the fresh air from the posterior air sacs move through the lungs and into the anterior air sacs, displacing the stale air that was there and moving it out of the organism. As the fresh air is moved through the lungs, small tubes in the lungs called parabronchi collects the air and performs gas exchange, taking the oxygen from the air and releasing carbon dioxide. It takes two full respiratory cycles for one inhale of air to move through the body and out.

Metabolic Waste Removal

Birds remove their waste through uric acid. Uric acid is insoluble in water which is unlike most forms of waste. Also, uric acid is non-toxic. Uric acid is able to be produced in a semisolid form with the use of little water which allows birds to hold their water for more time and use it more efficiently. Uric acid is essential to birds because shelled eggs, such as the ones birds use to reproduce, are not permeable to liquids. Therefore, if birds were to excrete in liquid form, it would build up in the egg and would harm the embryo. Instead, the semisolid is able to sit in the egg until it is hatched with no negative side effects. One big disadvantage of uric acid to offset all of these huge advantages is that it is more energetically expensive than the forms of excretion that most animals use. However, for birds, the pros definitely outweigh the cons.


Birds have a closed circulatory system, meaning that the blood stays in the vessels within the organism. The circulatory system of aves is one of the most complicated and powerful of all organisms, for it has a four chambered heart. This completely divided chamber consists of a left ventricle and atrium along with a right ventricle and atrium. The ventricle is the part of the heart that pumps the blood out to the body while the atrium returns the blood back to the heart. By completely splitting the ventricles and atria, birds are able to use one side exclusively for oxygen-rich blood and allow the other side to handle oxygen-poor blood. This allows the circulatory system to maximize power and efficiency by utilizing this double circulation method. The left ventricle pumps oxygen-rich blood to the pulmonary circuit, or the area where the lungs are held, where the capillaries, or small tubes that are able to perform gas exchange, deprive the blood of its oxygen. This blood then enters the right atrium and is pumped out of the ventricle to enter the systemic circuit, which replenishes the blood with oxygen by allowing the blood to move through systemic capillaries, which are similar to lung capillaries. The blood is then received by the left ventricle and the process repeats.
Diagram of a 4 chambered heart (KD)
Diagram of a 4 chambered heart (KD)

Self Protection

The most obvious form of self protection that birds take advantage of is flight. Most predators of prey are stuck on land, so birds are able to comfortably escape them simply by flying away. In addition, birds communicate by signaling to one another through auditory and visual cues. Birds are territorial so they signal to other birds where there territory is. One of the most common signals birds use is singing. Birds may also warn other birds of an incoming predator by singing. Birds such as the killdeer is able to trick potential predators. By feigning injury making it appear as if the bird has suffered a broken wing, the predator is attracted away from the nest and to the apparently injured bird. However, as the predator gets close, the bird simply gets up and flies away.

Some species of birds have lost their ability to fly through evolution. Flightlessness enables birds to become much larger, and some species of flightless birds such as emus and ostriches use their great size to ward off predators. (SF)
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This is a picture of an ostrich with its young. If you look at the size of the bird
compared to its wings it is understandable why the bird can no longer fly.(CP)

Killdeer feigning injury to protect its nest:

Osmotic Balance

Osmoregulation, the management of the body's water and solute content, is vital to birds, especially marine birds who spend most of their time drink sea water, which has a high salt concentration. Birds arrange there transport epithelia, layers of specialized epithelial cells that are able to regulate solute movements, into tubular networks that allows them to maximize their surface area. These tubules use a countercurrent system, flowing the blood in the opposite direction of the salt, which allows the blood to lose as much salt as possible to the tubules. The tubules then drain into a central duct which carries the high salt concentration to nasal glands. Birds are allowed to live for so long at sea because of these special nasal glands which are able to produce a fluid with a salt content even higher than that of salt water which is released through nostrils containing salt secretions. Therefore, the bird actually can release more salt than it takes in which allows it to gain a lot of the necessary water.

Temperature Balance

As many endotherms, or warm blooded animals do, birds are able to maintain temperature balance through vasodilation and vasoconstriction. Vasodilation increases blood flow by widening the blood vessels, while vasoconstriction reduces blood flow by narrowing the blood vessels. Blood is warm, so when more blood flows the organism gets warmer, and when less blood is allowed to flow the organism cools down. Birds are able to direct the blood flow in vasodilation through nerve signals telling the blood to flow and the heat it carries towards the area of the body that is cool. Birds are able perform basic homeostasis as well, including panting or finding a warm or cool area to settle in. Many species of birds migrate in the winter to find a more suitable location.

In addition, birds are one of the only classes of organisms that are able to change the rate of metabolic heat production. Physically, birds may increase their heat production by shivering, but there is also nonshivering thermogenesis or NST. NST is the ability to use hormones to alter the rate of mitochondria activity which can be forced to produce heat instead of ATP. Also, the feathers of birds provide significant insulation to trap the heat in and birds are able to fluff out their feathers in times of need to provide even more insulation.

Birds also make use of a countercurrent heat exchanger similar to the process the marine birds use to produce the salty substance they are able to secrete. Blood gets cooler as it moves further from the heart; therefore it is coolest in the outermost extremities such as the legs. Therefore, in these areas birds use a countercurrent heat exchanger. The blood flowing from the heart reaches the end of the extremity then loops back around to make its way back to the heart. Now, since the blood flowing to the leg is warmer than the blood flowing away from it, the two lanes are able to perform heat exchange as some heat moves from the warmer blood to the cooler blood. In this way, birds are able to maintain more of a constant temperature level.

Review Questions:
How do birds achieve temperature control? What is the significance of their ability to change their rate of metabolic heat production? (LS)
Describe the respiratory cycle of birds? (JJF)
Describe the countercurrent heat exchange process that birds use to contorl temperature. [Rabya S]
Describe the way in which some birds are able to live drinking seawater. (AZ)
How does bird anatomy explain the ability of birds to fly? [Ben Yudysky]
Why is the fact that birds remove their wastes through uric acid beneficial to both the bird and its egg? (LD)

Campbell, Neil A, and Jane B. Reece. Biology. Sixth Edition. San Fransisco: Pearson Education, Inc, 2002.
"Birds." Encyclopedia Britannica. 24 Oct. 2009. <>. (Sarah Fleming) (Sara Waugh) (Ali Kirsch) (Jesse Landy) (KNS)
Sparknotes AP Biology Book (Walker K.)
(Jake Schwartz) (KD) (Jackson Murphy) (Donna McDermott)
(APS) (DG)