Elisabeth Guenette


Diagnostic Characteristics:
-- honeycombed bone structure (strong but light). A honeycomb bone structure evolved in order to achieve lightness needed for flight, without affecting the overall integrity and strength of the skeletal system. As a result, most bones are not solid but are arranged into honeycombs of bone tissue.
external image hollow%20bone.jpg(JE)
As well as a having honeycomb bone structure, most birds capable of flight have a keel, a plate of bone that is attached to muscles needed for flight, while in flightless birds the keel is usually nonexistent (3 KA).
-- the beak is created by the upper and lower jaws extending outwards (EK)
-- pneumatic cavities that are filled with air and connect to the respiratory system. These cavities aid in both a supply of air and buoyancy during flight. (11 AN)
-- females only have one ovary
-- no teeth

feathers (SW 5) . They are the most distinguishing characteristic of birds; no other living animal has them. Feathers, in addition to allowing for flight, provide insulation and protection for birds. They also allow for cryptic coloration and communication with rivals or mates. Feathers are inexpensive to produce, and some birds grow a new coat each year.
--Unlike Mammals, Birds use a ZW sex determining system. This means that the ovum determines the sex of the offspring. A Male will have a ZZ pairing while a female will have a ZW pairing. (EK20)

Acquiring and Digesting Food / Metabolic Waste Removal
When food is not plentiful, such as during the winter, some birds migrate to warmer climates where food is easily found. (16 AL) Those that do not migrate have other methods of finding food during the winter. (16 AL) Amoung these are crows, nuthatches, chickadees, and jays, who store excess food. (16 AL)
In birds, digestive tubes, known as complete digestive tracts or alimentary canals, extend from the mouth to the anus. Food is ingested through the mouth, then travels from the crop to the stomach, both which are mostly used for food storage (but some digestion may occur), and then to the gizzard, which grinds up the food. Some birds lack the crop and/or the gizzard. Next the food goes to the intestine, where it is hydrolyzed (enzymatic hydrolysis is the splitting process of breaking covalent bonds by enzymatically adding water), and nutrients are absorbed into the blood stream. The remaining undigested waste continues and exits the bird through its anus.

Avian digestive system

Sensing the Environment
Many birds routinely migrate, which requires significant knowledge of the environment. There are three main types of migration: piloting, moving from one familiar landmark to another until reaching the final destination, orientation, detecting compass directions and then travelling in a straight line towards the destination, and navigation, which is a combination of piloting and orientation. Many birds species migrate to take advantage of global differences of seasonal temperatures to optimize the availability of food and breeding habitat. (6-SC)
Another example of how birds sense their environment is jays. Jays store food in caches, which they remember by using a cognitive map, an internal representation of the spatial relationship among objects in an environment.

In addition, birds have many familiar ways to sense the environment. These include eyes to see. Birds' visions is often mush better than our own. For example, the vision of the hawk is three times more detailed than our own (8T2).
The majority of birds’ main source of transportation is flight; however, there are some birds (penguins, ostriches, emus) that cannot fly. In birds that do, their wings are a shape known as an airfoil. Because the top of the wing is slightly convex and the underside is more concave or flat, there is a pressure difference between the air above the wing and the air below. The air pressure below is stronger than above, creating lift, which allows the bird to fly. Flying birds have evolved to be structured in a way that improves flight, for example, they do not have teeth, which makes their heads lighter, and they have honeycombed bone structure, composed of a central hollow shaft and vanes, barbs, and barbules that radiate out from the shaft, making the bones strong but light.
This image depicts the three types of feathers on the wing of a bird that contribute to its ability to fly. It also shows the anatomy of the feather. The barbules are what connect the feathers. (AW1)

Birds breathe faster than any other animals. (HL 19) The respiratory system is composed of eight or nine air sacs, none of which do air exchange (they just keep air flowing through the lungs), that penetrate abdomen, neck, and wings. The system of lungs and air sacs works as one complete circuit, with air flowing only in one direction, regardless of whether the bird is inhaling or exhaling. To maintain this one-directional path, air flows through parabronchi, tiny channels in which gas exchange occurs. Because each breath completely exchanges the air in the lungs, birds have higher maximum oxygen lung concentrations than mammals do. Unlike mammalian lungs in which the volume of air expands with every inhalation and exhalation, bird lungs provide a constant volume of air, which ultimately helps them maintain altitude during flight (1 MB).

This diagram of a bird's respiratory system stresses the air sacs that allow air flow to support energy production during flight (4 J. Stein).
This diagram of a bird's respiratory system stresses the air sacs that allow air flow to support energy production during flight (4 J. Stein).

Birds are endothermic, so they require an efficient metabolism and circulatory system, which is powered by their four-chambered heart. The bird's heart separates the atria and ventricles which results in the separation of the circulatory and systemic systems (3 JSun). Deoxygenated blood is pumped through the right atrum and ventricle; oxygenated blood flows through the left atrium and ventricle. Once the oxygen rich blood is pumped through the left ventricle, the blood flows through the whole body supplying nutrient and waste exchange. The average body temperature of a bird is 42°C (107.6°F) (CH). Their heart keeps tissues well supplied with oxygen and nutrients so these systems can be supported.

Self Protection
Birds have proportionately larger brains compared to reptilian and amphibian brain, and perhaps some of the results from that can be seen their defense mechanisms. In order to protect her young and pass on her genes to the next generation, a killdeer bird will deceive predators by distracting them and luring them away from the nest by pretending to have a broken wing, making her an easy target. When the predators get too close, she flies away and returns to the nest when it is safe.

Osmotic Balance
Marine birds depend on osmoregulation, management of the body’s water content and solute composition, because they need to be able to obtain food and water from the ocean. In these birds are nasal glands which secrete salt excretory fluid that is saltier than the ocean water. By excreting this salt when they consume salty water, the birds are left with a net gain of fresh water (salt water-salt fluid = fresh water). Each salt-excreting tubule is lined by a transport epithelium (a layer/layers of specialized epithelial cells that regulate solute movement) is surrounded by capillaries, and drains into a central duct. The salt in the blood enters the tubule via active transport, which is aided by a salt gradient (higher concentration near the central duct at the bottom, lower salt concentration near the top of the tubule, where, because the blood travels in the opposite direction of the gradient, the salt from the blood flows into the tubule).

Temperature Balance
Because birds are endothermic, (commonly known as warm-blooded) which means they use their high metabolic rate to maintain a stable, homeostatic body temperature. To uphold this rate, they must consume more food than a similarly-sized ectotherm, whose body temperature depends more on the environment than on their low metabolic rate. Birds are generally warmer than their surroundings but have systems for reducing their body temp in hot environments. These include feathers and sometimes a layer of fat to help hold in their heat. Some birds also have countercurrent heat exchangers, a special arrangement of blood vesicles to reduce heat loss, especially in the less insulated legs. The Canada goose has this system, so when it’s cold, the heat from the warm arterial blood coming from the body is transferred to the venous blood returning to the body before the heat travels down the bird’s leg and is released into the environment.
Other temperature-regulating systems include vasodilatation and its counterpart, vasoconstriction. In vasodilatation, superficial blood vessels (near body’s surface) dilate, increasing blood flow on the skin, which warms the skin and results in the transfer of additional heat from the body into the air. Vasoconstriction is the opposite; blood vessels constrict, reducing blood flow to them and thereby reducing heat transfer to the surrounding air.

Birds and Reptiles: Evolutionary Relations (AR 2)
Discoveries have been made suggesting that birds evolved from small, two-legged, feathered dinosaurs--thus a debate arose over whether birds should be classified as reptiles, as dinosaurs were. Some evolutionists think that birds diverged from reptiles after mammals did, while others argue that mammals and birds evolved from a common ancestor diverged from reptiles. Evidence for the latter hypothesis includes that of the four-chambered hearts present in both birds and mammals. For birds to have evolved from reptiles, which have three-chambered hearts, it would mean that birds and mammals underwent convergent evolution to end up with these more complex hearts. Many scientists think that this is unlikely, and it is more probable that birds and mammals evolved from a common ancestor that had a four-chambered heart. For more information about the opposite argument, see Reptilia .

review questions:
1)Name and explain two ways by which birds regulate their body temperature.(9DO).
2) Describe the evolutionary relationships between birds and reptiles. (10 VK).
3)What selection pressure caused ancestral birds to evolve honeycomed bones? (8RM).
4) What factors caused the evolution of feathers? (DPOD 12).
5) Why do birds migrate? What kind of behavior/behaviors is/are displayed in the migration of birds? (AS 18)
6) How does a bird's four-chambered heart make circulation more efficient? (CS 8)

Campbell, Neil A., and Jane B. Reece. Biology, Sixth Edition. 6th ed. San Francisco: Benjamin Cummings, 2001. Print.
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