Animalia Chordata - Aves: Birds

By Madi Bader

Birds are most likely descendants of feathered theropods, a group of small, carnivorous dinosaurs.


Diagnostic Characteristics that define the group:

Birds are a very different type of species, having more than typical mammilian characteristics of hair and efficient bodily systems. The main diagnostic characteristics that define the group are that 1) the structure is strong but light, and 2) that this can be due to the lack of certain organs, such as a) multiple ovaries and b) teeth.

Birds are known for their bird songs. It allows the usually male birds to communicate for long distances, and for a long time. The bird song has to functions: to attract a mate, and defend their territory. Along with birds songs, some birds have developed dances: (AR)

Species of the phylum aves have a unique bone structure. The skeletal structure is required to be strong but light weight, this is achieved through the hollowing of many of the long bones found in birds. The bones gain support through honeycomb like structures, which remove much of the internal structure of the bone mass but maintain the strength. Birds are also characterized by a their sternum which has a pronounced keel, an plate extending from the sternum allowing the muscles for the wings to adhere to. (MS 17)

Feathers are what distinguish birds the most from other animals. Feathers give birds their lightweight characteristic and ability to fly, and they also provide insulation and protection. (AM)


Acquiring and Digesting food:

Another difference and common misconception is that this species does not chew their food, but rather grinds it
in a digestive organ near the stomach called the gizzard. Beaks, made of keratin, pick up this food. It is a necessity
for survival that these are very adaptable and can take many different shapes to fit the animals needs.
Tongues of birds, like the beaks, are adapted to the type of food that the bird consumes. Most birds have salivary glands. The beak and the tongue help in manipulating food for swallowing. But, in reality, the bird mouth is unimportant in eating and digesting food in comparison to the mammalian mouth. (IL- source 7)
This is further
explained in the "metabolic waste removal" section. (
Birds can be carnivores, herbivores, or omnivores. Some birds, like vultures, are scavengers.Birds much eat a lot in proportion to their body weight so they have energy to fly! (LPE)
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Sensing the Environment:

Birds have the best eyes out of all of the vertebrates. This is because the visual areas of the brain were developed very
well. As for hunting and scavenging, the evolution of wings is a fantastic enhancer. Flight is also the best utilized to escape
from predators and for migration upon the changing of seasons.
Birds are social creatures, usually, who use calls and whistles to communicate. They often live in flocks. (LPE)

While they do have well developed brains, the primary, largest part seems to be dedicated to instinctive, innat behaviors. As such, birds tend to follow distinct patterns when dealing with their environment, as well as mating, migration and nesting. (JP- Source 13)


Motor areas of the brain developed very well, creating great coordination in birds. Almost every part of a typical bird's
anatomy is modified in some way that enhances flight. For example, the wings (flapped for usage) have the same
aerodynamics as the wings of an airplane.
Birds have various adaptations for flight: they possess a lightweight skeleton, two large flight muscles, the pectorals and the supracoracoideus, and have a modified forelimb which acts as an aerofoil (a mechanism which can control a vehicle in flight). About 60 extant bird species, especially those on isolated islands, cannot fly because of limited resources and the absence of land predators. Though flightless, birds such as penguins use the same muscular movements and limbs when swimming.(SP) The feathers of birds are also arranged in specific groups to increase efficiency in various aspects of flight. (SP)
Some species of birds can fly, while other species of birds cannot, even though they do possess wings. One reason some birds can't fly is because a lot of the flightless birds live on an island that doesn't have predators. Since flying is a way to get away from predators and these birds don't need to, they have developed a more specialized way to live on land that is less energy costly. However, the anatomy of all birds suggest that they all came from a common ancestor that could indeed fly. (MM)
Instead of hollow bones, many flightless aquatic birds have solid bones that are heavier to provide a counterbalance for diving. Diving birds, like ducks, use their legs, which are farther back on their body than on flying birds, for paddling. Diving birds use their wings as paddles for enhanced power and control. (JS)
Primaries are usually heftier than other wing feathers because they must be quickly rowed through the air to provide forward thrust. Secondaries provide a large surface area in which lift is generated. Covert feathers cover the underlying feathers to create a slick surface reducing turbulence in flight. (SP)

Types of Bird Feathers

There are 6 commonly recognized types of feathers:
1. Vaned or contour: Form the outer coverings of a bird's body, including the wing & tail feathers. (RW)
2.Down: Layer of loosely structured feathers beneath contour feathers which help to trap air near the birds body for warmth. (RW)
3.Semiplume: Loose & fluffy feathers similar to down feather; provides body insulation and increases the buoyancy of water birds.(RW)
4.Filoplume: Small hair-like feathers with a few barbs at the tip of the shaft; they occur among the contour feathers. (RW)
5.Bristle: Modified, vaneless contour feathers with only a few barbs at the base on a small, stiff rachis. They can occur around the eyes, nostrils, and in flying insect-catching birds (such as tyrant flycatchers & goatsuckers) around the mouth, called rictal bristles.(RW)
6. Powder down: Feathers that grow continuously and are never molted. The barbs at their tips constantly disintegrate into a fine, talc-like, water-resistant powder. Often abundant in birds that lack preen glands. (RW)


Birds have extremely efficient respiration techniques, for a four chambered heart assures that the tissues are recieving enough
nutrients and oxygen. The lungs contain tubes connected to air sacs that reduce density of the body and dissipate air. A sheet of
muscle, called the diaphragm, helps to ventilate the lungs. (
Apart from the lung birds have air sacs which allows air to flow in one direction through the lungs. This helps because there is a high percentage of oxygen (MP).
The bird heart weighs 6 times more than a human heart in proportion to body size. It is, obviously, the most powerful organ in the body, providing enough oxygen to the lungs so flying is possible. (LPE)
Birds have a two-stage system for breathing. A bird must breathe twice for one inhalation to finally exit the body. In the first inhalation of a cycle, air passes into the posterior air sacs. In the first exhalation, the air reaches the lungs. On the second breath, air goes from the lungs into the anterior (frontal) air sacs, and is finally exhaled out of the bird. (Matt B - Source 9)
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Metabolic Waste Removal:

Mammals have an active metabolism because efficient respiratory and circulatory systems support a high metabolic rate. (Hair and a
layer of fat under the skin help to retain metabolic heat). Most birds have 3 separate champers in the digestive system: the crop, the stomach,
and the gizzard, before the food is passed to the intestine before being excreted.
Digestion is very rapid in birds to avoid weight gain. They excrete waste (and eggs) through the cloaca. (PS Source 14)

Digestion of food in birds is extremely fast not only to avoid weight gain, but more generally, to be able to sustain a high metabolic rate. In order to be able to consume enough energy to meet these metabolic requirements, birds have extremely rapid digestive systems that can process large amounts of energy-rich foods very quickly. Once the bird’s food has been digested, it is excreted by an efficient excretory system, which, except in the case of the ostrich, lacks a urinary bladder. The structure of the bird’s excretory system also helps to keep the birds weight at a minimum, thus conserving energy. (MR; Source 19)


A four chambered heart ensures top-of-the-line circulation. (MB) The bird circulatory system is made of a heart and vessels that transport nutrients, oxygen and carbon dioxide, waste products, hormones, and heat throughout the body. The right ventricle pumps blood only to the lungs while the left ventricle pumps blood to the entire body. Since the left ventricle is doing so much more work, it has a thicker wall and the wall is more muscular. (MM- Source 3)

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Self Protection:

Flight and utilization of wings to scare or hide from predators are the most common defense mechanisms. Some birds will, however, mimic the sounds of predators.Other Birds will attack the oncoming predators with harsh kicks. Birds also use mimicry and aposomatic coloring, warning colors.(rj)

Some types of birds have developed long, sharp beaks that they use as weapons to attack predators. (CM)

Often times, birds express a behavior called Territoriality, which is a hostile reaction formed when another bird approaches the territory of a bird. This behavior is characterized as a defense mechanism and often leads to physical conflict.Some birds defend only their food supply, but other birds try to defend their whole home range. Most sea-birds simply defend the immediate vicinity of their nest. The size of the territory they defend varies from species to species and even within the species, as well as the type of defense mechanism the birds demonstrate. (RK)

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Osmotic Balance:

Cell survival depends on balancing water uptake and loss. Therefore, the movement of water across cell membranes are crucial to this
organism's survival.

Birds lose water when excreting uric acid, and when breathing; and gain salt ions by eating or drinking. The only way for them to get water is metabolically from the breakdown of carbohydrates, and by drinking. There are two ways that birds use in order to prevent net water loss and ion gain. Birds have special salt glands near their eyes to pump Cl- ions and Na+ out of the body. This way, birds are able to drink seawater while pumping excess ions out of their bodies, and still being able to retain the water. (CP source 5)

Temperature Balance:

Birds are an endothermic species. This means that they use their own heat (from food) to maintain homeostasis in temperature
regulation. Feathers also help with this process.

Birds are able to live in a large range of habitats due to their ability to maintain a constant body temperature. They are able to do this by taking advantage of both morphology and behavior. In low temperatures, birds must either raise their metabolic rate or use environmental heat to prevent a decrease in internal temperature. Due to the fact that birds do not possess feathered feet, the arteries and veins lie in contact with one another. This functions as a counter current system that the birds utilize to retain heat. In cold temperatures birds will also stand on a single leg and tuck the other leg into the chest, in order to minimize heat lose. Conversely, in a higher temperature environment, birds must lose body heat through evaporation, a process in birds that is similar to panting. In order to regulate heat, certain birds will continually rotate to face the sun; this effectively minimizes the surface area exposed to sunlight. (ZJ- source 6)

To add, there are several behavioral mechanisms birds can do to regulate their temperature and decrease their heat loss. One of the most common mechanisms birds use is to migrate to a warmer climate when the temperature drops below what they can survive in. Some birds, when placed in cold environments, will hunch down and tuck in their head, feet, or legs under their wings and body feathers to reduce surface area heat loss. The fluff of a birds feathers also help to trap heat. By sitting, birds are able to reduce anywhere from 20-50% of their heat loss. Some species, such as penguins, huddle together to protect themselves from the cold while other birds make use of their burrows. (VN)

Review Questions:

1). How does a 4-chambered heart make a bird's blood circulation more efficient? (AP)
2.) How has evolution had an impact on some bird species (i.e. - flightless birds)? (AM)


Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. San Francisco: Pearson Education, Inc., 2002. Print.

"Bird." Wikipedia, the Free Encyclopedia. 24 Oct. 2011. Web. 24 Oct. 2011. <>. (SP)

Source 3: "Avian Circulatory System." 25 Oct. 2011 (MM)

Davies, Gareth Huw. "The Life of Birds | Songs." PBS: Public Broadcasting Service. The Life of Birds. Web. 29 Oct. 2011. <>. (AR)
source 4:

5: "Excretion in Water." Marietta College. Web. 30 Oct. 2011. <>. (CP)
6. (LPE) (LPE)
"Digestion." Fernbank Science Center. Web. 30 Oct. 2011. <>.

8. (MM)
9. (Matt B)
10. (AM)
11. (JS)

12."Metabolism and Thermoregulation." Index to Ornithology. Web. 06 Nov. 2011. <>. (VN)

13. (JP) (TB)


15. Hughes, Janice M. "Flight Part 3: Wing Structure." Aves Vitae. Web. 06 Nov. 2011. <>. (SP)

16. Territoriality, Stanford University
<> (RK)

"Avian Anatomy." Feisty Feathers. Web. 13 Nov. 2011.

18. (RW)

19. "Aves." Animal Sciences. Ed. Allan B. Cobb. New York: Macmillan Reference USA, 2009. Gale Science In Context. Web. 15 Nov. 2011.