free_unique_birds_screensaver-66791-3.jpg Chordata- Aves

By: Yarden Avnor


The Diagnostic Characteristics
The Chordata--Aves, which are apart of the Animalia kingdom, are commonly known as birds. This specific taxa has several prominent traits that distinguish and separate it from the other taxa. Birds all have the unique characteristic of feathers which are made out of Keratin( a substance that seemed to be quite adaptable during the avian evolution). It is the shape and arrangement of the feathers that makes it possible for the bird to fly since they make the wing an airfoil. The honeycombed bone structure of the birds which is strong and light, also allows for successful flight.
It is the special characteristic of feathers that also provides an extra layer of fat on the body. This as a result provides insulation that enables birds to retain their metabolically generated heat, and are therefore known to be endothermic.( more about temperature regulation below). Birds all have toothless mandibles which are known as bills. Chordata--Aves all produce external eggs after mating, and they all have four chambered hearts, which supports a high rate of metabolism birds need ( more about the circulatory system below).
(KS)Another characteristic in relation to flight is that birds can be either precocial or altricial. Those that are precocial are born with feathers and learn yo fly on their own.Those that are altricial are born without feathers and attain a high parental investment.
Acquiring and digesting food /metabolic waste removal
As birds acquire food such as seeds with their beaks, the food enters their mouth but is not chewed because only bills are present. Next, the food enters the esophagus, crop, stomach, gizzard, intestine, and finally out the anus in one direction which therefore allows for the presence of specialized regions and organs to carry out digestion in an orderly and sequential manner. The complete digestive tracts and alimentary canals are considered to be the digestive tubes within the birds that extends between the mouth and anus. The crop, stomach and gizzard are the three separate chambers where acquired food is pulverized and churned before entering the intestine, where chemical digestion and the absorption of nutrients occurs. It must be noted though that some Aves are missing some of these digestive organs. This type of Extracellular digestion, which breaks down food outside the cells, is the main pathway the body receives its nutrients from the food it obtains
Birds excrete nitrogenous waste as urate (also known as uric acid) instead of urea like mammals or ammonia like fish. It is often excreted in a paste-like substance. Urate is mostly insoluble in water and thus requires very little water to excrete and is very concentrated unlike urea or ammonia. This allows birds to conserve water. (KL)(1)
The crop, which is not present in all birds, holds excess food while the stomach is digesting. The stomach of a bird is made up of two parts - the proventriculus and the gizzard. The proventriculus secretes an acid that breaks down food which then moves onto the gizzard. The gizzard's main function is to digest tough food. The muscles in the gizzard grind up the tough food, later moving onto the intestine. (SM)(6)

Below is a picture of a bird catching its prey, which, in this case, is a dragonfly. [MS] 20

Sensing the environment

Birds have the capability of acknowledging the climate change, and as a result they migrate to perhaps warmer environment where food resources are more available. During flight, the bird can chose from different types of movement mechanisms. During piloting, the bird tends to move from one familiar landmark to the other, until it finally arrives at its destination. During orientation, the bird will travel in a straight line path until it reaches its destination. Throughout the Navigation mechanism, the bird is able to determine its present location relative to other locations and surroundings, along with using orientation.
For basic survival, vision is paramount for all birds. Their importance is seen in their anatomy- the eyes of a bird in comparison to the rest of its body is much higher than say, most mammals. The phrase "eyes of a hawk" is no exaggeration- the eyes of birds are far more superior at viewing distance than humans. This is due to the anatomy of the eyes for birds. In the retina of a human eye, the number of receptors for daylight vision (cones) tops around 200,000 per square millimeter. House sparrows have double that number, and hawks can have as much as five times as the amount of humans! As a result, hawks can see details of distances two or three times the maximum distance of vision a human being could see. In addition to more cones, bird retinas actually have an extra type of light receptor, one that humans don't have. Humans only have rods and cones, where as birds have those, as well as double cones. This is thought to mean birds can perceive more colors than people, maybe including ultraviolet light. Bird vision is so advanced, the eyes of birds lack even blood vessels, which in humans, causes shadows and light scattering. One final anatomical optical advantage birds have is a layer at the back of the eye referred to as the tapetum lucidum, which acts as a mirror to reflect light back through the retina so that light is more likely to be "seen" by the eye. As a result, any bird, such as an owl, that has this has remarkable night vision. This helps birds be nocturnal. (TM)(4)

Although it may seem that all Aves have the capability of transporting by either flight or by leg, it is only true for some.
Birds, known as ratites are unable to use their wings for flight since their breastbone lacks keel and doesn’t have the large breast muscle that attaches to it. Such flightless birds include the ostrich, kiwi, and emu.
The contractions of the large pectoral breast muscles which are attached to a keel on the sternum breastbone, allow for the flapping of wings in the air. The rate of wing flapping depends if the bird is capable of continuous flying or soaring. In continuous flight as seen by the hummingbirds, wings are contacted at a rate of 170km/hr. Occasional wing flapping which is performed by the eagle and hawk, known as soaring, consists of birds simply soaring the air currents. No matter on the type of flying performed, Aves usually have better eyes than vertebrates, since the visual areas of their brain is more developed. To decrease the weight for flight, birds have honeycombed structured bones, and the loss of some organs and teeth. Females for example, only have one organ for this purpose.
Flight consists of an upstroke and a downstroke. When the pectoralis minor muscles are contracted, the wings move up, and when the pectoralis major muscles contract, the wings move down. This movement is generally done rhythmically, which propels the bird forward and provides lift. (CSR, 12)
Aves have a characteristic that is unique to their kind and aids in the process of flying. Aves have a bone called furcula, also known as the 'wishbone.' The furcula is located in the chest and functions to prevent compression of the chest cavity during a downstroke. (AC)(15)

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(the picture to the left depicts the upstroke and downstroke or Ave locomotion)

The 4 chambered heart of a bird supports a high rate of metabolism, which is actually an adaptation to the endotherms. This type of heart is composed of the right and left atrium, and the right and left ventricle, where the left side of heart receives and pumps ONLY oxygen rich blood, and the right side only handles oxygen poor blood. Because of this unique separation there is no mixing of oxygen poor and oxygen rich blood, which is very efficient during flight. This double circulation of birds allows for the flow of blood to the brain, muscles, and other organs because the blood is pumped a second time after it loses pressure in the capillary beds of the lungs. As blood passes through capillaries and organs there is an exchange of nutrients, gases, and waste products between the blood and body cells. As a result, the Closed Circulatory system (aka) Cardiovascular system transports nutrients, oxygen, carbon dioxide, waste products, hormones and heat.

Two cycles of both inhalation and exhalation are required in order for the air to pass through the body and out the Ave. During Inhalation, two air sacs expand. The Posterior sac fills with fresh air entering from the outside, as the Anterior sacs fill with stale air from the lungs. Throughout Exhalation, both air sacs deflate, forcing air from posterior air sac into the lungs, and air from the Anterior Sac out of the body through the trachea. The trachea splits into two primary bronchi at the syrinx. The syrinx is unique to birds and is their voicebox. The primary bronchi enter the lungs and are then called the mesobronchi. Branching off from the mesobronchi are the ventronronchi, which branch off into the parabronchi. (LW) (5) The parabronchi are the sites of gas exchange within the lungs. Along with these two major sacs, there are smaller air sacs that cover the abdomen, neck, and wings. They DO NOT participate in gas exchange rather they keep air flowing in the lungs. There are some other air sacs that are attached to the lungs by tubes that release heat and reduce body density during flight. It is also important to note that the lung oxygen concentration is higher in birds than in mammals, which is why Aves can fly at such high altitudes.

Stop at around 1:30.
Video says the air enters posterior air sacs during inhalation 1. During exhalation 1 the air moves through lungs into the anterior air sacs. When it moves through lungs gas exchange occurs with the circulatory system. During exhalation 2, the air leaves the anterior air sacs and exits the bird. This is advantageous because it allows air to move in 1 direction ensuring that only fresh air with a high oxygen gas concentration is moving through the lungs. (SD) (17)

Self Protection
Flight provides an easy escape from earthbound predators. A female bird will protect her young when predators are near by pretending she is injured and then quickly flying away as soon as her young have escaped.
A huge focus of birds is to protect their young from predators. Even their tricks such as the broken wing are to defend their young. Their most impressive mechanism is how they hide their nest. A bird tries to keep the nest hidden from predators by blending it in with the surroundings. This is similar to mimicry because it is appearing like something else. The African Palm-Swift uses its saliva as glue to hold its nest in place. Some birds intentionally build their nests next to an organism that does not prey on them, but preys on their predators. This keeps them from being eaten or attacked. An example of this is in the Siberian Tundra, where the red-breasted goose and the falcon nest at the same time. The goose is kept safe from the fox by the falcon. (NG)
Fight is one of the most prominent tactics for aves to protect themselves. However, some aves are flightless. In these cases their size has enabled them to escape predation. For example, ostriches can weigh from 140- 290 pounds and height of 5- 9 feet and may not be able to fly, but they have survived many years. Why is that? Their wings may be small but they help to balance the birds while running, turning, and swerving. An ostrich acting in self-defense will extend first one wing and then the other in repeated, threatening motions resembling those of a boxer. In a charge, ostriches fully extend their wings, creating an awesome spectacle. Ostriches and emus are strong, fast runners. They can run up to 4O miles an hour, emus covering 9 feet, ostriches 25 feet in a stride. Their toes--emus and other flightless fowl have three and ostriches have two, compared to the four toes of flying birds--are made for speed, as fewer toes mean less ground contact, adding more speed. The long, powerful legs of these birds are their main weapon of defense, enabling them to flee or fight. Ratites fight with kicks, kicking from the knees (technically the ankles) forward and down, instead of backward. (7,8)(ZS)
Feathers are an important protection mechanism for birds. Birds are able to grow feathers easily and some birds can produce a new coat
every year. The coloration of a bird’s feathers provides camouflage as well as a way to communicate with mates and rivals. These feathers are both lightweight and strong which make flying easier for a bird. This allows them to get allow from predators. In addition, a bird’s feathers act as insulations that help the birds stay warm. Also, the feathers are used for physical protection for the bird’s body. (9)(MF)
Birds have claws or nails to enhance self-protection. They cover the toes and fingers, are usually curved in shape, and are often sharply pointed. Birds use these claws for grabbing potential food off the ground, and can also use them for self-protection. (CC) (13)

Osmotic and temperature balance
Aves maintain a body osmotic pressure that is different than their environment, which is known as homeostasis.
homeostasis is a dynamic state and an interplay between outside forces, such as weather, that tend to change the internal environment and internal control mechanisms that oppose such changes. As mentioned before, Aves are endothermic, meaning that they use metabolic heat to maintain a warm and constant body temperature. Their feathers and layers of fat provide insulation that enables birds to retain their metabolically generated heat.
Because aves are thermoregulators, they have developed a variety of ways to both increase and decrease their bodies' temperatures in relation to that of the environment. Unfeathered, or unsinsulated, areas are extremely important for this. Legs, for example, are unsinsulated. When attempting to lower its body temperature, aves will expose their legs. Many species will also excrete waste onto them, so as to increase heat loss by evaporation. When it is cold, however, legs have special adaptations to keep body heat from being lost. Veins and arteries in the legs are kept very close together, usually touching, and function as a countercurrent exchange system to keep from losing heat. (ZXU) (3)
Salt glands help maintain osmotic balance in birds that have a high intake of salt or low intake of water, such as, marine birds or desert birds. In such conditions a bird's kidneys cannot keep up with such high levels of salt ions so the salt glands, which are positioned above the eyes, excrete a highly salty fluid. Although the bird losses some water through the excretion from the salt glands there is an overall increase in free water. (RL)(18)

Males have paired testes but females often only have left ovaries.(ORS 11) After mating, the ave embryo is found in the amniotic egg of the female, where it's inflexible shell is made out of calcium carbonate and retains water. For this reason the eggs can be laid on land. There are 4 Major Extra embryonic Membranes located in the amniotic egg. The Amnion protects the embryo in a fluid filled cavity, prevents dehydration, and decreases mechanical shock. The Allantois is a disposable sac for metabolic waste that works with chorion as a respiratory organ. The Chorion thus exchanges gas between the embryo and surrounding air and allows for its survival. The Yolk sac transports nutrients from the yolk into embryo.Almost immediately after insemination, embryonic development begins. The egg is usual formed and laid in 24 hours.(ORS 11)
After the egg is laid, the female (usually), will sit over it, at least for a certain amount of time. This allows the eggs to develop at the right temperature (often the usual 98 degrees Fahrenheit). This period, known as incubation, usually lasts at least 10 days in smaller birds, but may last much longer in larger birds.(GR)(10)

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A chick embryo, magnified six times with a microscope. (NI) 14

Male ducks fight for dominance during mating season. (MC)

Review Questions
1. Describe the anatomical characteristics that enable species of aves to fly. Why can’t all species of birds fly? (MT)
2. How many chambers do Aves have in their hearts and why is this beneficial aerodynamically? (RG)
3. Describe in detail the osmotic balance of birds and why waste removal is crucial for flight? (SR)
4. How is it possible that birds are able to lay their eggs out in the environment? What mechanisms have developed to protect the egg? (NG)

Main source: Campbell, N.C., Reece, J.R. (2002). Biology. (Sixth Edition). San Francisco: Benjamin Cummings
1. (KL)
2. (NG)
3. (ZXU)
4. (TM)
5. (LW)
7. (ZS)
8. (ZS)
9. (MF)
10. (GR)
12. (CSR)
14 (NI)
15. (AC)
16. (AC)(picture)
17. (SD) (video)
18. (RL)
20. (MS)