BIRDSArielle R

Kingdom: Animalia
Phylum: Chordata
Class: Aves

edited by: Meru Nangia, Omer Zaidi, Hilary Stepansky, NK, Ethan Richman, Hanna Zhu, Rachel Kornetsky, Becca Levenson, Jesse Carmen, SS, Brittany Marcus-Blank, Sarah Vlach, Kevin Nayer, Josh Czik, Daisy Joo, Sam Blatchford, DP

Birds first evolved during the great reptilian radiation of the Mesozoic era. There are currently 8,600 known species of birds (28 orders). 60% of living bird species belongs in an order. What makes a bird, a bird? There are several defining characteristics of aves, all of which are designed to enhance flight. The first is bone structure. Aves’ bones are honeycombed, much like a honeybee’s nest, to provide a strong yet light structure. To see what a "honeycombed" bone looks like, look at the broken edge of the bone in the picture below (16 DJ). An example of light structure is the frigate bird, which has a wingspan of more than 2 meters but only weighs 4 ounces. Another characteristic of birds is the absence of some organs. Because birds need to be light, modern birds are toothless. Also, in females, there is only one ovary, compared to the usual two in most chordates. As result of having no teeth, birds are also specially adapted in how they grind their food. Food breakdown takes place in the gizzard, a digestive organ near the stomach. Another defining characteristic of birds is keratin. Birds’ beaks and legs are made of this scleroprotein, which is extremely adaptable in taking on a variety of shapes. This sustains diet changes and evolution. In addition, birds are endothermic. They use their own metabolic heat to maintain their body temperature.
The "honeycomb" structure of bird bones responsible for strength and light-weight (16 DJ).

Flying Adaptations
In addition to the specializations you just read about, birds have other mechanisms that help them fly. Birds’ flight senses are very acute. Their eyesight is considered the best of the vertebrates. Their motor areas, coordination, and visual areas of their brains are extremely well-developed. Birds follow the principle of aerodynamics. They flap their wings by contractions of large pectoral (breast) muscles attached to a keel on the sternum (breastbone). Some birds are adapted to soaring on air currents and only flap occasionally. The arrangement of feathers also helps birds fly. Feathers are very light and strong. They are made of keratin like their beaks and feet (1). It also seems that the shape of a bird's tail has something to do with the flying, since different species of birds have different shape, size, and use of their tails and tail feathers (9).
The bird's tail plays an important role in flying. It acts as the rudder of the bird. It helps the bird turn and keep its balance during flight. It also helps the bird slow down and stop during flight. When it wants to stop, the bird just tilts its tail down. (JAC) (14)
The bones of birds are adapted for flight. The long bones are thinner and lighter than human bones and other skeletal adaptations have occured. The skull is lighter, there are not as many bones in the wrist, and many bones are fused together. This helps the bird because it does not have to use as much energy to keep its body straight. (HS 4)

The Benefits of Flying
This bird, the Nestor notabilis, has an advantage over other organisms because
it can spot prey without scaring it away. The ability to fly also makes it difficult for
predators to hunt it, greatly increasing its chances for survival. (11) (SS)
There are many benefits of flying. The first is enhanced hunting and scavenging. Also, flying provides an escape mechanism from earth predators. Third, flying allows birds to partake in large-scale migration, where they can find different climates, food sources, and breeding areas.

Birds are Closely Related to Reptiles
You may have heard people say that birds are reptiles. They are partially right. Modern birds are not reptiles, but they share many of the same characteristics. In fact, birds began as feathered reptiles. The closest reptilian relatives of birds were theropods, a group of small, bipedal (two feet), carnivorous dinosaurs. An example of an ancient reptilian-bird is the Archaeopteryx. This bird flew the earth 150 million years ago during the Jurassic period. It is not an ancestor of modern birds, but it is rather a side branch deviating from the same ancient ancestor. It had clawed forelimbs, teeth, and a long tale containing vertebrate- all of which are reptilian features. The skeleton suggested it was a weak flyer and a tree-dweller. Other characteristics that birds share with reptiles are scaled legs and amniotic eggs (a membranous, fluid-filled sac which contains the embryo). Keratin is a similarity between reptiles and birds as well. Keratin is a protein the composes birds’ feet, beaks, and feathers, but also composes reptile scales. Also, gizzards were found in some ancient dinosaurs and in modern reptiles such as crocodiles. A difference is that birds have bigger brains than reptiles.

Flightless Birds (Ratites)
In flightless birds, the breastbone lacks a keel, a bone extension of the sternum (OZ), and the large breast muscles that attach it found in flying birds. The powerful breast muscles are used for other forms of movement. For example, in penguins, they’re used for swimming. Other ratites include emus, ostriches, and kiwis.
New research is disproving the idea that ratites share a common, flightless ancestor. Ratites are instead thought to be a product of parallel evolution (different species in different environments evolve similar adaptations), meaning that each species individually lost its flight after diverging from ancestors who could fly. This also means that ratites are more closely related to their "cousins" who fly than to other ratities (8). (RK)
Examples of species of flightless birds include Ostriches, Penguins, Kiwis, Emus, and Rheas. ( 3 KN)

Thermoregulation in Birds
Birds are endothermic, meaning they use their own metabolic heat to maintain a warm body temperature of 39-42 degrees Celsius. Aves use a circulatory adaptation called countercurrent heat exchanger, an arrangement of the blood vessels that traps heat in the body’s center, to minimize heat loss. Because birds’ extremities usually face cold conditions, they have specialized arteries that use countercurrent exchange. Arteries carrying warm blood down to the extremities contact veins carrying cold blood back to the body’s core. Heat is exchanged between the two, rather than being lost to the environment. Birds have several other mechanisms to aide in retaining heat. The first is nonshivering thermogenesis. Here, hormones are released which cause mitochondria to increase metabolic activity and produce heat instead of ATP. Another mechanism is brown fat. This tissue is located around the neck and specializes in rapid heat production. A third device birds use for thermoregulation is their insulation. Birds not only have fat and hair, but they also have feathers which trap warm air in their layers. Collectively, thermoregulation helps birds increase their metabolic heat activity 5-10 times more than it would regularly.

Osmotic Balance
Like all animals, birds’ bodily fluids depend on osmoregulation, the management of the body’s water content and solute composition, to remove and stop accumulation of metabolic wastes. Because birds have a closed circulatory system, the cells are immersed in an interstitial fluid. The composition of this fluid is controlled by managing the composition of blood. Because marine birds can spend up to months- or even years at sea, they have specialized devices for helping them remove the excess salt. Layers of specialized cells that regulate solute movement, called transport epithelium, allow birds to maintain a diet at sea. In marine aves’ nasal glands, they secrete a fluid saltier than ocean water. Thus, when they drink saltwater from the ocean, there is a net gain of water. The birds’ salt glands empty into the nostrils and release the salt by dripping or via an exhaled mist. The transport epithelium lines the salt-excreting tubules, which drains into a central duct. These epithelium cells actively transport the salt from the blood into the tubules. A countercurrent exchange takes place between the salt in the tubules and the blood flow.

Waste Removal
Kidneys are the primary mechanism in birds that accounts for waste removal and aides in osmoregulation. These compact organs contain thousands of organized, small tubules. The capillary network, which work with the tubule network, carries urine out of the bird through the tubules, kidney, and eventually outside the bird. Birds have specialized kidneys containing nephrons, filtering units of the kidney, which help preserve water. Also, the loops of Henle, parts of the nephron responsible for transporting water and concentrating urine, are shorter than in mammals. Their urine is therefore not as concentrated. Instead, birds use uric acid as the nitrogen excretion molecule, which preserves water.

Circulation System
Birds have a four-chambered heart. In a circulation system with a four chambered heart, the lungs have tiny tubes connecting to/from elastic air sacs that help disperse heat and lower the density of blood (ER). This supports increased rates of respiration needed for flying.

The figure above demonstrates the circulatory system of birds, specifically the four chambered heart. (5. Sharma)
In a four chambered heart, deoxygenated blood never meets up with oxygenated blood. Blood passes from the right atrium to the right ventricle. Blood from the right ventricle is pumped throigh the vena cava to the lungs of the bird. From the lungs of the bird, the blood, now oxygenated, passes into the left atrium. From the left atrium, blood passes into the left ventricle. The left ventricle pumps blood through the aorta to the body of the bird. (RJS)

Respiratory System
Birds have very adequate respiration systems for flying. Collectively, there are 8-9 sacs that penetrate the abdomen, neck and wings and reduce the density of the bird. In inhalation, the anterior and posterior sacs expand. The posterior sac fills with fresh air from the outside, while the anterior fills with stale air from the lungs. In exhalation, both sacs deflate. The air is forced out from the anterior sac via the trachea and out of the posterior sacs into the lungs. In the lungs, the air passes through tiny tubes called parabronchi, where gas exchange occurs in the walls. This system is extremely effective because it exchanges all the air in the lungs with every breath. Subsequently, maximum lung oxygen concentrations are higher in birds than mammals, allowing them to perform better at higher altitudes.
Campbell Biology
Campbell Biology

Acquiring Food(ER)
Birds employ a wide variety of methods to obtain their food. Some bird species are carnivores. Some carnivorous birds like eagles and hawks are at the top of the food chain for their particular habitats. Other species of birds are herbivores. Still other species of birds are detrivores. A vulture for example obtains most of its food through scavanging rather than hunting (6, NK).

Alimentary Canals (Digestive System)
Birds have a highly developed digestive system, which is also referred to as the bird’s alimentary canals. Birds have a high digestive rate because they are small animals that have high-energy needs. Many birds have the ability to completely metabolize their food in 45 minutes.
Birds take in food through their beaks. The beaks can be used for grinding and crushing the food. However, because birds do not have teeth, most of their ‘chewing’ is done later in the digestive process. Birds also do not have the ability to swallow. They have to tilt their head back to transfer the food to the Oesophagus, which is the first part of the bird’s throat. From here, the food travels down the digestive tract through peristaltic contractions, in which the muscles that line the digestive tract contract and expand to force the food onward. Within the oesophagus, most birds have highly developed salivary glands, glands that excrete saliva. In some birds, the saliva is used to ‘glue’ the food together in balls to aid in digestion. In other birds, the saliva is used as an adhesive in catching insects or in building nests.
After the food leaves the oesophagus, it enters the crop. The crop serves as a large storage container where food that was eaten in a hurry can be stored for later digestion.
After the crop, the food enters the stomach, which is where digestion begins to occur. The stomach is made up of two organs, the proventriculus and the gizzard. The proventriculus is similar to a tube-like stomach. It produces digestive juices that break down the food chemically, including pepsin and hydrochloric acid. From the proventriculus, the food passes into the gizzard. The gizzard is where all the food is physically broken down and ‘chewed’.
After the gizzard, the food, now called a bolus, enters the small intestine. The small intestine of a bird is broken up into four parts, respectively, the duodenum, the pylorus, the jejunum, and the ileum. The pancreas is more developed in birds than in mammals and secretes digestive enzymes into the small intestine. The liver of the bird secretes bile via to distinct ducts. Many birds do not have a gall bladder and the bile that is secreted in birds is acidic rather than basic.
As the bolus travels down the small intestine, nutrients are absorbed into the bird’s body via the circulatory system.
At the end of the small intestine are several colic caeca. The caeca contain bacteria that are essential in the breakdown of cellulose, which is then absorbed by the bird.
The digestive tract ends in the cloaca, where waste is then removed through the anus. [2 Nangia]

external image digestion.jpg
In this diagram of the digestive system of a bird, the path that the food follows can be clearly traced. You can see that from the esophagus the food enters the crop, which is like a pouch so that it can store food. From the crop, the food passes into the proventriculus and then to the gizzard to be broken down. From the gizzard, the food passes through the small intestine so that nutrients can be absorbed into the blood stream. Afterwards, the goes through the large intestine and leaves the bird through the cloaca [HZ].

Behavior/ Mating in Birds
Birds are known for their diverse mating rituals (ER), which usually involve displaying their feathers and repertoires (songs). It has been proven that there is a relationship between fitness and repertoires. This is because females prefer males with longer repertoires, and the longer the repertoire, the older the male. Longer repertoires also stimulate females more. If a female chooses a more experienced male, they will mate earlier in the season, giving their offspring a better chance of survival. The video below illustrates the mating ritual of the Lyre bird, which imitates the songs of surrounding birds. (ER) In many species the skill of a males' nest building ability is a sign of his suitability as a mate. Some species, such as the European house wren, will build up to 12 nests to attract a female mate.(13) (BMB)

Birds build nests in which to incubate their eggs. There are four basic parts to an egg: the yolk, the germinal disc, the albumen (egg white), and the shell. The yolk has a lot of fat and proteins, making it a rich source of nutrition for the bird embryo (an embryo is a developing organism). The yolk is surrounded by membranes with blood vessels that transport nutrients from the mother's body to the young. The germinal disc is on the surface of the yolk. It has the female parent's nucleus and the embryo's DNA, if the egg is a fertilized one (if it is not, it only has the mother's DNA). The albumen is made up of proteins that provide the developing chick with water and protein. The shell, in addition to protecting the embryo, also has pores that allow water to excess water to leave the shell (7) [HZ]. Because the size of an egg is large, it is difficult for a female to retain more than one egg at a time; it would make flying harder and require much more energy from the mother. It is commonly misbelieved that birds’ eggs are fragile. In reality, birds' eggs are engineered to withstand a lot pressure so they can survive under the weight of their mother. (13) (BMB)

Imprinting is learning limited to a specific time period in an animal’s life and is generally irreversible. An example is in Graylag geese. They have no innate sense of mother, so they therefore respond to and identify with the first object they encounter. Scientist Konrad Lorenz experimented with these geese and was identified as their mother during their sensitive period. They immediately recognized themselves as humans, Lorenz as their mother, and behaved much like him. Also, most birds must learn their repertoires during the sensitive period, a limited phase in an animal’s development where learning specific behaviors take place. If a bird never learns its repertoire during this period, it fails to learn the song of its species. Song sparrows for example need to hear and observe an adult's song during a specific sensitive period (SES 15). This allows the bird to create a mental template of the adult song (SES 15). After this, they use the template to start singing the song themselves (SES 15). They must be able to hear themselves practicing the song, or else the song will not come out correctly in the end (SES 15).

Birds have many mechanisms of defense against danger. This first of these is flying. If in danger, a bird can simply fly away, considering the threat is earth-stricken. Birds are also very territorial. Some birds will have their niches be within a few meters of their breeding ground, while others will only be territorial during breeding season. Birds use pair-systems or large flocks to ward off danger. Males will also sing to ward off other birds from their territory. The goal is the sender (the bird) to produce a noticeable behavioral difference in the receiver (threat). Birds also have an elaborate immune system, protecting them from any foreign particles. Their immune system consists of lymphatic vessels and lymphoid tissue. Lymphatic tissues and lymphatic vessels are separate from the circulatory system; however, these tissues and vessels do return fluid and protein to the blood. The lymphatic tissues and lymphatic vessels help to eliminate anything that may be harmful to the bird's body. (Jesse Carmen) (10)

When you look up, especially around fall, it’s hard not to see birds in the sky. This is because they partake in seasonal migration, regular movement over relatively long distances. Birds use three mechanisms to help in migration. Piloting is used for short distance migration, where birds recognize familiar landmarks to find their way. Orientation is when birds detect compass directions and travel in a straight line until they reach a certain point or their destination. The most complex of the migration skills is navigation. This is the mechanism most commonly used by birds, where they determine their present location in relation to others and use orientation to get there. The bird that travels the farthest is the Arctic Tern, which completes a roundtrip between the north and south pole each year. No two species of birds every follow the same migration route from beginning to end. The distance traveled, speed of the bird, and direction of flying all play a great role in which route the species are bird takes. For North American birds they are four major "flyways", geographic regions in which migrations routes can be found, the Atlantic, the Mississippi, the Central, and the Pacific.(12, GR)

Review Questions
  • What is the evidence that birds and reptiles are closely related? (SV)
  • List 4 ways birds protect themselves from danger (VM)
  • Define orientation and how birds use it to their advantage(SJB)
  • What parts of a bird's brain are highly developed? Why do you think this is? (DP)

Campbell, Neil A., and Jane B. Reece.
[2] Ramel, Gordon. "Digestion." The Alimentary Canal in Birds
(4) "Birds." Phylum Chordata
(8) "Long-held Assumptions Of Flightless Bird Evolution Challenged By New Research." 7 September 2008. 7 December 2008 <­ /releases/2008/09/080903172152.htm>.
(9) "bird." . 2008. Encyclopædia Britannica Online. 07 Dec. 2008 <>.
(10) “Anatomy of Animals.” 7 December 2008. <>
(11) "Aves."
. 2007. 7 Dec. 2008 <>.
(12) "North American Migration Flyways.". 2001. 7 Dec. 2008 <>.
(13) Davies, Gareth H. "Parenthood."
(14) "Birds." National Business Aviation Association. 14 Dec. 2008 <>.
(15) "Types of learning >> Perpetual learning >> Song Learning." Animal Learning. Encyclopædia Britannica Online.16 Dec. 2008. <>
(16)Tumlison, Renn. "Skeletal Adaptations of Birds for Flight."
Nature Trivia__. Henderson State University. 17 Dec. 2008