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Kingdom: Animalia, Phylum: Chordata, Class: Aves, Order:
Falconiformes, Family: Accipitridae, Genus: Buteo, Species: Buteo jamaicensis

Habitat: North America

This species of bird is the most common type of hawk across all of North America. The red tailed hawk uses its eyesight to spot prey while circling open fields, it then swoops down using its specialized wings to catch its prey. The scream of the red tailed hawk is unlike any other species in the animal kingdom. These birds are fairly large while still being nimble allowing them to catch prey like small rodents and snakes. These unique qualities make this species different from the rest of the animals in the class aves.

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Kingdom: Animalia, Phylum: Chordata, Class: Aves, Order: Sphenisciformes, Family: Spheniscidae, Genus: Aptenodytes, Species: Aptenodytes forsteri

Habitat: Antarctica

The emperor penguin is the largest species of penguin in the world, averaging around 51 pounds. This species is extremely unique in the class aves because it is a great swimmer instead of being able to fly. These birds also live in some of the most brutal climates in the world; as a result of this, they have some unique features that are not found in any other organism. For example, they huddle together in order to keep their warmth; along with this they have special fat that allows them to keep their core body temperature up.

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Kingdom: Animalia, Phylum: Chordata, Class: Aves, Order: Piciformes, Family: Woodpecker, Genus: Dryocopus, Species: Dryocopus pileatus

Habitat: Northern parts of America and many parts of Canada

The pileated woodpecker is a  very large species of woodpecker. The distinct red coloring on the head of this bird sets it apart from anything else in its heavily wooded  habitat. This family of bird has a special beak that allows it to drill holes into wood in order to find food like small insects or to make its home. The pileated woodpecker's red coloring along with its strong beak and way of acquiring its food make this species of bird unique.

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The taxonomy of organisms can show when each animal branches off into its own species. This will show how closely related the two organisms are. The red tailed hawk is closely related to kites, harriers and eagles because they are in the Accipitridae family, but all of these organisms have evolved into completely different birds with different characteristics and genetic material making them separate species. Another example of this is the finches located on the Galapagos Islands; there are 13 different species but they are all in the same genus. This shows that they branched off in order to be the best fit for their habitat while still being very closely related in both physical and genetic characteristics.

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Internal
- Lightweight bones allow for flight of most species
- Fused clavicle bones
- Warm blooded
- Highly developed chest muscles
- No urinary bladder
- High metabolic rate
External
- Feathers
- Beak or bill
- Toothless
Behavioral
- Many species in the class aves migrate during certain mating seasons
- Most species in the class aves have the ability to fly
- Lay hard shelled eggs (varying in size)
- Reproduce sexually

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The phylogenetic tree shows how all of the species within the class aves are related. Due to the diversity of this class there are many different sub categories as shown in the tree below. An example of this is the land dwelling ostriches, kiwis, and emus; these species of bird all live in Australia, and as a result they have acquired characteristics that are unlike any other organism in their class. They are all next to each other on the phylogenetic tree because they have these characteristics, however due to certain unique characteristics they are very far away from any other bird showing that they are not closely related to any other species in the class aves.

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Over the course of history birds have gone through both slow and fast forms of speciation. When birds evolved into an avian species there was a punctuated form of evolution, they evolved from land dwelling reptiles into organisms with wings in a very short period of time. More recent splits are most frequently an example of gradual speciation. For example, the many species of finches located on the Galapagos Islands were created because of a lack of resources causing competition between species. As a result the 13 species of birds on the Galapagos Islands formed over a few million years. Due to birds territorial behavior there is rarely enough competition to cause speciation to occur in a short amount of time.

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There are many different species that are able to fly; not all of these organisms are in the class aves. A wing like structure has developed in many organisms like the red tailed hawk, Mexican free- tailed bat, and honey bees. None of these species evolved from a common ancestor, making them an example of convergent evolution. All of these organisms needed to adapt to the environment that they were in and had to be the most fit in order to survive, causing the change in their limb structure to that of a wing. Another example of this are flippers that penguins have acquired; these structures resemble a fin on a fish or other sea creature. The emperor penguin has evolved this way because of the habitat that it lives in; Antarctica has a sparse amount of food on land so penguins have evolved into excellent swimmers in order to catch fish and other prey in order to survive.

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Microevolution has been crucial to the survival of many members of the class aves. For example sparrows in North America have changed in both size and color since they were introduced to the America's in the 1850's. Sparrows in the northern regions have a higher body weight in order to keep their body heat up in the cold temperature, they also have a darker color in order to blend into their forrest surroundings. Sparrows in the southern regions show a considerably lighter tone along with a lower body weight in order to be best fit for their environment; these micro evolutions are the result of natural selection.

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Geographic Isolation is not seen a lot in the class aves because many species in the class have the ability to fly. There are however some rare examples of geography playing a role in the process of evolution within the class aves. One example of this is the emperor penguin; due to their remote habitat on Antarctica this species of penguin was not able to exchange genetic material with other birds on different pieces of land and as a result had to adapt to this area. There are many reproductive barriers in the class aves that result in speciation. One of these is the mating call used by different species, for example the mating ritual of the pileated woodpecker is completely different than that of the red tailed hawk. Due to this difference these species would not be attracted to one another which would result in these two organisms not being able to share genetic information with each other.

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Stabilizing selection can be seen in many bird's eggs. For example, robins typically lay four eggs because less eggs might result in no viable young while too many eggs may result in the malnourishment of all of the eggs. An example of directional selection in the class aves is in the size of emperor penguins. Emperor penguins must be able to keep their body temperature up in the sub freezing temperatures; as a result of that the phenotype for small lightweight emperor penguins has died off because the larger penguins are more fit for their environment.

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Descent with modification is the passing down of hereditary traits from parent to offspring. This process is essential to the survival of many species within the class aves; one example of this is the finches located on the Galapagos Islands. The birds on these islands had competition for food; as a result the birds with the best fit beak for their habitat were able to survive and therefore reproduce more effectively. The traits that were best suited for certain places were passed down to the offspring and eventually caused the birds with other beaks to die off. As a result, over the span of millions of years certain phenotypes are far more likely to be passed down from parent to offspring.

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Organisms in the class aves all share a trait that is unique to birds, reptiles, and some fish. This trait is that their sex chromosomes are opposite of mammals. For example
female mammals are homogametic with XX chromosomes and male mammals are heterogametic having XY, but female birds are heterogametic ZW and male birds are homogametic ZZ. This trait for birds occurred when the split from mammals to reptiles was made millions of years ago. As a result, reptiles passed down the ZW system of sex identification to birds; this shows that both reptiles and birds have a common ancestor dating back millions of years. As seen in the image below the W chromosome is much smaller than the Z chromosome; this is the result of evolution over millions of years. The female birds tend to be more attracted to males with bright colors, due to this attraction, the Z chromosome carries more information causing males to be more likely to pass down sex linked traits. This can be seen in the pileated woodpecker as it has a bright red spot on the back of its head which helps the males attract females during mating.

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The beaks of many species in the class aves have a high level of the protein keratin present. This protein creates an extremely strong beak that allows animals such as the pileated woodpecker to use its beak as a digging tool. This protein can be found in the beaks or bills of all species of birds, even those that do not use their beaks in such a rough way. While the beaks of birds all show resemblance in their proteins, there are some differences based on the needs for each species of animal. For example the finches on the Galapagos Islands showed an increase in the protein Bmp4; this is a result of competition for resources and survival causing their genetic make up to shift over time. In recent years scientists have also found some protein similarities between the claws of birds, feet of reptiles, and the flippers of current turtles. This is significant because it shows the common ancestry between the reptile family and birds. Over the course of millions of years the shape of bird's feet have changed as a result of decent with modification. Most species in the class aves no longer have a need for feet with webbed toes like they did when they were just coming onto land.

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All members in the class aves that are capable of flight have a short genome, consisting of only around 1 billion base pairs (humans have 2.8 billion base pairs).  They received this characteristic from a prehistoric animal called a saurischian; this organism eventually evolved into the Archaeopteryx which was the gateway into modern birds. An example of this is the modified genes in the genome of the penguin for feathers, this is the result of evolution; allowing them to survive in sub freezing temperatures. The genome of the red tailed hawk shows an emphasis on their receptors in their genome for vision allowing them to spot their prey and survive off of the resources that are available in their habitat. As a result of the first species within the class aves, all organisms now have this characteristic; evolution modifies their genome over time as those organisms who are best fit will survive and pass down their genotype.
This image shows the difference in size of a bird's genome to a reptile or mammal.

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All organisms in the class aves have certain physical features that either allow for their survival, or are left over from their prehistoric ancestors and do not harm or benefit them. One example of a feature that is beneficial to them is wings; while not all animals in this class have the ability to fly they all have a structure that resembles a wing. Those organisms that do not use their wings to fly use them for other tasks; for example the emperor penguin uses its flippers that evolved from wings to swim as it does not have the ability to fly due to its weight. In their habitat there is an abundance of water as they mainly live on Antarctica causing them to need the ability to swim more than flying. Wings are an example of a homologous structure because they are seen in different organisms but they do not do the same task in all organisms.
This is a model showing the similarity in the two structures while having different functions based on what the animal needs.

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This fossil was found in southern Germany, it is believed to be around 150 million years old (Cretaceous period). This organism is a strong candidate to be the transitional species between non- avian dinosaurs and birds. In recent years scientists have now found many reptile characteristics in this fossil making it hard to classify as it either avian or reptile.

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This fossil is another organism that is thought to be part of a transitional species from dinosaurs to modern avian birds. This paravian dinosaur roamed the earth around 160 million years ago (Jurassic period) and was dug up in south west China.

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This fossil is the first organism that looks very similar to avian creatures today. This species was dug up in western China and is thought to have lived around 150 million years ago.

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This fossil was found in the Germany, far away from where it was thought to live in Asia. Xiaotingia roamed Earth around 155 million years ago (Jurassic period).

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Zhouirnis is a fossil that is around 130 million years old (Cretaceous period). This fossil was found in China and is part of a group of birds that look exactly like modern birds called Enantiornithes.