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-Feathers
-Beak/Bill
-Wings
-Lightweight skeletons

The first characteristic all birds share is having feathers. Feathers give the birds insulation which keeps them warm in all types of habitats. Their feathers also give them their unique coloration which helps them when choosing a mate or for other activities. The beak/ bill of a bird is another major characteristic. The beak is a modified version of a mouth that doesn't have teeth which makes it more lightweight. Birds need to be lightweight to fly so having a beak instead of a mouth helps with that. They also all have specific beaks that help them obtain the type of food they eat. All birds also have wings which help some birds fly. Flightless birds still have wings but they do not have a major use besides possibly for protection. Lastly, all birds have lightweight skeletons which help some birds have an easier time flying because it is easier to fly with not a lot of weight to hold.

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Habitat:
-Nearctic Areas
U.S/Canada/Mexico/Central America
Open areas (easy to see prey)
Can live almost anywhere
Characteristics:
-48-65 cm in size
-Wing span= about 4 ft.
-Females are larger than males
-Plumage ranges from light auburn to deep brown
-Excellent vision
-See in color
-Strong, Sharp Talons
-Hooked beaks
Taxonomy:
Kingdom= Anamalia
Phylum= Chordata
Class= Aves
Order= Accipitriformes
Family= Accipitridae
Genus= Buteo
Species= jamaicensis

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Habitat:
-North America
-South America
-Warm climates
-Lakes or Lagoons
Characteristics:
-Pink (range from light to dark)
-Feet= yellow to orange to pink-red
-Ankle is half way up the leg
-Knee is not visible
-Long and sinuous neck
-Very social
-Use displays before, during, and after breeding
-Feeding, preening, resting, bathing
Taxonomy:
Kingdom= Anamalia
Phylum= Chordata
Class= Aves
Order= Ciconiiformes
Family= Phoenicopteridae
Genus= Phoenicopterus
Species= ruber ruber

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Habitat:
-Forests, mountain valleys, swamps, gardens
-Cuba
Characteristics:
-Bright breeding plumage
-Fiery reddish pink feathers
-Bluish green feathers on upper part
-Bright irredesceng coloration
-Small

-Flying
-Solitary(except when breeding)
-Don't migrate generally
Taxonomy:
Kingdom= Anamalia
Phylum= Chordata
Class= Aves
Order= Caprimulgiformes
Family= Trochilidae
Genus= Mellisuga

Species= helenae

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Taxonomy is the classification of organisms into a system that indicates natural relationships (evolutionary relationships); the theory and practice of describing, naming, and classifying organisms. Taxonomy alone can show the relatedness between organisms because the organisms are grouped based on their similar characteristics from a broad/ general group (kingdom) all the way down to the specific group (species). The more taxonomic levels that organisms share the more similar they are. For example all three of the organisms in class aves, red tailed hawk, American flamingo, and the bee hummingbird all have the same Kingdom, Phylum and Class which shows the level of relation they all share. However, all three organisms break off into a different order, family, genus and species which proves that all three organisms are unique and different from one another. Taxonomy breaks the organisms into groups based on similarities so that people can easily identify how related they are.

Phylogenetic tree of the Class Aves breaking into different species as their taxonomies change.

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-127 million years old
-Creataceous time period
This fossil was of a class aves animal that lived during the dinosaur period. This fossil showed that the organism had teeth and claws on it's wings which modern birds do not have. This is similar to other characteristics but shows the evolution of birds away from those characteristics into new ones.

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-Cretaceous period
-150.8 mya- 125.45 mya
-Transitional form between birds and reptiles
The Archaeopteryx had a full set of teeth, a rather flat sternum, a long, bony tail, gastralia, and three claws on the wing. All of these are characteristics that birds today do not have which proves that birds have evolved. This fossil was know as the earliest form of a bird however, it has many different characteristics which shows birds have evolved to have new characteristics to help them better survive.

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-160 mya
-Jurassic period
This fossil is similar to the Archaeopteryx fossil in that it is know as a transitional species between dinosaurs and modern day birds. It has many closely related features to modern birds which shows how the birds have evolved from a dinosaur to their modern form.

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-160 million years ago
-Jurassic period
This organism was one of the earliest avialans found. This fossil is the closest related fossil to class aves today.  The organism had clawed wings and a long bony tail but most of it's other features were similar to modern bird features today. This shows evolution because birds were similar to this organism but evolved some traits to better survive.

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-Late Jurassic to Cretaceous period
-161 mya-100 mya
This fossil also had differing traits from modern birds but also similarities which again shows evolution. The forearm of Confuciusornis was shorter than both its hand and upper arm bone. It also retained the feature of having three free fingers on the hand, like Archaeopteryx. This fossil is slightly different from the fossil before but still shows modern day bird characteristics which shows evolution.

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Birds have a unique characteristic when it comes to their chromosomes. Normally, female mammals are XX and male mammals are XY. Females are usually homogeneous and males are heterogeneous when it comes to sex chromosomes. However, birds are the opposite females are heterogeneous, ZW, and males are homogeneous, ZZ. This unique characteristic is similar to that of reptiles and fish thus proving they have a common ancestor that they have evolved from. The unusual trait of chromosomes shared by these animals shows they are all related and must evolve from a common ancestor. However, since they are all very different organisms it shows that they have evolved from their common ancestor and broken off into different organisms who still share a few common traits, like the chromosome similarity.

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Birds all have similar proteins in their beaks. Birds beaks are made of a hard shell of keratin which contains proteins like glycine, tyrosine, and serine. The birds beaks are made this way so they can be strong and help the bird with their daily needs. The fact that birds share this characteristic shows evolution because they've all evolved their beaks with the same protein in slightly different ways to perform different tasks. For example, the red tailed hawk uses it's strong beak to pierce into its prey and kill them for food. The flamingo has a different use for the strong beak and that is to be able to move it in different ways when preening in order to attract a mate. Lastly, the bee hummingbird has this protein in it's bill to make it strong enough to go into flowers and get food out. All three organisms have protein keratin in their beaks but they all need the protein for different reasons which shows how they have evolved. Overtime it has also been discovered that there are protein similarities between the claws of birds, feet of reptiles, and the flippers of current turtles. This shows that they all most likely shared an ancestor and evolved as time went on into different organisms in different classes.

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Organisms in Class Aves have many similarities in anatomy like wings and light skeleton structures. However, these structures are homologous and have evolved different uses overtime based on the needs of the organism. For example, wings help many class aves organisms fly, like the bee hummingbird and the red tailed hawk. However, flamingos have wings but they cannot fly so the wing has a different purpose. The wings on a flamingo help keep them warm and allow them to swim. This shows evolution because they have similar structures to each other but they have evolved to use the structures in different ways in order to best suit the individual organism.  Birds also have different uses for their light skeleton structure. Some birds have the light structure to help them fly easier. Whereas other birds have this light structure for easy land movement or swimming. This again shows evolution because the structures have evolved different uses.

This photo is the anatomy of the wing structure that birds have. The wing is a homologous structure which shows evolution.

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Originally when birds first formed from reptiles and dinosaurs there was punctuated equilibrium. The birds evolved from these other organisms in a relatively quick amount of time. However, now the class aves organisms go through speciation at a more gradual rate. Class aves is a part of gradualism because it takes a long period of time for them to evolve new characteristics that make new species. Birds are born with a new advantageous trait that then continues to be passed on throughout a long period of time showing their gradal evolution/speciation. An example of gradual speciation for birds was the finches on Galapagos Island. The finches evolved overtime, not immediately. They gradually changed and evolved new species with new characteristics.

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Birds are part of convergent evolution. Convergent evolution is the process whereby organisms not closely related independently evolve similar traits as a result of having to adapt to similar environments. Birds, insects, pterosaurs, and bats all have the wings that are usually used for flight. All four organisms evolved independently from each other but evolved the same trait of wings because they all lived in similar environments where wings were helpful.
Birds are also part of divergent evolution. One of the best examples of divergent evolution is Darwin's finches. The species of finches started splitting into different groups with different characteristics, usually based on the food they ate, which lead to new species being formed. Even though they were in the same environment, they were isolated from gene flow so new species formed.

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Class aves has examples of geographic and reproductive isolation. An example of geographic isolation could be birds that live on islands and therefore are isolated from mainland birds so they evolve different traits. This could be like the finches on Galapagos Island
and how they were isolated so therefore evolved different traits to adapt to their environment. Also, this could be similar to birds like penguins who are isolated in extremely cold areas so they evolve different traits to stay alive in the cold temperatures. Reproductive isolation occurs with mating calls, Many species of birds have different mating calls so it creates a barrier between them and other species with different calls. Species with different mating calls aren't attracted to each other and wont mate which means they can't share genetic information and create a new type of bird.

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Birds migrate seasonally and for mating. This means that there is a pretty regular immigration and emigration of birds from a variety of places. With all of this movement to and from different environments it contributes to evolution. When birds move to a new place they have to adapt to that environment whether it be to the climate, the food sources, etc. When the birds do adapt it creates evolution because new traits are being formed and it can even lead to a new species being formed if the characteristics become different enough. Also, when birds emigrate out of their environment it creates changes in that environment which affects the other organisms that live there. This could be other species of birds which then they have to adapt and evolve to fit the altered environment. Migration in any group of organisms has great affects and can lead to evolution.

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Descent with modification is passing traits from parent to offspring. This contributes to evolution because the organisms with the best traits to survive reproduce and pass on those good traits to their offspring until those traits are the new normal. This leads to birds evolving new traits due to being passed them through heredity. This was very prominent for Darwin's finches on Galapagos Island because they evolved better traits for survival like eating food then they passed these on to their offspring and eventually the whole species evolved into different traits. The main change was in their beaks because they all evolved different beak shapes and sizes in order to be able to eat different foods so that they could survive. Eventually only the birds best fit survived and reproduced more birds with the newly improved trait.

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Natural Selection in birds can occur in all three types. However, directional natural selection is most common and has been observed with the finches on Galapagos Island. Birds on the island had different beak sizes for eating different sized seeds. It was shown that when the island would experience a drought the weaker seeds would die out leaving only large seeds as food. This created directional natural selection because the birds with the very large beaks that ate the large seeds (an extreme form of the trait) are the only ones who could survive because only large seeds were available. This is one instance of many where directional natural selection has occurred within the bird population.

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http://www.ucmp.berkeley.edu/diapsids/birds/archaeopteryx.html
http://www.ucmp.berkeley.edu/diapsids/birds/birdmm.html

http://animals.mom.me/5-major-characteristics-birds-5792.html

http://www.nhptv.org/natureworks/redtailhawk.htm

https://en.wikipedia.org/wiki/Bee_hummingbird

https://en.wikipedia.org/wiki/American_flamingo

http://www.iucnredlist.org/details/22695933/0

http://www.iucnredlist.org/details/22729706/0

http://www.iucnredlist.org/details/22688214/0

https://en.wikipedia.org/wiki/Anchiornis

https://en.wikipedia.org/wiki/Enantiornithes

https://www.britannica.com/animal/Confuciusornis

https://www.ncbi.nlm.nih.gov/pubmed/9354761

https://en.wikipedia.org/wiki/Bird_anatomy

https://www.flamingos-world.com/flamingo-anatomy/

https://www.ncbi.nlm.nih.gov/pubmed/1023860

https://biologydictionary.net/divergent-evolution/

https://evolution.berkeley.edu/evolibrary/article/evo_44