Avian Classification and Number of Species
Diagram 1. The scientific classification of birds.
Birds are part of the Animalia Kingdom and the Chordata phylum, which refers to vertebrates and a few closely related invertebrate species. The class Aves has two superorders with twenty-nine orders. Across these orders, there are ten thousand species and twenty-two thousand subspecies. 
There are many highly adapted avian species across the world. Some, like the Arctic Skua (Stercorarius parasiticus), live in the harsh climate of the polar regions, spanning from Alaska to Siberia and as far south as Scotland; some live off the southern coasts of Australia, South America, and southern Africa.  Other avian species, like the Torrent Duck (Merganetta armata), have webbed feet to hunt in water and live on land. 
General Description of Size
Avian species vary in size, the smallest bird being the bee hummingbird (Mellisuga helenae) at 5.7 cm (2.24 in) and the heaviest, tallest bird being the ostrich (Struthio camelus) with the maximum weight of 156 kg (345 lbs.) and maximum height of 2.7 m (9 ft.). The bird with the largest wingspan is the wandering albatross (Diomedea exulans), with a maximum of wingspan of 3.63 m (11 ft. 11 in.). 
Survey of Avian Biology
The majority of bird species are omnivorous, although some are primarily vegetarian or carnivorous. Birds of prey, also known as raptors, vary in size and shape, and they play a vital role in mythology and symbolism throughout human history. Raptors have several distinct families, including the following:
|Accipitridae||Diurnal||buzzards, eagles, harriers, hawks, kites, old vultures |
|Cathartidae||Diurnal||new vultures (i.e. condors) |
|Falconidae||Diurnal||falcons, caracaras |
|Pandionidae||Diurnal||Single Species: Osprey |
|Sagittariidae||Diurnal||Single Species: Secretary Bird |
|Strigidae||Nocturnal||typical owls |
|Tytonidae||Nocturnal||barn owls, masked owls |
While avians are well-known for their ability to fly, there are many flightless birds, including members of the order Sphenisciformes, such as the Rockhopper Penguin (Eudyptes chrysocome), which has flippers instead of flight-capable wings.  Other flightless species, such as the ostrich (Struthio camelus), evolved feathers for insulation but not for flight.
Flying birds have a wide range of wing structure, generally adapted in relation to their flight patterns. Aerial performance has some standard measurements that indicate maneuverability and speed.  Two parameters include aspect ratio  and wing loading. 
Aspect ratio is defined as the total wingspan of the bird, including the shoulder width, divided by the wing breadth.  Birds with high aspect ratios generally have patterns of long distance, soaring flight with few flaps, such as the albatross. 
Birds with low aspect ratios have short, tapered wings. These wings require more flapping but also provide munch more maneuverability.  Falcons are a good example of birds with low aspect ratios.
Wing Loading is another standard measurement, examining the property of lift given a certain wing design. It is calculated as the ratio of the animal's body weight versus the surface area of the wing.  Wing Loading is also used to assess the capability of a gliding animal's movement through the air. 
Avian anatomy varies with the adaptation of the forelimb, such as flippers for swimming and wings for flying. Flying birds have additional adaptations, such as lighter bones and, in some instances like the hummingbird, fewer organs, in order to maximize flight capabilities.
Illustration 1. General Avian Anatomy. © Kylie 'drago' McCormick.
While avian anatomy varies from species to species, especially from flying species to non-flying species, this illustration presents a generalized view of the anatomy of small, flying birds. Because the carpals and metacarpals of birds have been fused together, the metacarpal and carpal bones are sometimes referred to as carpometacarpus. 
Birds have a special adaptation for flight, called the furcula, sometimes call the wishbone, which helps protect the animal's thorax during flight. Other specific bone structures of interest include the fused bones in the skeleton, including tibiotarsus, hypotarsus, tarsometatarsus, and pygostyle.
Considerations on Dragon Anatomy and Physiology
Birds are an important consideration in terms of dragon anatomy, primarily because of the wide variety of wing styles in both flight-capable and non-flying birds. Dragons and birds have long been associated in mythology and folklore, even though many modern artists associate dragons with bats or bat anatomy in terms of wing appearance.
The fusion of multiple bones for the specialization of flight is interesting for comparative anatomy, especially when attempting to identify possible configurations for flight-capable dragon anatomy.
However, one of the most important aspects of bird anatomy to dragon anatomy is the size of flight-capable birds. The wandering albatross has the largest avian wingspan, with a maximum size of 3.63 m (11 ft. 11 in.).  It is likely that dragon anatomy based upon bird anatomy would have a similar limit in size.