The Form and Motion of Real Birds: Morphology of Aves
Bird wings are very diverse because of the various modes of flight seen in birds.
There are, in general, four wing shapes attributed to flying birds: long and narrow (soaring birds, like albatrosses), short and round (like in grouse which are good for quick takeoff and maneuvering), slim and un-slotted (like in falcons for speed), and intermediate dimension slotted wings (like in hawks for gliding ability) [Gill 1995]. Despite the great diversity in birds and their wings, they still have a common overall bone and feather structure which can be seen in most flighted birds [Sibley 2000].
Bones: Bird wings are a modified forelimb, similar to the human arm: Scapula, humerus, radius and ulna, carpus and metacarpus, and the phalanges fused together forming the “hand”. The shoulder consists of the scapula (shoulder blade), coracoid (projecting part of the shoulder blade), and humerus (upper arm). The humerus joins the radius and ulna (forearm) to form the elbow. The wrist bones, carpus and metacarpus are fused together forming the carpometacarpals and the digits (fingers) are fused together in three digits. The alula (thumb) is also known as the “bastard wing” and moves independently of the rest of the wing [Gill 1995]. Due to the similarities with a human arm and for simplicity, the terms used for a human arm (“wrist”, “elbow” and “shoulder”) will often be used throughout this work.
Feathers: There are three main types of feathers: vaned feathers, down feathers and filoplumes. Filoplumes are hair like and monitor movement and position of adjacent vaned feathers. These feathers are distributed throughout the plumage and are normally not visible due to their small size. Down feathers have no rachis (or central shaft) and provide insulation to the bird. Down feathers lie under the vaned feathers and thus are also usually not visible, except on baby birds whose vaned feathers have yet to grow in. Because of this, down feathers and filoplumes are not seen in computer graphics except for rare instances (like baby birds with down) and are not be taken into consideration in this project.
Vaned feathers are the feathers that cover a bird’s body and have of a rigid center (rachis) and soft barbs on the side. There two different types of vaned feathers found in the wings: remiges and coverts. The remiges are the flight feathers and consist of the primaries, or outermost remiges, and secondaries, the innermost remiges [Gill 1995]. These feathers make up a large portion of the shape of the wing, so much so that the length and shapes of the primaries and secondaries are used as field marks to identify different species of birds [Sibley 2002]. Coverts are, as their name suggests, feathers that cover the wings. These feathers can visually be broken up into several groups. The arrangement of these feather groups is similar across all species of flighted birds even though the shape of individual feathers varies [Sibley 2005]. Often when trying to identify a bird by appearance, birdwatchers will use attributes of these feather groups (such as color and shape) to tell one bird from another. Within each group, feathers grow in orderly rows, overlapping like shingles on a roof, and plumage patters tend to follow the contours of these feather groups Sibley 2000]. These feather groups are primaries, secondaries, greater primary coverts, greater secondary coverts, median secondary coverts, lesser secondary coverts, and alula [Sibley 2000]. (See image below).
The main area of interest in this project is feathers in the wing that interact like venetian blinds or fans. These are the primaries, secondaries, alulas, and primary, secondary and median coverts. These feather groups are the main focus because they are the ones with the most movement and expression to them, making up the main body of the wing. They are often the feathers that are over-simplified which degrade the realism of the wings. Feathers such as the marginal covert feathers act more as scales instead of fans or venetian blinds and have very little range of motion to them; their portrayal in film is often reasonably accurate and believable because of their simple nature and limited motion. There has also been previous work in this area such as in the papers by Seddon et al.  and Weber and Goronwicz . Due to the fact that these feathers have a considerable amount of previous work on them and are often treated accurately in wings, unlike the main feather groups, they are considered outside of the scope of this project.
Tutorials and Wing References
– GILL, F. B. 1995. Ornithology (2nd ed). New York: W.H. Freeman and Company.
– SIBLEY, D.A. 2000. The Sibley Guide to Birds. New York: Knopf.
– SIBLEY, D.A. 2002. Sibley’s Birding Basics. New York: Knopf.