Shoulder girdle

1. Sternum / breastbone
2. Coracoid
3. Clavicles / furcula
4. Scapula
5. Joint with the wing
6. Foramen trioceum

The shoulder girdle is formed by a set of five bones: two coracoids, one furcula (wishbone) and two scapulars or shoulder blades. The shoulder joint is located at the point where the head of the scapular, coracoid and the furcula meet. The coracoids are connected tot the sternum. The shape and built of the shoulder girdle cannot be seen apart from the shape of the sternum and depends in a similar way on flight and diving behaviour.

Coracoids

The two coracoids form pairwise a solid strut between the shoulder (and wings) and the sternum. All birds and many other vertebrates have coracoids, but mammals don’t.
At the head (proximal) end it is connected to the scapular and both articulate to the humerus to form the actual shoulder joint. The shoulder joint itself is a rather shallow concave surface in which the head of the humerus can move freely. This enables the bird to move the wing in various positions to the body: up and down for normal flight, but also back and forth for complex manoeuvring and hovering. The limitations to the wing movements are not determined by the shape of the joint, but by the muscles, tendons and ligaments. Albatrosses and some petrels, mainly soaring on outstreched wings, have a tendonous layer in their breast muscles – the so-called shoulder lock – preventing the outstreched wing in moving above the horizontal plane. Birds with flapping flight don’t possess this feature.
The foot (distal end) of the coracoid is firmly connected to the sternum and fits neatly into a socket at the top of the sternal plate. It is kept in place by strong ligaments, allowing only slight movement.
In most birds the coracoids are separate bones, except in the Frigatebirds and the Hoatzin Ophistocomus hoazin. In Frigatebirds the coracoids are fused to the scapulars and the furcula at the shoulder but not to the sternum.
Coracoids are variable depending on the species. Short and stubby in soaring birds like Albatrosses and many Petrels with their short flight muscles but longer in species that beat their wings deeply up and down, such as Gulls, Cormorants and Frigatebirds. In Shearwaters, showing considerable differences in flight and diving behaviour among the species, the differences are clear: short in the ‘soarers’ and longer in the ‘flappers’ with their longer breast muscles.
Short coracoids are also found in the wing propelling divers like Alcids. In these species the length of the powerful breast musculature sits along the elongated sternum. A long and thus more fragile coracoid would not be beneficial to the bird.
In aerial and very light species like Frigatebirds the coracoids are pneumatic but in the more heavy built Divers solid.
The shape and built of the coracoid depends very much on flight an diving behaviour and is always the result of the concertation of many different variables.

The wishbone or furcula

This V-shaped bone is another part of the framework to which the breast muscles are attached. It is in fact a pair of clavicles that have fused at the distal end. It is located in front of the sternum en connects the head of the coracoid and scapular to the apex of the carina. Im most birds there is some space between the carina and the furcula and is the connection formed by a strong ligament, but in Pelicans and Frigatebirds the sternum and the furcula are fused together. In Diving Petrels and most other members of the Pelicanfamily (Cormorants, Gannets/Boobies and Tropicbirds) the furcula is so thightly connected to the carina that is functions as a single piece. Sternum Magnificent Frigatebird Fregata magnificens
Extraordinary behaviour requires special adaptations. Frigatebirds have a very different lay out of the shouldergirdle. In this family furcula, the long coracoids and short sternum are fused as well at the should as to the sternum. The coracoids are conected tot the sternum in the usual way, thigtly fixed in a socket, but not fused. In this way it provides a rigid framework for strong breast muscles enabling the bird to flap with deep wing beats and quick manoeuvring with their extremely long wings.
Another family of seabirds with a different shoulder girdle are the Pelicans. The furcula is fused to the sternum, but the coracoids are 'free'. In as the Frigatebirds as well as the Pelicans the construction is strong and light, in spite of the large air sacs in the pneumatized bones. These air sacs may also play a rol in heat regulation. The fusion may contribute to the ability of soaring with relatively small breastmuscles. Incidentally the furcula vcan be fused in species in which this normally is not the case. 
A strongly curved furcula and long coracoids bring the point of gravity forward an provide space for the long muscles nescessary for wing propulsion in subaquatic hunting or powerful deep wing beating. In gliders the furcula is less curved and less heavy built. In diving seabirds the furcula is more U-shaped and in birds that only fly more V-shaped.

During flight the fucula acts like a spring synchronous to the wing movements. Whether this spring action plays an active role to the wing movement is unclear. It is also possibly that it plays a role in respiartuion.

Shoulder blade or scapula

The scauplas sit pairwise at either side of the vertebral collumn between the shoulder and the pelvis. It is connected tot the thoracal vertebrae and the ribcase by a number of muscles. In most birds it is narrow, flat sabre shaped or spatule bone. In general it is not very specialized but in Penguins it is very wide and provides a solid structure for the strong muscles. It probably also plays a role in the distribution of the water pressure to the body.

Foramen trioceum

This is the name for the gap at the meeting point of the three bones. It forms the passage for the tendon of the small breast muscle for the upstroke of the wing. This muscle is conected to the upper side of the head of humerus. This foramen functions as a pulley.