Sternum
The sternum or brestbone varies in shape depending on the species' life habits, because it's not only the flying that determines the shape of the brestbone. Also it's foraging strategy and a few other variables play an important role.
The sternum forms the basis for two large muscles: one for the upstroke m. supracoracoideus and one for the downstroke m. pectoralis major. The latter is the largest and strongest of the two, but depending on the flight and diving technique considerable differences exist. The sternum is short and well keeled in species that fly with slow movements or a gliding flight. For this type of flight small pectoral muscles are suffficient. It is longer in short winged birds that need long and well developed musculature to fly with rapid wingbeats.
Albatrosses are the extremes on one end of the scale, guillemots Uria and loons Gavia represent the latter group. Gulls are found in the middle of the range, though this group also shows a good deal of variation. As a consequence birds that are strongly adapted to a gliding or soaring flight are not well equipped for diving. In general: the more a glider, the less a - wing propelled - diver.
Diving with wing propulsion (subaqueous flying) requires short wings and powerful wing movements. The long and rather high keeled sternum of a guillemot for instance reflects its flight as well as its foraging technique: rapid wingbeating in flight combined with powerful underwater pursuit with wing propulsion. On the other hand, foot propelled divers such as the Divers or Loons also have a long sternum but a rather low keel: they are not provided with the powerful muscles needed for wing beating under water. The flightless Galapagos cormorant has an almost keelless sternum. Penguins fly under water by using their flippers and have long sternums with a well developed keel.
In the tubenoses the whole range of adaptations can be found to a certain extent. The Sooty shearwater Puffinus griseus, one of the best pursuit divers among the petrels, has a rather long and well keeled sternum compared with the similar sized Flesh-footed shearwater Puffinus carneipes that has a shorter sternum and is known to be more a glider which makes shallower dives. The larger and longer winged Streaked shearwater Calonectris leucomelas also makes only shallow dives and has a even smaller and short but well keeled sternum. The anterior pillar of the keel is inclined backward in the more gliding species and shows a less backward to a forward inclination in species that dive also.
The sternums of seabirds are often increasingly pneumatized in the larger species with gliding flights to decrease their wing loading and are not pneumatized in diving species that are better off with  heavier skeletons to enhance their diving capacities. In many diving seabirds the plate of sternum is elongated or has one or more notches and/or extensions that form the framework for a membrane. This elongation provides a protection the internal organs against increasing water pressure.

Shoulder girdle
The shoulder girdle consists of a set of five (in fact six) bones connected to the sternum: the coracoids, clavicles (fused into a furcula at the front) and the scapulas. The meeting point of these three bones forms on each side the joint with the wing. The lay out of the shoulder girdle cannot be seen apart from the shape of the sternum and is likewise depending on the way of flight and diving habits.
The development of the coracoid and clavicles is correlated with each other, occupying a position between upper arm (humerus) and sternum, but the scapula is rather an accessory bone to fix the joint of the wing to the dorsal surface of the ribs. The scapula is a sword-like bone with variations predominantly varying from triangular, rounded to hooked. It is generally not a very specialized bone, but in penguins it is broad.
The articulation of the clavicle, coracoid and scapula forms a hole, the foramen trioceum through which the tendon of the m. supracoracoideus passes to attach on the outer side of the humerus. The size of the pectoral muscles is dependent on the variation of sternum, clavicles and coracoids.  In  the pelicans the sternum and the clavicles are fused and in Fregata the sternum and clavicles are fused and the coracoids to the clavicles at the shoulder joint.
The curve of the clavicles and the position and shape of the carina determines the postition of pectoral muscles. In the diving species the position of the musculature has moved the point of gravity more forward, but in the non-diving species the point of gravity is located more to the centre of the body. As a consequence most divers are poor walkers, and if they do they have to maintain a rather upright posture.
 

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

Sternum and shoulder girdle of Fulmarus glacialis

Species with a gliding and soaring flight have short and sturdy coracoids and 'flat' clavicles. The albatrosses, gliding shearwaters and fulmars are good examples of this group. Northern Fulmar Fulmarus glacialis

Birds with a flapping flight can be roughly divided in three categories. A gallinaceous type in which the coracoid bearing part of the sternum protrudes forward, articulating with a long coracoid, being adapted to a short and straight flight. This type is not found among seabirds.
The second group is a general type with a medium sized sternum with fairly long coracoids. Gulls and cormorants are representatives of this group. In cormorants the sternum is short and the clavicles strongly curved forward, while in gulls the sternum is rather long and the clavicles not very curved. Depending on specific behavioural aspects there are of course differences between the species.

On the following pages the sternum and bones of the shoulder girdle of different groups of seabirds are shown.

• Divers: seabirds: penguins, divers, grebes, alcids
• Pelecanoidae: pelicans, gannets and cormorants
• Tubenoses : petrels, shearwaters and albatrosses
• Gulls, terns and jaegers