Breastbone or sternum

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

The breastbone or sternum is one of the largest bones of a bird’s skeleton and is part of pectoral girdle. It consists of a convex/concave basal plate and the keel perpendicular to it, protruding ventrally. From the breastbone  two coracoids form the connection to the shoulder,  articulating to the breastbone in the slot between the Labium ventrale and dorsale. The furcula is attached to the apex of the keel by a ligament or is fused to the apex in Frigatebirds and Pelicans. The ribs are connected to each side of the breastbone by small flexible ligaments.

The shape of the sternum depends very much on the life habits of the species. It is the basis for two important flight muscles: the Musculus pectoralis major for the downstroke, and the smaller Musculus supracoracoideus for the upstroke. The first is, as the name suggests, the larger and the most powerful. Depending on flight or diving behaviour there is a considerable variation in built and size of these muscles. The pectoralis major lies over the supracoracoideus and together they fill the angle between the keel and the basal plate.

The topography of the breastbone

1. Spina externa / Manubrium sterni
2. Labium ventrale
3. Labium dorsale
4. Crista sterni / carina or keel
4a Anterior pillar
5. Apex cristae sterni
6. Metasternum
7. Facets for the ribs
8. Processus lateralis
8a Lateral sternal notch / outer posterior notch
9 Processus intermedius
9a Medial sternal notch / inner posterior notch

A complex of variables

Apart from the way of flying a complex of variables determines the shape of the sternum, such as the foraging strategy, type en behaviour of the prey and the skills needed to catch it. A birds that uses it wings for under water propulsion poses different demands to its sternum than a species that uses its wings only for flight. Different flight styles are also determining factors. Climatological circumstances play an important role as well. The tropical doldrums require another flight technique than the stormy seas of the Roaring Fourties. Beside that also the fysical laws of aqua- and aerodynamics and mechanics play their part.

Flight style

In species that fly with slow or shallow wingbeats or have a soaring flight, the sternum is short and rather small. For this flight behaviour short muscles, predominantly adapted to static forces, are sufficient. The surface of the sternum – basal plate and keel – and its length increases in species with a high wing load (wing load: ratio bodymass : wing surface) that fly and/or dive with small wings and rapid wingbeats, like the Alcids. These have long flight muscles, nescessary for deep and powerful wing beats. The importance of a good streamline is expressed by the slenderness of the sternum in most diving species and very obvious in the Alcids and Gannets.

Sternum Common Guillemot
Common Guillemot
Uria aalge Fulmar Fulmarus glacialis
Gannet Morus bassanus

Sternum of a Giant Petrel Macronectes giganteus woth pneumatic holes

Pneumatic holes

In larger species the sternum often shows cavities: pneumatic holes. This lowers the wing load, without loss of strength. In diving species these cavities are absent, because solid - and thus heavier - bones improve the diving capacities. Penguins have no pneumatic holes, but Albatrosses, Giant Petrels Macronectes (right), Frigatebirds and Pelicans have very pneumatized breastbones.
Shearwaters represent the whole range of adaptations in this respect. The typical gliders such as the Cory's Shearwater have pneumatized breastbones but the divers in this group, the Sooty Shearwaters have solid breastbones.

Extended breastbones

In many diving species the basal plate is extended and shows more or less deep notches. These form a frame for a membrane, that, together with the large basal plate, provide protection to the intestines from the high water pressure during diving. Good examples are the Divers, Penguins (above left: Macaroni Penguin Eudyptes chrysolophus) and Grebes (above right: Great Crested Grebe Podiceps cristatus).

Sternum Magnificent Frigatebird Fregata magnificens

A varied spectrum

Albatrosses are the extremes at one end of the spectrum with their short and 'squarish' breastbone,  Penguins, Alcids and Divers sit at the other end. All Gull-related species are somewhere in the middle of the scale. Frigatebirds with their long and narrow wings are able to combine soaring and gliding with a strong flapping flight and great agility. They have a very peculiar sternum with partly fused coracoids and furcula (right). This forms a very strong structure to support the extreme large flight muscles. The fusion of the sternum with the furcula also exists in pelicans. In Cormorants, Gannets and Diving Petrels the furcula is tightly connected to the apex of the keel, but not fused.
Sternum Flightless CormorantThe flightless foot propelled Galapagos Cormorant Phalacrocorax harissi (left) has a virtually keelless breastbone and almost rudimentary flight musculature. Penguins ‘fly’ under water and dive very deep. As a consequence they have a sternum with a long protecting basal plate and a well developed keel. Gannets fly with shallow wingbeats and and glide a lot. To catch their prey they use gravity to obtain a high attack speed by plunging like a spear into the water. Below the surface they use their wings only for a short pursuit. Their arrowhead-shaped body has but room for a slender, rather long breastbone that leaves relatively little room for large flight muscles.

Skuas have breastbones superficially similar to those of the Gulls. In the Skua it is comparatively much larger because of their large and powerful muscles needed to outfly their victims. Above breastbones at the same scale of the Herring Gull Larus argentatus (left) and the Pomarine Jaeger Stercorarius pomarinus (right), the first being considerably larger than the latter, but with similar sized breastbones

In the Tubenoses, Shearwaters in particular, the whole range of adaptations is present to a certain extend. The large Streaked Shearwater Calonectris leucomelas (left) is a typical glider and dives less deep. This species has a short sternum with a good number pneumatic holes. On the other hand the Sooty Shearwater Ardenna grisea (right), one of the deepest diving petrels, has a much longer basal plate with deep notches, a wel developed keel and no pneumatic holes. The Great Shearwater Ardenna gravis dives less deep and has a more gliding flight. Its sternum is shorter and has a few pneumatic holes and sits in the middle of the 'shearwater spectrum'.

Always a compromise

The shape and built of the sternum of (sea)birds is always a compromise between the factors that determine the birds life style and habits. This compromise defines also the limitations. An albatross will never be able to make a deep pursuit dive and a Guillemot is capable of gliding for only a few meters.