Divers/Loons: osteology

The skeletons of all Divers are very similar and differ mainly in size. Divers are foot propelling diving birds that don't use their wings underwater. The body is typical for a diving waterbird, well streamlined and legs placed very backwards to the body.

Wings

Divers don't use their wings under water and keep them close to the body while hunting. As a result of this the wings are not designed for the purpose of underwater 'flying' like in the Alcids or the diving Shearwater species but well developed for aerial flying. Divers are powerful flap-flyers that fly fast with speeds up to 75 km/h close to the surface of the water over considerable distances during their winter and spring migration. The wing bones are rather light built. Cross cut the humerus is round and not flattened as in birds that use their wings for underwater propulsion.

Legs

The legs however are extremely adapted to their locomotive fuction underwater. They are located far backwards. Underwater propulsion is done by more or less sideward strokes of the legs and feet. In Divers the anatomical adaptations implicate that they can hardly walk but shuffle when on land on their nest. Divers land on the water ontheir belly; the legs streched backwards instead of forward like ducks do.
femur common loon gavia immer tibia common loon diver gavia immer

The femur is a strong, short and curved bone which is very short to the other leg bones. The tibia shows an extremely long cnemial crest or processus rotularis - a spike shaped extension at the knee - to which the very strong thy muscles are connected. It functions as a lever in the paddling motion of the lower leg: the tibia itself, the tarsometatarsus (tarsus) and the large webbed feet.
The tarsus in Divers is extremely laterally compressed, almost like the blade of a knive. This is due to the fact that the soft parts are located postyerior to the also flattened bone.
The feet are large but with closed toes also rather flat. In this way the 'upstroke' offers very little resistance whereas in the 'downstroke' the spread toes and somewhat turned feet and tarsus offer maximum surface. The aquadynamic properties of the feet are thus developed to an optimum.

Body and vertebral column

The body shows the properties of a typical waterbird, very streamlined but still rather wide at the chest. The breastbone has a long, broad basal plate with deep notches to support the membrane that protects the intestinals against the water pressure. The breastbone is also wide. This is found in many waterbirds like ducks. A wide chest provides good initial stability when floating at the surface. The keel is well developed, but by far not like those of the Alcids and Penguins, both wing propelled divers.
The coracoids are neither short or long. The furcula is strongly curved and shows a flattened section near the shoulder joint.
The pelvis is long and slender. The ilium and ischium provide a large surface for the large thy musculature. The illium is much longer than the ischium.
The vertebral collumn consists of XX vertrebrae. Ventrally, in the chest cavity, the thoracal vertabrae show processes forming a large, partly flared, hypapophyse extending to the synsacrum. These extensions are interconnected by liganments and sheets of connexting tissue. The hypapophyse is a reinforcing structure often seen in diving birds. It is supposed to limit bending and twisiting of the spine.