Legs

The legs of seabirds consist of femur, tibio-tarsus (tibia), fibula, patella, tarso-metatatarsus (tarsus) and the toe bones. The relative length and shape of these bones reflect very much the habits of a species. All seabirds are more or less adapted to an aquatic life, but do use their legs and feet also for walking and digging burrows.

Leg of Northern Fulmar Fulmarus glacialis 1.   Femur 2.  Patella 3.  Fibula 4.  Tibio-tarsus (tibia) 4.1 Processus rotularis or processus cnemalis 5.  Tarso-metatarsus (tarsus) 6.  Toes

In shearwaters the aquatic adaptation is more clearly visible in the elongation of the tarsus than in the size and built of the toes, and especialy the elongation of the tibia and tarsus in regard to the femur in aquatic shearwaters than in less swimming species such as albatrosses, fulmars and storm-petrels. The latter have longer, more slender and straighter femurs. Puffinus griseus, which is a very aquatic shearwater, has a shorter, thicker and curved femur. This femur character is is noticed among other seabirds, reaching the extreme in the most powerful foot-propelling divers such as the divers (Gavidae), grebes and cormorants. In the alcids, which are wing-propelled divers the femur is longer than in the shearwaters and less specialized. That shearwaters in general have markedly shorter femurs than other swimming birds might be an adaptation to their burrowing habit in which the same movement of the legs is performed as in diving.
For the back stroke in diving not only the muscles that pull back the femur are strong and wel developed, also the muscles of the tibia to the tarsus are needed. Divers and cormorants also have very large leg muscles. In the divers the tibia is very much elongated and shows an enormous triangular process processus rotularis at the knee joint offering a long surface for the attachment of these muscles. The processus rotularis is highly developed processus cnemialis as it is called in other bird groups. This process is also found in the tubenoses and its shape correlates perfectly with their aquatic-life differentiations. It is the least developed in albatrosses, gadfly petrels, fulmars and Calonectris shearwaters and shows an intermediate development in Puffinus carneipes and is long in the divers among the shearwaters Puffinus griseus and the shearwaters of the Manx' group, such as Puffinus huttoni.
The patella is a small bone embedded in the tendon at the knee joint. It is well developed in birds with a short processus rotularis but almost absent in species in which it is long.

Sooty Shearwater Puffinus griseus. Note the short curved femur and long tibia and tarsus compared with Fulmarus glacialis and Calonectris leucomelas. The tarsus is slightly flattened.
Flesh-footed Shearwater Puffinus carneipes. A longer and slenderer femur than in the above species. The processus rotularis shows an intermediate form compared with Calonectris and P. griseus
Streaked Shearwater Calonectris leucomelas is less aquatic than Puffinus griseus and has a longer femur and rather small processus rotularis.
Northern Fulmar Fulmarus glacialis. A longer and straighter femur and a less developed processus rotularis. The tibia and tarsus are relatively short.
Red-throated Diver Gavia stellata has a very short femur and an extremely long processus rotularis, typical for foot proppeling divers. The tarsus is laterally flattened which is also seen in many other diving seabirds. The toes (not pictured) are long.
Great Crested Grebe Podiceps cristatus is also a foot propelled diver. It has also a short femur, an elongated tibia and laterally flattened tarsus. The claw is extremely adapted to foot propulsion by being disc like.
Great Cormorant Phalacrocorax carbo. A short and curved femur. The processus rotularis is not elongated but offers sufficient surface for the attachment of the muscles. The pattella is well developed.
Northern Gannet Morus bassanus. A plunge diver, not adapted to foot propelling: with a relatively long and straight femur. The tibia has a small processus rotlularis and the tarsus is short. The patella is not as large as in the cormorants. Note also the broad tarsus compared with Gavia stellata
Common Guillemot Uria aalge. As a wing propelled diver it has a rather long femur and tibia and a short tarsus.
Herring Gull Larus argentatus. Not a diver, but swims good and is a good walker. It has a relatively long tarsus and tibia, and a poorly developed processus rotularis. The femur is straight, as is the case in other good walkers.