The flight muscles on the breast of a bird are large and better known. These consist of one depressor, one elevator, two accessory depressors and four tensor muscles. The largest of these is the pectoral muscle (also called pectoralis major). This enormous triangular muscle occupies the whole space of the ventral half of the keel of the sternum. Its broad flat tendon is inserted on the ventrolateral surface of the humerus. It is the great depressor muscle, i.e., it pulls the wing to the lower side. For elevating the wing there is another smaller subclavius muscle (also called deep pectoral or pectoralis minor). This also originates on the sternum beneath the pectoralis major, and inserts on the dorso-medial aspect of the humerus. It is the only muscle that elevates the wing. Among the accessory depressors are the coraco-brachialis longus and coracobrachial brevis.
Then there are some muscles that keep the wing fully spread (tense) during flight. These are the tensor muscles. The longest among these is the tensor longus. A thin sheet of muscles, the tensor brevis, spreads between the biceps muscles and the posterior margin of the proximal portion of the tensor longus. There is a comparatively broader muscle, the tensor accessorius. A thin strip of muscle, the tensor posterius, runs along the outer edge of the postpatagium and keeps it fully stretched during flight.
The movements of a bird are restricted, there being no movements of abduction and adduction, which a man makes at the hip during walking. These movements are replaced by movements of rotation at the knee. It will be seen that a bird is “sitting down” while it is standing up, since the knees are directed forward horizontally. When a bird perches the leg bones jack-knife together, thus pulling the tendons attached to the toes so that the phalanges clinch around the perch. This pulling of the tendon is brought about by the body weight, as such to unlock the foot the body has to be raised.
Muscles of Thigh
The muscles around the hipjoint form a system of couples (or braces) that help in balancing and locomotion. The lateral and medial (abductor and adductor) muscles are very poorly developed. The anterior muscle (protractors) are well developed and include the following: (i) the sartorius (ii) the ilio-tibial (iii) the femorotibial muscles. The lateral side of the hipjoint has rather small muscles including the ilio-trochanteric acting as median rotators and the ischio-femoral and obturator muscles that act as lateral rotators. The posterior muscles (retractors) of the hip-joint are the main locomotor muscles. These lie behind the hip-joint and include several muscles.
The knee-joint comprises three sets of ligaments and a pair of cartilages called menisci (singular meniscus). The three ligaments are known as the lateral, medial and cruciate according to their positions. Following are the muscles of the joint: the gastrocnemius along with the flexor muscles of the toes lying behind the tibia; the tibialis-anterior on the front of the tibia and acting across the intertarsal joint, and the extensors of the toes also lying on the front of the tibia.
Pigeon, like other flying birds, has the ability to perch on the branches of the tree. Some muscles in the legs are modified in such a fashion that the toes can close round the twig automatically when it sits on the tree. There are four digits in the hind limbs which are flexed by two sets of tendons. The tendons of the hallux arises from the flexor perforans muscle. The tendons of the three forwardly directed digits are formed by the trifurcation of the tendon coming from peroneus muscle. The tendons are so oriented that a pull upon any tendon flexes the toes. When the bird settles on the branch of a tree, the legs are bent and puts the flexor tendons on the stretch. With the exertion of the pull, the toes are bent spontaneously around the perch. A bird can go to sleep in this position without any fear of falling off. To unlock the feet, it is obligatory for the bird to raise its body to straighten the leg and loosening the tendons which have been pulled tight over the ankle during perching. It is notable that the foot contains no muscle, but the working of the digits is controlled by tendons coming from the muscles situated in the upper sector of the legs.