Growth of the pectoralis muscle of the house sparrow (Passer domesticus).

The pectoralis muscle from growing house sparrows was investigated using linear, gravimetric, histological and ultrastructural techniques. The structure of the muscle at different stages of development was discussed in relation to known behavioural activities, such as the onset of regular wing exercising and thermogenic shivering in the nestlings. Fibres in the pectoralis muscle of birds about to fledge (15 day nestlings) were similar in composition to fibres in muscles from fully fledged juveniles, with two exceptions. Firstly, numerous lipid droplets had accumulated adjacent to mitochondria in fibres from the pectoralis muscle of the nestlings. This lipid could act as a readily available source of energy for the birds on fledging. Secondly, masses of ribosomes and irregularly shaped myofibrils were present, indicating the continued elaboration of the contractile apparatus in fibres from the pectoralis muscle of the 15 day nestlings. The inefficient flight of newly fledged house sparrows may be accounted for by the size of the pectoralis muscle relative to total body weight. The total contractile material in the muscle of the birds about to fledge was estimated to be 60% or less of that found in fully fledged juveniles, while the body weight was nearly that of an adult bird.