We examined the relationship between function, water content, and growth rate of skeletal muscles in the European Starling (Sturnus vulgaris). Function was measured as the ability of nestlings to increase their rate of oxygen consumption in response to cold stress. Water content and growth rate of pectoral and leg muscles were determined for tissues dissected from a series of known-age nestlings. The maximum metabolic response to cold stress increased from 0 at 4 days of age to 4.5-5.5 cm 3

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... •.h -• at 15-16 days. During this period, the mass of the leg muscles increased 5-fold and that of the pectoral muscles, 12.5fold. The water fraction (WF) of the pectoral muscles decreased from about 0.86 to 0.72, while that of the leg muscles decreased from 0.83 to 0.72. Of the variation in metabolic response to cold stress per gram of muscle, 89% could be related to the WF of skeletal muscle by a linear relationship. Over the ages surveyed, the metabolic response varied from 0 at WF = 0.85 to an increase above resting metabolism amounting to 60 cm 3 O2.gram of pectoral and leg muscle •.h -a at WF = 0.72. Also during this period, the growth rate of both muscle masses decreased with age and proportion of water, from a mass-specific growth rate of 0.5/day at WF = 0.85 to near 0 at WF = 0.72. If these relationships represent a balance between growth rate and functional maturity in developing tissues, then even small changes in function, associated with a change in water content of only a few percent, could have large consequences for the growth rate of the individual. IN comparisons among species of birds of the same size, the growth rate of chicks varies over an approximately 10-fold range (Ricklefs 1979b). Much of this variation is associated with the mode of development. Chicks of precocial species, which are relatively independent of their parents from an early age, grow more slowly than do those of altricial species, which depend upon their parents for food and warmth during much of their development. Such comparisons, and the observation that mass-specific growth rates of individuals slow as the individual matures functionally, led Ricklefs (1969Ricklefs ( , 1973Ricklefs ( , 1979a to suggest that growth rate is inversely related to functional maturity at the tissue level. As a tissue differentiates and begins to function at an adult level, its proliferation by cell division and growth by cell enlargement decrease. Skeletal muscle provides a model for this constraint in that as mesen-