A Model of Growth in Altricial Birds Based on Changes in Water Content of the Tissues

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Abstract
Ricklefs (1969b, 1973, 1979) has proposed that the key to understanding the variation in growth rate among birds lies in physiological constraints, principally changes in developmental maturity of tissues. To examine this hypothesis I have developed a simple mathematical model of growth based on changes of water index (water weight/lean dry weight) as an indicator of the level of maturity. The growth curves obtained from the model are S-shaped and for altricial species they fit changes in body weight as well as Richards growth curve does. They can also describe weight recession in growing nestings, which occurs prior to fledging. Values of parameters used in the model are consistent with values computed on the basis of experimental results. Thus these parameters are biologically relevant. Results obtained in the present paper supports Ricklef''s suggestion that physiological constraints limit growth rate. Hence these results support the suggestion that growth rate is maximized. The relevance of water index and water fraction (water weight/lean wet weight) as indices of maturity is also considered.