Postnatal Development of the Northern Gannet Sula bassanus, and the General Relationship between Growth Rate and Adult Size in Birds

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Abstract
The changes in the relative sizes and water contents of body components are described during the postnatal development of the northern gannet, a large altricial seabird. During the 13-wk nestling period, body proportions change markedly as mass increases .apprx. 40-fold. As in other altricial species, proportions of the appendages, associated skeletal muscles and feathers increase with growth while those of the viscera and the head decrease. Percentage of water in the body decreases linearly with age from .apprx. 85% at hatching to 70% at 50 days of age. The relationships of organ size to body size, and of the percentage of water in a component to the proportion of its growth accomplished at a particular age in gannet chicks appear similar to those of smaller altricial nestlings. Hence pattern of growth and development does not differ greatly among species with adult sizes spanning 2 orders of magnitude. The slower growth rate of the larger gannet apparently reflects a general prolongation of the altricial pattern of development with little qualitative change. The earlier achievement of homeothermy in the gannet than in smaller species appears related in part to its larger size and resulting greater thermal inertia and lower mass-specific conductance. Although skeletal muscles may acquire function at a somewhat lower relative mass in the gannet than in smaller altricial species, the slower growth of larger species cannot be linked to any major difference in pattern of anatomical or physiological development.