Consequences of Egg-Size Variation in the Red-Winged Blackbird

Abstract

Life-history theory predicts that as parents increase their investment in individual offspring, the survival rates of those offspring should also increase (Winkler and Wallin 1987, Stearns 1992, Bernardo 1996). For oviparous taxa, parental investment may be directed toward eggs or hatchlings. Birds are ideal organisms to test general hypotheses of parental investment owing to the substantial interspecific variation in relative investments made by parents toward eggs and hatchlings and because of the ability of researchers to isolate and quantify these investments. The altricial-precocial continuum, based on the development of young at hatching (Nice 1962), provides a convenient description of interspecific patterns in relative energy investment directed toward eggs versus hatched young. Precocial young hatch from energy-rich eggs and require little parental care after hatching, whereas altricial young hatch from relatively energy-poor eggs and require considerable parental care after hatching (Sotherland and Rahn 1987). We can use these patterns of energy investment in eggs versus young to make predictions about the relative importance of each of these stages in determining offspring performance in different taxa along the life-history continuum. Parents of altricial young spend a large amount of effort during the breeding season feeding and caring for young relative to parents of precocial young. Therefore, we predict that variation in the quality of parental care after hatching should have a larger influence on offspring performance in altricial species than in precocial species. However, because altricial young hatch from energy-poor eggs, factors associated with eggs should have a smaller influence on the performance of altricial species relative to precocial species. Thus, based on interspecific variation in patterns of energy investment, we predicted that the importance of the egg stage to subsequent performance of young birds will increase with increasing precocity of the young. One measure of parental investments in eggs is size, with large eggs presumably requiring a greater energy investment by females than do small eggs.