Parental investment and avian reproductive rate: Williams's principle reconsidered.

Beginning with George Williams's concept of present and residual (future) reproductive value, life-history theory has considered that the optimized level of parental investment (i.e., assumed risk) should increase in proportion to the annual mortality rate of parent individuals. However, when the survival of young from independence to maturity is separated from parental reproductive success, optimized parental investment is proportional, instead, to prereproductive survival when reproductive and nonreproductive components of adult mortality are additive (simultaneous risk) or to the ratio of prereproductive survival to adult nonreproductive survival when the adult mortality components are multiplicative (independent risk). Applied to the lower, and largely nonoverlapping, brood sizes of tropical compared to temperate and boreal birds, estimates of both adult and prereproductive survival do not predict different levels of reproductive investment. Accordingly, the pervasive increase in clutch size with latitude would appear to reflect increasing availability of food resources to provision offspring.