On the Evolution of Reproductive Strategies in Birds: Reproductive Effort

Abstract

A model is developed for the optimization of fecundity (B) with respect to adult mortality (M) in populations of birds. Special assumptions of the model include constant age-specific survival and fecundity and adult risk due to reproduction (m) related to fecundity by the expression m = (B/fz. The term f is interpreted as the level of resources available to reproduction. For large values of Z, optimized fecundity is insensitive to variation in M, although risk is always a constant proportion of (1/Z) of M. Values of Z estimated for natural populations as the ratio of M to m were found to vary between 4-10 with an average of about 6. The direct relationship between fecundity and adult mortality observed in natural populations is interpreted as reflecting primarily the density-dependent feedback of adult survival on resources for reproduction (f). Optimization of reproductive risk with respect to adult mortality exerts a minor influence on fecundity and explains little of the variation of reproductive rates among species of birds.