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/f)Z. 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 four and 10 with an average of about six. 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.