Nomadism and Site Tenacity as Alternative Reproductive Tactics in Birds

Abstract

Adult nomadism is compared to site tenacity, using a model which relates the fitness of an individual to its clutch size and mortality rate, and to the pattern of food production. The model predicts that the relative merit of nomadism is higher with cyclic than with random fluctuations in food abundance. The merit increases with the interval between succesive good years in an area. With cyclic food production, nomadism is favored by large clutch size, high juvenile survival and low adult survival. Although the relation between juvenile and adult survival is important, the general level of the survival rate does not influence the optimal tactic. Empirical results from birds for which food abundance varies strongly between breeding seasons support several predictions, but the need for further studies of movement patterns in banded adults is stressed.