BIRD MIGRATION - AN ENERGY-BASED ANALYSIS OF COSTS AND BENEFITS

We look at a model of migration as part of a whole ecological system. The purpose is to investigate the mutual relationship of population dy namics and migration strategies. In particular, we deal with the quest ion of justifying the cost and danger of migration. We evaluate the fl ight costs in terms of energy and try to model annual events that infl uence migration rather than emphasizing single aspects. We apply estim ates of the total annual energy budget to assess the motivation for mi gration. We give simulation runs of our model for the migration behavi our of one bird-of-prey species (Houbara bustard, Chlamydotis undulata macqueenii) and of one passerine species (stonechat, Saxicola torquat a). In our model the most important factors determining migration succ ess are overwinter survival and reproductive success. This is in agree ment with experimental studies. The model allows strategies to be chec ked with respect to evolutionary stability in simulating species with very small initial values for the population size. During a period of warm winters a nonmigratory behaviour can be an evolutionary-stable st rategy (ESS), but species which do not migrate are endangered and they can easily be extinguished during seasons with food supply below the average minimum (strong winters), even if such seasons are rare.