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 dynamics and migration strategies. In particular, we deal with the question of justifying the cost and danger of migration. We evaluate the flight costs in terms of energy and try to model annual events that influence migration rather than emphasizing single aspects. We apply estimates of the total annual energy budget to assess the motivation for migration. We give simulation runs of our model for the migration behaviour of one bird-of-prey species (Houbara bustard, Chlamydotis undulata macqueenii) and of one passerine species (stonechat, Saxicola torquata). In our model the most important factors determining migration success are overwinter survival and reproductive success. This is in agreement with experimental studies. The model allows strategies to be checked 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 strategy (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.