Partial bird migration and evolutionarily stable strategies

Abstract

Partial migration in birds is analyzed from an evolutionarily stable strategy (ESS) point of view and it is argued that frequency-dependent selection of tactics (i.e. migration and residency) should be considered. Since the empirical evidence for the condition-dependent choice due to individual asymmetries cannot be ignored, the possibility that partial migration is a mixed ESS at the individual level is excluded. Nonetheless, a mixed ESS at the population level (i.e. a genetic dimorphism) is possible, albeit that the experimental and logical evidence recently put forward are ambiguous. It is suggested that partial migration can be regarded as a conditional strategy with frequency-dependent choice. The individual asymmetries (e.g. age, sex, dominance position), acting against a complete fitness balancing of tactics, and the frequency-dependent choice, tending to equalize fitnesses, are analyzed. Whether this system will result in tactics with equal fitness payoffs is further discussed in relation to the life history consequences of migration and residency.