The role of parasites in ecology and evolution of migration and migratory connectivity

All organisms are adapted to their native environment, posing problems in terms of fitness costs for individuals that leave this environment. Other species constitute an important coevolving component of this native environment, with more than half of all living organisms being parasites. All host species have evolved behavioral and physiological defenses against parasitism, and all higher organisms have evolved immunity that rely on exposure to novel substances derived from other organisms (antigens) for developing efficient defenses. The evolution of dispersal must have been affected by host–parasite interactions because dispersers and migrants inevitably encounter novel parasite strains to which they are not adapted. We provide an overview of bird migration and migratory connectivity viewed in the light of host–parasite interactions. Migratory birds generally show strong site fidelity to both breeding and wintering locations, as evidenced from studies of individuals, estimates of adult survival rates based on breeding or non-breeding captures, and studies of migratory connectivity. Connectivity is closely linked to development of the immune system and regression of immune defense organs. Empirical and theoretical evidence suggests that site fidelity is under stabilizing selection, and that offspring resemble their parents in terms of site fidelity. Populations may diverge in connectivity when the fitness benefits in terms of parasitism exceed the costs. Many species of birds have evolved migratory divides from glacial and post-glacial isolation that may constitute incipient speciation linked to divergence in parasite faunas and hence in local co-adaptation of hosts and parasites in the breeding and wintering quarters. Migration may play a role in speciation when interbreeding among hosts causes divergence in fitness costs and benefits of parasitism due to local adaptation in breeding and wintering areas. These ecological and evolutionary scenarios for migration and migratory connectivity provide a number of testable assumptions and predictions that can form the basis of future research.