The evolution of bird migration--a synthesis

Area of Science:

• Ornithology
• Evolutionary Biology
• Ecology

Background:

• The evolution of bird migration is often debated, with theories focusing on either expansion to new breeding areas or survival in non-breeding regions.
• Existing hypotheses, such as the "southern-home-theory" and "northern-home-theory," present ambiguities in explaining migratory behavior.

Purpose of the Study:

• To propose a unified theory for the evolution of bird migration.
• To investigate whether migration primarily evolves for enhanced survival during the non-breeding season or for increased reproductive success in new breeding areas.

Main Methods:

• A synthesis of existing theories using a threshold model.
• Analysis of microevolutionary processes driving changes in migratory traits.
• Incorporation of genetically non-directed events like dispersal and colonization.

Main Results:

• Migration evolved through gradual dispersal to seasonal habitats, enhancing non-breeding season survival.
• The threshold model explains rapid evolutionary changes in migratory traits.
• The theory integrates dispersal, colonization, and selection as key drivers, moving beyond assumptions of competition or solely increased reproductive success.

Conclusions:

• Bird migration evolves through gradual selection following dispersal and colonization, primarily for survival benefits during the non-breeding season.
• This model reconciles previous theories and explains diverse migratory phenomena, including route selection and seasonal movements.
• The theory applies universally across latitudes and directions, emphasizing microevolutionary processes over sudden changes.