Mechanistic principles of locomotion performance in migrating animals

This chapter presents some basic principles of locomotion and various migration strategies that result from them. As an animal accumulates fuel for migration, it actually accumulates potential migration distance, which typically has diminishing returns to added fuel mass. Depending on the mode of locomotion – running, flying, or swimming – the exact shape of this range function for migration distance may vary. Paired with assumptions about the immediate selection force acting on the migration episode (e.g., minimising energy cost, time, or mortality risk), the range curve can be used to derive optimal strategies and behaviours, including optimal fuel loads, migration step length, detours, and travel speeds. The chapter also discusses problems of drift, compensation, and altitude/depth associated with travel in a medium (air, water) that is itself in motion. Future theoretical work on optimal migration strategies will need to take into account variation in the foraging strategies of migrants and incorporate more precise estimates of individual mortality and fitness.