Migrating birds alternate between two main phases of (1) flight when distance is covered and energy (fuel) is consumed, and (2) fuel deposition when energy is accumulated by intensive foraging. Fuel deposition takes place during stopover periods between flights as well as during a premigratory fuelling period before the initial flight. Thus, the total duration of migration T is the sum of flight time T flight and fuel deposition time T dep: $$T = {T_{flight}} + {T_{dep}}.$$ (1)