Many migratory bird species fly mainly during the night (nocturnal migrants), others during daytime (diurnal migrants) and still others during both night and day. Need to forage during the day, atmospheric structure, predator avoidance and orientation conditions have been proposed as explanations for the widespread occurrence of nocturnal migration. However, the general principles that determine the basic nocturnal-diurnal variation in flight habits are poorly known. In the present study optimal timing of migratory flights, giving the minimum total duration of the migratory journey, is evaluated in a schematic way in relation to ecological conditions for energy gain in foraging and for energy costs in flight. There exists a strong and fundamental advantage of flying by night because foraging time is maximized and energy deposition can take place on days immediately after and prior to the nocturnal flights. The increase in migration speed by nocturnal compared with diurnal migration will be largest for birds with low flight costs and high energy deposition rates. Diurnal migration will be optimal if it is associated with efficient energy gain immediately after a migratory flight because suitable stopover/foraging places have been located during the flight or if energy losses during flight are substantially reduced by thermal soaring and/or by fly-and-forage migration. A strategy of combined diurnal and nocturnal migration may be optimal when birds migrate across regions with relatively poor conditions for energy deposition (not only severe but also soft barriers). Predictions about variable timing of migratory flights depending on changing foraging and environmental conditions along the migration route may be tested for individual birds by analysing satellite tracking results with respect to daily travel routines in different regions. Documenting and understanding the adaptive variability in daily travel schedules among migrating animals constitute a fascinating challenge for future research.