A juvenile arctic tern Sterna paradisaea setting out on its first epic migration towards the Antarctic Ocean. The diagram shows the power-speed relationship of bird flight and the graphical solution to the optimal flight speed (Vmt) associated with time-selected migration and the overall migration speed (Vmigr). Based on T. Alerstam 1990. Trends. Ecol. Evol. 6: 210-215.

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