Weather effects and other aspects

They [birds] are usually able to choose a period of mild and favouring winds. North winds [in autumn] either lateral or from the rear are favourable, and they wait for them with the same sagacity that sailors exhibit when at sea. (Frederick II of Hohenstaufen, 1244-1248; from an English translation of 1943.)

Weather has obvious effects on bird migration. It influences the times when birds can travel, the energy costs and risks of the journey, and the visibility of any celestial or ground-based cues that birds might use for navigation. This chapter is concerned with how birds behave in different weather conditions, with the altitudes at which they fly, with day-night patterns of migration, and with the influence of social factors, such as the sizes and formations of flocks. In all these aspects of migration, the interest is in seeing how birds of different kinds adjust their behaviour to prevailing conditions so as to minimise the costs and risks of long-distance travel.

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