Flying in the Dark

[A] Most small songbirds that cross continents do so at night, which looks like the harder option. The obvious explanation is safety, since the hawks that hunt them are day hunters, and safety is certainly part of it. But a bird that migrates at night must also feed by day, and the hours it spends aloft are hours it cannot spend eating, so the arrangement costs something as well. [B]

[C] Two further advantages are usually offered. The air after sunset is cooler and stiller, and a bird that flies in it loses less water and meets fewer of the rising currents that make level flight tiring. Night skies also carry information: experiments with caged birds under artificial skies show that many species take a bearing from the pattern of stars around the pole, which turns at a rate that keeps the pole itself fixed. [D] The difficulty is that each advantage was demonstrated in a different species, and species that migrate by day face the same heat and the same hawks without adopting the habit. What the evidence supports is that night flight pays for itself in several unrelated ways at once, which is a weaker claim than a single cause and probably a truer one.