A. Birds have many unique design features that enable them to perform amazing feats of endurance. They are equipped with lightweight, hollow bones; intricately designed feathers providing lift and thrust; navigation systems superior to any that humans have developed; and a heat-conserving design that concentrates blood circulation beneath layers of warm, waterproof plumage, allowing life in the harshest climates. Their respiratory systems must work efficiently during sustained high-altitude flights, so birds extract oxygen from their lungs far better than other animals. Late in the breeding season, when food is plentiful, their bodies can lay down considerable fat to power long migratory flights.

B. The fundamental reason birds migrate is to find adequate food during winter when it is scarce. This applies especially to species breeding in temperate and Arctic regions of the Northern Hemisphere, where food is abundant only during the short growing season. Many species tolerate cold temperatures if food is plentiful, but when it is not, they must migrate. However, intriguing questions remain.

C. One puzzling fact is that many birds journey much farther than necessary simply to find food and good weather. For instance, British swallows could presumably survive the winter in equatorial Africa, yet they fly several thousand miles farther to South Africa’s Cape Province. Another mystery involves huge migrations by arctic terns and mudflat-feeding shorebirds that breed near the poles. In general, the farther north a migrant breeds, the farther south it winters. For arctic terns this means an annual round trip of about 25,000 miles. En route all these individuals overfly apparently suitable habitat across two hemispheres. We may not fully understand why birds choose particular places, but we can marvel at their feats.

D. One of the greatest mysteries is how young birds find traditional wintering areas without parental guidance. Very few adults migrate with juveniles, and youngsters may have little or no idea what their parents look like. The cuckoo, for example, lays its eggs in another species’ nest and never meets its young. Yet once raised by the host species, the young cuckoo makes its own way to ancestral wintering grounds in the tropics before returning alone to northern Europe next season to seek a mate of its own kind. The implication is that it inherits an in-built route map and direction-finding capability, as well as a mental image of another cuckoo. How this is possible is unknown.

E. Mounting evidence confirms that birds use the positions of the sun and stars to obtain compass directions. They also seem able to detect the earth’s magnetic field, probably through minute crystals of magnetite in the brain. True navigation, however, also requires awareness of position and time—especially when lost. Experiments show that after being taken thousands of miles over unfamiliar land, birds can still return rapidly to nest sites. Such phenomenal powers result from computing many cues, including an inborn map of the night sky and the pull of the earth’s magnetic field. Most small birds migrate at night and take their direction from the setting sun, but they also see the plane of polarized light it creates, which calibrates their compass. Night travel avoids daytime predators, reduces dehydration in warm, sunlit skies, and benefits from cooler, less turbulent air.

F. Nevertheless, all journeys involve risk, and part of arriving safely is setting off at the right time—requiring accurate weather sense and use of favourable winds. Birds are adept at both; in laboratory tests, some detect minute differences in barometric pressure between floor and ceiling. Often birds react to weather changes before any visible sign appears. Lapwings flee west from the Netherlands to the British Isles, France and Spain at the onset of a cold snap, returning to Holland ahead of a thaw as pressure changes signal improvement.

G. In one instance, a Welsh Manx shearwater carried to America and released was back in its burrow on Skokholm Island, off the Pembrokeshire coast, one day before a letter announcing its release arrived. Conversely, each autumn a few North American birds are blown across the Atlantic by fast-moving westerlies; ringing evidence shows some make it back the following spring, probably after wintering with European migrants in Africa.