Bird movements have been studied by observations (made directly or with radar), by bird counts at particular localities in different seasons, by widescale distribution surveys, by use of ring recoveries to elucidate routes, and in recent years by the use of radio-transmitters or position locators fixed to individual birds which can then be tracked day by day on their journeys.

Most bird migration occurs at heights too great to be seen only with binoculars, and many species travel by night, so counts of migrating birds seen from the ground represent a small and variable proportion of those passing overhead. For most species, visual counts cannot, therefore, reveal the true volume of migration, or the weather conditions that favour it. At night, birds can be seen through binoculars or a telescope as they cross the lit surface of the moon, or through a powerful upwards-directed light beam. Night migrants can also be heard passing overhead, especially with a parabolic reflector and amplifier. The best measures of the volume of bird migration are made using radar, which can be used day or night in all weathers, but can seldom give precise identification of species.

Ringing is the main means by which the migration routes and wintering areas of breeding birds have been studied, together with any age or sex differences within populations. Ringing identifies individuals unequivocally, but tends to be concentrated in particular regions with high human interest. Similarly, recoveries tend to be biased towards areas with high human populations and literacy. Live birds in the hand can also be measured and weighed, providing information on weight gain and fat deposition; they can also be tested in orientation cages for directional preferences. Colour rings and other conspicuous tags enable birds to be identified at a distance without their being recaptured or killed.

Satellite-based radio-tracking can be used to follow individuals precisely on their journeys, wherever in the world they may go but, because of their weight, PTTs can be used safely only on large birds. Analyses of isotope or trace element signatures in bird feathers or other tissues have provided additional insights, linking birds from particular breeding areas with particular wintering areas, or vice versa.

Laboratory work has revealed relevant physiological processes and controlling mechanisms, and wind tunnels have been used to study various aspects of bird flight, including energy consumption. Studies of migratory restlessness and migratory orientation on captive birds have provided details of migratory timing and directional preferences in particular populations. Such studies are being increasingly extended to free-living birds.

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Part One

The Migratory Process

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King Eiders Somateria spectabilis on migration
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