The numbers of diurnal migrants seen on the move or on the ground by day constitute a variable proportion of the total participants. This is because most migration occurs at night or too high by day to be seen through binoculars. Radar is therefore the best method for studying its day-to-day volume, and relationship with weather.
As found mainly by radar studies: (1) the intensity of migration is influenced positively by clear skies and following winds, and negatively by mist, rain and opposing winds; (2) to some extent most migrants adjust their flight altitudes to prevailing winds; (3) birds can compensate for weak lateral winds, and remain on course, but they drift increasingly off course with stronger winds or at high flight altitudes, and more by night than by day; (4) birds that migrate by flapping flight usually travel on a broad front, although low-flying migrants may be temporarily deflected by topographical features into apparent streams; and (5) among nocturnal migrants adjustments to coastlines increase during the night, as landbirds become more reluctant to strike out over water. Drifted birds can re-orientate after skies clear and wind becomes more favourable. Raptors and other landbirds that migrate mainly by soaring flight provide exceptions to some of these generalisations, because of their dependence on thermals and other updrafts.
In spring at temperate latitudes in the northern hemisphere, more migration occurs on warm days (when winds blow from the south), and in autumn on cold days (when winds blow from the north), but temperature may be a correlate of winds and other conditions that favour migration, rather than an immediate causal factor. Nevertheless, temperature can have a direct effect on bird energy needs and food supplies, with potential repercussions on migration timing.
Within their normal limits, birds seem to fly at altitudes that offer the best wind conditions for their progress, although some large species - apparently for energetic or physiological reasons - seem restricted to relatively low altitudes (<1 km). With increasing height above ground, travel speeds increase and predation risks decline, but the atmosphere thins and temperature plummets. The height of bird migration varies with geographical location, topographical situation, weather, day or night, type of flight (flapping or gliding), and species. Some species have been recorded at heights of several kilometres above sea level, and at least one at more than 9 km.
Some types of birds migrate primarily by day and others primarily by night. In general, all species in particular bird families (if they migrate at all) behave the same way in this respect, but some closely related families behave differently from one another, and within some passerine genera, most nocturnal flight occurs among species that make the longest journeys. By migrating at night, birds can avoid predators and loss of feeding time by day, and travel in less turbulent (less energy-demanding) conditions.
Some bird species migrate singly and others in flocks, the structure of which varies consistently between different types of birds. The flight times and behaviour of birds are clearly influenced by those of other individuals, leading to waves in the passage of birds, characteristic flock sizes and formations. Migration in a temporarily reversed direction can occur for various reasons, both in autumn and spring.
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