Question

why birds fly in straight line?​

Answer 1

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• The contrast between visual observations of diurnal topography-related migrants and radar screens showing nocturnal broad-front migration promoted the idea that birds flying at night might be independent of topography and rely exclusively on compass directions at fixed angles to celestial or magnetic cues. The present paper shows: (1) topography-related changes in flight paths are more frequent at night than currently believed; (2) drift compensation is possible with reference to structures on the ground, but not to stars and hardly to the magnetic field: (3) in opposing side winds, flight paths coincide most obviously with linear structures below; (4) there are indications of the use of topographical elements ahead, suggesting that this might be the normal procedure to maintain a selected course in favourable winds and good visibility; (5) flights towards single beacons may result in a simple correction for drift, and may be the most efficient way for short sea crossings. A comparison of directions along the northern border of the Alps shows a general occurrence of WSW directions, deviating from the expected SW/SSW direction. Towards the west this general effect of the Alps declines, while towards the east S-bound and SE-bound populations become more important.

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