Soaring

In travelling between two points, soaring migrants typically cover much longer distances than flapping migrants. The route extension is made up of: (1) distance added as a result of circumventing large water bodies and using longer overland routes; (2) distance added due to circling and climbing in thermals, as well as moving between them, which is not necessarily along the shortest and straight-est route. These various distances were calculated for four species in Israel using data collected by following birds with a motorised glider (Leshem & Yom-Tov 1996b). The four species were White Pelican Pelecanus onocrotalus, White Stork Ciconia ciconia, Lesser Spotted Eagle Aquila pomarina and Honey Buzzard Pernis apivorus. Between the centres of their breeding and wintering areas, these four species had extended their migration distances by 48-91% compared with the straight line distances. The increased distance caused by circumventing sea areas was estimated at 22-34% in different species, while the increase resulting from use of thermals accounted for an additional 23-57%. Presumably, the energy saved by use of soaring-gliding flight more than compensated for the energy consumed by covering the extra distances.

Avoidance of long water crossings results in some curious detours, as exemplified by Short-toed (Snake) Eagles Circaetus gallicus nesting in central Italy (Agostini et al. 2002). Instead of migrating south in autumn, and making the oversea flight from Sicily to North Africa, these birds start by migrating northwest up the Italian peninsula, then move westward and southwestward around the Mediterranean to cross the 14 km of sea at Gibraltar. In order to reach 37°N in North Africa they travel a roundabout route of 2000 km rather than take the shorter 700 km direct flight from their breeding areas through southern Italy and over the sea to North Africa. This gives a forceful demonstration, in this species, of the amount of extra travel undertaken to avoid a long sea-crossing.

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