<P> Some large broad - winged birds rely on thermal columns of rising hot air to enable them to soar . These include many birds of prey such as vultures, eagles, and buzzards, but also storks . These birds migrate in the daytime . Migratory species in these groups have great difficulty crossing large bodies of water, since thermals only form over land, and these birds cannot maintain active flight for long distances . Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points . Massive numbers of large raptors and storks pass through areas such as the Strait of Messina, Gibraltar, Falsterbo, and the Bosphorus at migration times . More common species, such as the European honey buzzard Pernis apivorus, can be counted in hundreds of thousands in autumn . Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants . This is a notable factor in the Central American migratory bottleneck . Batumi bottleneck in the Caucasus is one of the heaviest migratory funnels on earth . Avoiding flying over the Black Sea surface and across high mountains, hundreds of thousands of soaring birds funnel through an area around the city of Batumi, Georgia . Birds of prey such as honey buzzards which migrate using thermals lose only 10 to 20% of their weight during migration, which may explain why they forage less during migration than do smaller birds of prey with more active flight such as falcons, hawks and harriers . </P> <P> From observing the migration of eleven soaring bird species over the Strait of Gibraltar, species which did not advance their autumn migration dates were those with declining breeding populations in Europe . </P> <P> Many long - distance migrants appear to be genetically programmed to respond to changing day length . Species that move short distances, however, may not need such a timing mechanism, instead moving in response to local weather conditions . Thus mountain and moorland breeders, such as wallcreeper Tichodroma muraria and white - throated dipper Cinclus cinclus, may move only altitudinally to escape the cold higher ground . Other species such as merlin Falco columbarius and Eurasian skylark Alauda arvensis move further, to the coast or towards the south . Species like the chaffinch are much less migratory in Britain than those of continental Europe, mostly not moving more than 5 km in their lives . </P> <P> Short - distance passerine migrants have two evolutionary origins . Those that have long - distance migrants in the same family, such as the common chiffchaff Phylloscopus collybita, are species of southern hemisphere origins that have progressively shortened their return migration to stay in the northern hemisphere . </P>

Where do birds go when they fly south