Summary

Because soaring birds migrate entirely (or almost entirely) by day, often at heights low enough for the birds to be identified by ground-based observers, and because they pass certain points in concentrated streams, some aspects of their migrations have been studied in detail.

On migration, soaring raptors, storks and pelicans depend mainly on updrafts where crosswinds are deflected upwards from cliffs or slopes, or on thermals where columns of rising air provide lift, enabling the birds to glide to the next thermal losing height, and then rise again. Such species are constrained to make as much of their journey as possible overland, making detours to avoid or minimise water crossings.

On a world scale, five major flyways for soaring birds are currently recognised, along which enormous numbers pass each year: one converges through Panama, the second across the Straits of Gibraltar, the third along the Great Rift Valley in the Middle East, the fourth through Southeast Asia and western Indonesia, and the fifth through the islands of eastern Asia to the Philippines and eastern Indonesia. Particularly large numbers of raptors have been counted at Eilat in Israel (more than one million in spring), at Corpus Criste in Texas (840 000 in autumn), at Panama (2.5 million in autumn and spring) and at Veracruz in Mexico (>6 million in autumn).

At various sites in Israel and elsewhere, different species pass in the same sequence each year, both autumn and spring, and passage dates are remarkably consistent from year to year. In general, species that pass northward earliest in spring pass southward latest in autumn. In some species, autumn passage is spread over a longer period than spring passage, and in others the reverse. Spring passage tends to be longest in large species in which the non-breeding immatures migrate north much later than the adults. Otherwise the timing and duration of passage depend on the extent of the breeding range (longer passage with wider latitudinal spread) and diet (bird-eating and mammal-eating species spending longer in breeding areas than reptile-eaters and insect-eaters). Some species take slightly or markedly different routes in autumn and spring, depending largely on wind conditions.

Many raptor species are known to feed on migration, especially bird-eating falcons and accipiters which migrate at the same time as their prey, and insectivores which also encounter food en route. Eagles and buteos that migrate long distances feed more episodically, and probably make large parts of their journeys without eating. At least some raptors are known to accumulate migratory fat (with up to 30% increase in body weight in Steppe Buzzards Buteo b. vulpinus). Because of their dependence on topography, the wind and other conditions that favour heavy passage differ to some extent from one site to another. However, low cloud and rain are largely inimical to migration by soaring birds, as well as by other birds.

Wandering albatross Diomedia exulans, one of the fastest of all migrating birds
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