Info

From Leshem & Yom-Tov (1996a).

From Leshem & Yom-Tov (1996a).

Comparing the different species of raptors, three general trends emerged:

(1) species that eat warm-blooded prey (birds and mammals) generally passed earlier in spring and later in autumn than species that eat cold-blooded prey, with the insectivores being last to move north and first to move south (Chapter 14);

(2) species that travel in obvious flocks (Lesser Spotted Eagle Aquila pomarina, Western Honey Buzzard Pernis apivorus, Levant Sparrowhawk Accipiter brevipes, Red-footed Falcon Falco vespertinus) passed within a shorter period each year than non-flocking species; (3) species that breed over large areas of Eurasia had the longest passage periods, presumably because birds from different localities had started at different dates and travelled different distances; and (4) whereas for some species the spring passage period was shorter than the autumn one (as in many other birds, Chapter 14), in most species the spring passage period was longest. Longer spring passage was most marked in large species in which individuals do not begin nesting until they are more than two years of age. Their populations therefore contained a larger proportion of immatures, which usually migrated later in spring and over a longer period than the adults, spending a much shorter period in the breeding areas (for Steppe Buzzard Buteo b. vulpi-nus, see Gorney & Yom-Tov 1994; for Lesser Spotted Eagles Aquila pomarina see Meyburg et al. 2001). The same held for the White Stork Ciconia ciconia in spring when adults began passing over in February, while immatures appeared in April-May. Year-to-year consistency in passage periods seems usual in the migrations of other obligate migrants, but counting difficulties make them harder to assess. Typically, however, whatever the length of the total migration period, the bulk of the birds may pass on a small number of days within it.

Food is important to the timing of raptor migration, not so much on the route itself, but in breeding areas (Chapter 14). At the end of summer the first animals to disappear with the onset of cold weather at high latitudes are large insects, followed by reptiles and amphibians, while fish retreat to deeper water. By then, many small birds have begun to migrate, and mammals begin to disappear, some hibernating and others spending increasing periods in sheltered sites where they are unavailable to raptors. During spring, the situation is reversed, with mammals appearing first and large insects last. The Steppe Eagle Aquila nipalensis, which eats chiefly mammals, is the first to migrate north, passing through Israel mainly in early March, while the Western Honey Buzzard Pernis apivorus, which eats insects, migrates last, in May. The Steppe Eagle spends six months in its breeding areas, passing south through Israel mainly in mid-October to mid-November, while the Western Honey Buzzard spends only three months in its breeding areas, passing through Israel in early September. The eagle also has a longer breeding cycle than the Honey Buzzard, with longer incubation, nestling and post-fledging periods.

Was this article helpful?

0 0

Post a comment