Raptors and other soaring birds

if they had continued for a fortnight in the same strength as on that day, we could surely have said that they were in greater number than all the men living on the earth. . . . they are seen to pass in this way as thick as ants, and so to continue for many days. (Belon (1555), writing of the migration of Black Kites Milvus migrans on the Black Sea coast.)

In some large bird species, the spread wings span a large area relative to body weight, and provide good lift in rising air currents. This is true of broad-winged raptors, pelicans, storks and anhingas, and to some extent of cranes. Such species therefore depend more heavily upon soaring-gliding flight (as opposed to flapping flight) than do most other birds (Kerlinger 1989, Hedenstrom 1993). They migrate primarily by day, when thermals are best developed, and most avoid long water crossings (Chapter 3). They usually travel low enough to be seen with the naked eye, and in certain places, as determined by geography and topography, they form predictable migration streams. They can therefore be studied and counted at such sites by ground-based observers equipped only with binoculars or telescopes in ways that other, higher flying or night-flying birds cannot.

The seasonal timing of their migrations can be assessed accurately and day-to-day passage can be related to weather and other conditions. In some species, the different age groups can be distinguished, enabling their movements to be examined separately. Study has been helped by the publication of special field guides showing the birds from below, and facilitating the identification of species, age groups and sometimes sexes, as they pass overhead (Porter et al. 1974, Forsman 1999, Clark 1999, Clark & Wheeler 2001). Some aspects of soaring bird migration result from the mode of travel itself, and do not entirely apply to other types of birds, but other aspects hold more generally. Hence, some aspects of bird migration have been better studied in soaring species than in many others (see also Chapters 3 and 4).

No sharp division separates species that travel by soaring-gliding flight from those that travel by continued flapping. Different species form a continuum of variation between the two extremes, depending on their body size and wing shape, and in all soaring species the ratio of flapping to gliding varies with air conditions at the time. Among the birds of prey, vultures and eagles are most dependent on soaring-gliding, followed in descending order by Buteo hawks, Milvus kites, Accipiter hawks and Pernis honey buzzards, and then by Circus harriers and Pandion ospreys. Falcons are more active fliers, less dependent on updrafts, but making use of them when available. This order of listing broadly follows the sequence of wing-loading, from lowest to highest, and the variation in wing shape from long and broad, with slotted primary feathers, to narrow and pointed, with little or no slotting. It also reflects the dependence of these various species on updrafts, and hence the extents to which they form into concentrated migration streams and avoid long sea-crossings. It is chiefly the falcons that regularly make long (<100 km) overwater flights, but other species (Pandion, Pernis, Butastur and others) do so in some parts of the world. The most extreme is the Amur Falcon Falco amurensis, which each autumn is thought to cross the Indian Ocean between India and East Africa on a journey exceeding 4000 km. This cross-water flight occurs in late November or early December, and is assisted by prevailing winds. On the return spring migration, winds are less favourable, so migration is largely over land, running west and north of the outbound passage, up the east side of Africa into Asia.

Soaring landbirds use routes where topography favours the development of thermals and other updrafts. Well-known observation points for watching soaring migrants include Hawk Mountain in Pennsylvania, Cape May Point in New Jersey, Veracruz on the Gulf coast of Mexico, Panama in Central America, Falsterbo in Sweden, Gibraltar and the Bosphorus at either end of the Mediterranean Sea, the Black Sea coast in northeast Turkey, various localities in the Rift Valley in Israel, Suez in Egypt, Chumphon in Thailand and Kenting on the southern tip of Taiwan (Figure 7.1). At these points, large numbers of raptors and other soaring species pass in spring or autumn, with total numbers typically varying between tens of thousands and hundreds of thousands, even millions, depending on site.

Because they depend on geography and topography, birds take the same routes each year, but many take somewhat different routes in spring and autumn, depending on wind and other conditions. Thus the passage at Hawk Mountain in Pennsylvania is marked in autumn, but barely noticeable in spring, while at Eilat in Israel the spring passage is much bigger than the autumn one. Daily counts

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Birds East Africa

Figure 7.1 Main flyways used by soaring birds. 1. Trans-American Flyway; 2. Western European-West African Flyway; 3. Eurasian-East African Flyway; 4. East Asian Continental Flyway; 5. East Asian Oceanic Flyway. Note that no major flyways for northern hemisphere raptors extend into New Guinea and Australia. Watch sites mentioned in the text: B - Bosphorus, BM - Bab el Mandeb, BP - Belen Pass, E - Eilat, F - Falsterbo, G - Gibraltar, H - Hawk Mountain, K - Kenting, M - Messina Strait, P - Panama, S - Suez, T - Corpus Cristi, Texas, V - Veracruz, Mexico. Modified from Zalles & Bildstein (2000).

Figure 7.1 Main flyways used by soaring birds. 1. Trans-American Flyway; 2. Western European-West African Flyway; 3. Eurasian-East African Flyway; 4. East Asian Continental Flyway; 5. East Asian Oceanic Flyway. Note that no major flyways for northern hemisphere raptors extend into New Guinea and Australia. Watch sites mentioned in the text: B - Bosphorus, BM - Bab el Mandeb, BP - Belen Pass, E - Eilat, F - Falsterbo, G - Gibraltar, H - Hawk Mountain, K - Kenting, M - Messina Strait, P - Panama, S - Suez, T - Corpus Cristi, Texas, V - Veracruz, Mexico. Modified from Zalles & Bildstein (2000).

throughout the autumn or spring migration seasons have been made at several of these sites, and at some such sites counts have been made repeatedly over several to many years (for Hawk Mountain see Bednarz et al. 1990; for Cape May Point in New Jersey see Dunne & Clarke 1977; for various sites in western North America see Hoffman & Smith 2003, for Falsterbo in southern Sweden see Kjellen & Roos 2000; for Israel and elsewhere see Shirihai et al. 2000). Observers at well-watched sites now publish count data on the web, updated each year as fresh data become available.

For parts of their migrations, most soaring species travel on a broad front (Bednarz & Kerlinger 1989), but for other parts, they form into concentrated streams. Such streams typically occur along landscape features that favour soaring-gliding flight, such as mountain chains, narrow valleys and coastal plains, but also on the east or west sides of large water bodies, as birds hug the shoreline rather than crossing the water. Worldwide, most migration corridors coalesce into one of five principal flyways, two of which extend to the southern parts of the southern continents (Figure 7.1; Zalles & Bildstein 2000). All flyways tend to converge on narrow land bridges (such as Panama or Suez) or on short sea-crossings

(such as the Straits of Gibraltar or Bosphorus). At these points, the migration streams may be only a few tens of kilometres across (or less).

1. The Trans-American Flyway: Each autumn, more than six million raptors travel along one or other part of this 10 000 km overland system of corridors that stretches from boreal Canada to central Argentina (Figure 7.1). At least 32 species, including eight buteos, migrate along the flyway's central land corridor that connects North and South America, reaching its narrowest point at Panama. Once the birds reach South America, the flight-line turns south, and many of the migrants follow the Magdalena Valley south through Colombia and onwards. The central part of the route is dominated by Turkey Vultures Cathartes aura (>two million), Broad-winged Hawks Buteo platypterus (>one million) and Swainson's Hawks B. swainsoni (>one million) (Table 7.1, Bildstein 2004). The biggest numbers of birds are seen near Corpus Christi (Texas, 840 000 birds in autumn), Veracruz (Mexico, more than six million birds in autumn) and Panama City (more than 2.5 million birds, both seasons). In some parts, the route also carries substantial numbers of other species, including Ospreys Pandion haliaetus (5000), Mississippi Kites Ictinia mississippiensis (200 000), Peregrines Falco peregrinus (5000) and others.

2. The Western European-West African Flyway: Each autumn, at least 200 000 raptors travel along one or other part of the 5000 km overland system of corridors that stretches from northern Europe to West Africa, via the short (<14 km) sea-crossing at the Straits of Gibraltar (Figure 7.1). At least 22 species use this flyway, which for most of its length is dominated by European Honey Buzzards Pernis apivorus (117 000) and Black Kites Milvus migrans (39 000). The biggest concentrations of birds are seen in both spring and autumn at the Straits of Gibraltar, where some 195 000 birds were counted in 1972 (Bernis et al. 1975; Table 7.2).

3. The Eurasian-East African Flyway: More than 1.5 million raptors travel along this 10 000 km system of largely overland corridors that extends from northeastern Europe and western Siberia through the Middle East into southern Africa (Figure 7.1). At least 35 species use this flyway that for much of its course follows the Great Rift Valley, and that includes narrow water crossings at the Bosphorus, Suez or Bab el Mandeb Straits. Two main known routes converge on Africa. In the western route, birds pass in autumn, east or west of the Black Sea, over the Bosphorus and on to cross Jordan and Israel, and then Sinai, entering Africa at the northern end of the Red Sea at Suez. In the eastern Caspian-Arabia route birds pass either side of the Caspian Sea, move on south through Arabia and cross into Africa via the Bab el Mandeb Straits at the southern end of the Red Sea. Six raptor species make up the bulk of the flight through the Middle East, namely the Western Honey Buzzard Pernis apivorus (up to 852 000), Black Kite Milvus migrans (37 000), Levant Sparrowhawk Accipiter brevipes (60 000), Eurasian (Steppe) Buzzard Buteo buteo vulpinus (466 000), Lesser Spotted Eagle Aquila pomarina (142 000) and Steppe Eagle A. nipalensis (75 000), along with White Storks Ciconia ciconia (530 000), Black Storks C. nigra (17 000) and Great White Pelicans Pelecanus onocrotalus (66 000). Major concentrations of birds are seen at several sites in Israel (see later), and at the various water crossings mentioned above, and on the east and west sides

Table 7.1 Autumn counts (August-November) of soaring birds at Veracruz, Mexico, 2002-2005

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