From Bilsma (1983), Welch & Welch (1988), Wimpfheimer et al. (1983).

aA count is available for Bab el Mandeb in spring but covered only a few days (Welch & Welch 1998).

From Bilsma (1983), Welch & Welch (1988), Wimpfheimer et al. (1983).

aA count is available for Bab el Mandeb in spring but covered only a few days (Welch & Welch 1998).

than those in Africa and South America. At least 33 raptor species migrate along portions of this flyway through eastern Asia, with the bulk of the flight dominated by Oriental Honey Buzzards Pernis ptilorhynchus, Grey-faced Buzzard Eagles Butastur indicus, Chinese Sparrowhawks Accipiter soloensis and Japanese Sparrowhawks Accipiter gularis. Eurasian (Japanese) Buzzards Buteo b. japonicus use the northern half of the flyway, and Black Bazas Aviceda leuphotes the southern half. Major concentrations of birds are seen in autumn at Chumphon in Thailand (>170 000), at Taiping (>30 000) and the Selangor Plain (121 000) in Peninsular Malaysia, and at the Straits of Malacca, including Singapore, suggesting that most of them winter in the Indonesian Islands (DeCandido et al. 2004). More than 90 000 individuals of several species have been counted crossing from Bali to Lombok, presumably to winter in the islands to the east (Germi & Waluyo 2006).

5. The East Asian Oceanic Flyway: More than 225 000 raptors travel along this 5000 km largely overwater flyway that stretches from coastal eastern Siberia and Kamchatka to Japan, the Philippines and into eastern Indonesia (Figure 7.1). At least 26 species migrate at least part way along the main flyway, which extends from southern Japan through the Ryukyu Islands and Taiwan to the Philippines and beyond. The bulk of the flight is dominated by Grey-faced Buzzard Eagles Butastur indicus (30 000 at Kenting on the southern tip of Taiwan) and Chinese Sparrowhawks Accipiter soloensis (up to 201 000 at Kenting). The long sea-crossings of up to 300 km may restrict the variety of species that use this flyway (Lin & Severinghaus 1998 updated). However, some of the overwater crossings fall within the tradewind zone, so it is possible that raptors can take advantage of the weak thermals that develop there, although they are not known for sure to do so.

Some species use only certain parts of the major flyways, while other species use other parts, depending on the locations of their breeding and wintering areas. Along each flyway, however, the numbers of birds seen in autumn tend to increase southwards towards the tropics, as more and more individuals join the migration, outnumbering those that stop. Numbers reach a peak in the northern tropics and, as the streams continue southwards, numbers then gradually decline, as birds progressively stop migrating and settle in their wintering areas. Nevertheless, substantial numbers of birds continue to the southern parts of the major flyways in South America or Africa. In spring, the reverse geographical trend in numbers occurs as the birds return northwards. Observation sites in temperate latitudes tend to produce seasonal counts up to tens of thousands of individuals, whereas those at lower latitudes can produce hundreds of thousands or millions, with the record from Veracruz in Mexico (about 22°N) where more than 6.6 million raptors and other soaring birds were counted in autumn 2001 (website; for earlier counts see Ruelas Inunza et al. 2000).

Other major flyways may remain to be discovered. In particular, only minor routes have yet been described around or through the Himalayas, yet India forms an important wintering area for raptors and other soaring birds. Similarly, Lake Baikal in Siberia is the largest body of freshwater in the world, yet to my knowledge only minor concentration points (<10 000 birds) on its edges have yet been described. Moreover, little is known of the routes taken by raptors through South America or Africa, although the radio-tracking of individuals suggests a continuation of narrow corridor routes in some species (see Fuller et al. 1998 for Swainson's Hawks Buteo swainsoni in South America (see Figure 8.6), Meyburg et al. 2002 for Lesser Spotted Eagles Aquila pomarina, Berthold et al. 2002, 2004 for White Storks Ciconia ciconia in Africa). In addition to the major flyways, many minor ones can be discerned, such as the Strait of Messina between Italy and

Sicily which is crossed by thousands of raptors in autumn and spring en route between Europe and North Africa (Corso 2001).

Many species perform 'loop migrations', taking mainly different routes on their outward and return journey, as mentioned earlier for the Amur Falcon Falco amu-rensis. There seems to be at least two loops for migration between Europe and Africa. In one, the southward movement occurs through Gibraltar and the northward one through Sicily (an anticlockwise loop). In the other, a southward route occurs across Arabia, down the east side of the Red Sea and crossing to Africa at Bab el Mandeb, and the northward route occurs up the west side of the Red Sea to cross from Africa at the Gulf of Suez (a clockwise loop). Both loops are used by many raptors, as well as by passerines and other kinds of birds. They have been demonstrated by counts on the different flyways, by ringing recoveries and by the satellite tracking of radio-marked birds (see Meyburg et al. 2003 for Steppe Eagle Aquila nipalensis). Similarly, in North America, many Peregrines Falco peregrinus migrate southward down the east coast, crossing the Caribbean Islands to South America. But on their return in spring, most Peregrines travel up through the centre of the continent, again like many other species (Chapter 8).

The numbers of birds counted at established watch sites vary greatly from day to day in the migration season, and from year to year. Even in the commoner species, counts may vary by more than two-fold from one year to the next. Much of this variation may be due to fluctuations in observer effort and to variations in weather, which influence the volume of migration on particular days, its altitude and route. Lateral displacement of the centre of the migration stream by only a few kilometres can mean that most birds are missed by observers based at a fixed site. Occasional high flights can also reduce the proportions of passing birds that are readily visible to ground observers. Compared with such effects, the proportion of the year-to-year variation in counts that is due to annual variation in population sizes is probably small, but such counts have nevertheless proved useful in revealing long-term trends in populations. As the birds are usually drawn from a wide area, any long-term trends they might show are likely to be widespread, overriding purely local changes. The counts from Hawk Mountain in Pennsylvania, and from Falsterbo in Sweden, which span several decades, have been used to assess long-term population trends (Roos 1978, Bednarz et al. 1990, Kjellen & Roos 2000), as have the shorter runs of counts from other sites elsewhere (for Israel see Shirihai et al. 2000). In some species, annual counts also indicated changes in the ratio of juveniles to adults during the 1950s-1970s, as reproductive rates were lowered by the effects of DDE (a metabolite of the insecticide DDT, known to cause reproductive failure through shell-thinning) (Dunne & Sutton 1986, Bednarz et al. 1990, Bildstein 1998). The numbers of raptors counted at many concentration sites in the past 50 years were probably small fractions of the numbers present several hundred years ago, before their populations were so greatly reduced by human activities.

Was this article helpful?

0 0

Post a comment