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Measurements made from a motorised glider and light aircraft, as well as by radar. Species listed in decreasing order of body weight. From Leshem & Yom-Tov (1996a). aEstimated time taken to travel between the centre of the breeding range and the centre of the wintering range, assuming that migration occurred on every day over the daily distance shown. These estimates are lower than those uncovered by radio-tracking of the same species, mainly because migration did not occur on every day (Chapter 9).

Measurements made from a motorised glider and light aircraft, as well as by radar. Species listed in decreasing order of body weight. From Leshem & Yom-Tov (1996a). aEstimated time taken to travel between the centre of the breeding range and the centre of the wintering range, assuming that migration occurred on every day over the daily distance shown. These estimates are lower than those uncovered by radio-tracking of the same species, mainly because migration did not occur on every day (Chapter 9).

Figure 7.3 Route and altitude in relation to ground level for a flock of Great White Pelicans Pelecanus onocrotalus followed by glider through Israel on 12 October 1987. Each dot represents a point where the first birds started to climb or descend in a thermal. Time of day and average velocity are given along the bottom, and the route from take-off to landing that day is shown in the map on the right. From Leshem & Yom-Tov (1996b).

Figure 7.3 Route and altitude in relation to ground level for a flock of Great White Pelicans Pelecanus onocrotalus followed by glider through Israel on 12 October 1987. Each dot represents a point where the first birds started to climb or descend in a thermal. Time of day and average velocity are given along the bottom, and the route from take-off to landing that day is shown in the map on the right. From Leshem & Yom-Tov (1996b).

In Steppe Eagles Aquila nipalensis studied in Israel, cross-country speed was related to the climb rate in thermal circling. Over the whole diurnal cycle, the mean climb rate in thermals was 1.9 m per second, but rates up to 5.0 m per second were reached around noon. Mean gliding air speed between thermals was 56 km per hour which, allowing for climb times, gave a mean cross-country speed of 45 km per hour. The upper limit of migration was about 1600 m above ground, but was mostly below 1000 m (Spaar & Bruderer 1996). Smaller Honey Buzzards Pernis apivorus and Steppe Buzzards Buteo buteo vulpinus achieved lower crosscountry speeds, at 37 and 35 km per hour. However, in a 12-hour day, Steppe Eagles soared for only 6 hours and covered 270 km, whereas the two smaller species migrated for 10 hours and covered 360 km. Hence, although larger birds can travel faster and further between thermals, they do not necessarily cover more kilometres per day (Spaar & Bruderer 1996). In cooler climates, where thermals occur during a smaller part of each day, migration times are shorter, and larger raptors, such as Golden Eagles Aquila chrysaetos, travel for at most a few hours per day, chiefly between 12.00 and 14.00 hours.

Figure 7.4 Gliding speed (left) and cross-country speed (right) relative to the air in soaring-gliding flight in raptors of different body mass. BE, Booted Eagle Hieraaetus pennatus; BK, Black Kite Milvus migrans; EV, Egyptian Vulture Neophron percnopterus; GV, Griffon Vulture; HB, European Honey Buzzard Pernis apivorus; LS, Levant Sparrowhawk Accipiter brevipes; LSE, Lesser Spotted Eagle Aquila pomarina; MPH, Montagu's/Pallid Harrier Circus pygargus; MH, Marsh Harrier Circus aeruginosus; SB, Steppe Buzzard Buteo buteo vulpinus; SE, Steppe Eagle Aquila nipalensis; SF, Small falcon (ca. 220 g), StE, Short-toed Eagle Circaetus gallicus. Cross-country speed = 2.67 x log body mass + 1.37; 95% confidence interval of the slope: 1.38-11.23; r11 = 0.63, P < 0.05. Note that with a following wind, all speeds would be higher. From Spaar (1997).

Figure 7.4 Gliding speed (left) and cross-country speed (right) relative to the air in soaring-gliding flight in raptors of different body mass. BE, Booted Eagle Hieraaetus pennatus; BK, Black Kite Milvus migrans; EV, Egyptian Vulture Neophron percnopterus; GV, Griffon Vulture; HB, European Honey Buzzard Pernis apivorus; LS, Levant Sparrowhawk Accipiter brevipes; LSE, Lesser Spotted Eagle Aquila pomarina; MPH, Montagu's/Pallid Harrier Circus pygargus; MH, Marsh Harrier Circus aeruginosus; SB, Steppe Buzzard Buteo buteo vulpinus; SE, Steppe Eagle Aquila nipalensis; SF, Small falcon (ca. 220 g), StE, Short-toed Eagle Circaetus gallicus. Cross-country speed = 2.67 x log body mass + 1.37; 95% confidence interval of the slope: 1.38-11.23; r11 = 0.63, P < 0.05. Note that with a following wind, all speeds would be higher. From Spaar (1997).

As expected, weather conditions also affect progress. On one favourable occasion, White Storks Ciconia ciconia over Israel were able to climb to 1550 m and then glide for 36 km before needing to climb again. Their average flight speed was 57 km per hour, about 47% faster than average. The crossing of short stretches of sea, as at the Bosphorus or Gibraltar, ideally needs to be accomplished in one long descending glide, in order to avoid laborious flapping flight. Early in the day, when conditions are not ideal, birds sometimes start a crossing, and having reached a few kilometres from shore, turn back to try again later.

Comparisons between the various species of soaring raptors in Israel revealed that: (1) the average climbing rate in thermal circling was independent of body size, at 1.5-2.1 m per second, although smaller species had a smaller turning radius, so in theory could benefit more from the faster currents in the centre of the column; (2) in inter-thermal gliding, air speed was positively related to body mass, and gliding angle negatively related to body mass - heavier species glided faster, and at shallower angles, losing less height per unit distance; (3) overall cross-country speed relative to the air was positively related to the species body mass, the larger species travelling faster but for fewer hours each day (Figure 7.4, Table 7.5; Spaar 1997). The better gliding performance of larger raptors fits theoretical considerations about gliding flight (Chapter 3). The same principles apply to sailplanes, and to increase gliding speed, pilots sometimes add water as ballast. The faster a bird glides, the more lift it gains from the airflow below its wings, over and above any obtained from localised rising air currents.

In some topographic situations, given appropriate wind and temperature conditions, updrafts called leewaves are sometimes formed on the downwind sides of mountains, reaching much greater altitudes than thermals. Because of their restricted distribution, leewaves are unlikely to be of widespread importance in soaring bird migration, but they could be used by raptors in some situations, as when crossing the Strait of Gibraltar (Evans & Lathbury 1973) or moving through Panama (Smith 1985a). Migrants using leewaves would normally be invisible to ground-based observers.

When migrating over land or water, where no thermals are available, some raptors can make use of a following wind, using dynamic soaring. The birds glide downwind, picking up speed, but gradually losing height, then turn into the wind to gain lift, thus regaining the altitude lost in the glide, before turning again into the next glide to continue their journey. They continue this looping flight for long distances, but are constrained to travel more or less downwind. Progress can be quicker than in thermal soaring, because the birds spend less time in gaining altitude. They can also fly in moderate-angled crosswinds which provide the lift, but if conditions turn against them, they have to switch to energy-demanding flapping flight if they are not to be blown off course. Using these flight modes, birds are less restricted to the middle parts of the day, and flights may begin at dawn, as at Kenting on the southern tip of Taiwan. The main participants at this site are Grey-faced Buzzard Eagles Butastur indicus and Chinese Sparrowhawks Accipiter soloensis, but many other species are seen in smaller numbers. The Chinese Sparrowhawks, detected over the sea by radar, occurred in long straggling flocks up to 21 km long, travelling at an average of 20 km per hour.

While most soaring bird migration occurs by day, for the reasons given earlier, some species have been seen flying over the sea or other hostile terrain at night. For example, Western Honey Buzzards Pernis apivorus have been recorded by radar migrating over Malta at night (Elkins 1988), and have appeared on dark nights in autumn at the lighthouse on Heligoland Island off Germany (Gatke 1895). In addition, Levant Sparrowhawks Accipiter brevipes have been found to enter tree roosts at night in Israel after descending from a daytime desert flight (Yosef 2003). Other raptor species that have occasionally been seen flying at night include Northern Harrier Circus cyaneus, Osprey Pandion haliaetus and Peregrine Falco peregrinus, and other overwater migrants, such as Chinese Sparrowhawk Accipiter soloensis, Grey-faced Buzzard Eagle Butastur indicus and Amur Falcon Falco amurensis (DeCandido et al. 2006).

Weather affects soaring landbirds in some of the same ways as other birds. Wind strength and direction influence the number of soaring birds that migrate on particular days, their travel routes, flight altitudes and speeds. Low dense cloud and rain suppress migration altogether. To provide ideal migration conditions, however, different wind directions and temperatures are needed at different sites, depending on local topography, and on the importance of thermals as opposed to updrafts from slopes. For compared to other birds, soaring species are more dependent on the interaction between wind and topography than on wind alone. In addition, thermals cannot form in strong winds which, in the absence of other updrafts, bring soaring bird migration to a standstill, however favourable the wind direction. Hence, on some days that are ideal for most migrating birds, soaring species may be grounded (Box 7.1).

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