Use Of Thermals And Other Updrafts

Birds that travel over land by soaring-gliding flight mostly gain lift from rising air, whether in the form of thermals or other updrafts (Chapter 8). On many days, updrafts are formed when wind strikes a slope or cliff and is deflected upwards. Long mountain ridges thus provide excellent flyways for soaring migrants in those (relatively rare) places where the ridge lies roughly north-south, in a direction appropriate for migration. One such site is the Kittatiny Ridge in eastern North America, which provides a long unbroken source of lift on which hawks can glide with hardly a flap for up to 300 km, beginning in New York State and continuing past Hawk Mountain in Pennsylvania (Kerlinger 1989). The ridge deflects the prevailing west-north autumn winds upwards, at rates often exceeding 3-4 m per second, and sometimes to heights exceeding 300-400 m above ground. The hawks are mostly moving northwest-southeast when they encounter the ridge running northeast-southwest, at right angles to their path. For a time, they ride the upcurrents generated by the ridge, even though this activity takes them off their main direction. It gets them to lower latitudes, but eventually they must leave the ridge to continue their journey towards the southeast. Other long ridges used by migrating raptors occur in the Rockies of western North America, the Andes in South America, and in various mountains in the Middle East, including the Rift Valley which extends southwards into Africa. With knowledge of the needs of migrant raptors, topographic maps can be examined for likely-looking sites, parts of the world being largely unexplored in this respect.

Thermals are localised columns of rising air created mainly through the uneven heating of the ground by the sun (Box 7.1). These columns rise to high elevation, until they have cooled to the temperature of the surrounding air, where they often produce a cumulus cloud, marking their position. They usually begin in the morning once the ground has heated sufficiently, but gather strength during the day. They climb gradually faster and higher, often reaching more than 1000 m at noon, and then wane in the evening as the ground cools. They typically rise at 1-4 m per second (Kerlinger 1989). Birds progress on migration by circling in one thermal to gain height, and then gliding with loss of height to the next thermal where they rise again, repeating the process along the route (see Figure 3.5). This enables birds to travel across country at around 30-50 km per hour, depending on the rate and extent of rise within thermals and the distance covered between thermals (in turn dependent on the 'glide coefficient', which is the ratio between the horizontal distance covered by the bird and its altitude loss over that distance). Small species with light wing-loading ascend more rapidly so spend less time in each thermal; but in travelling between thermals they glide less rapidly than large species and so lose more height per unit distance; they can therefore glide less far before having to climb again (Chapter 3; Figure 7.2, Table 7.5). Yet small species can get underway earlier in the morning and continue later into the evening than large species with heavier wing-loading that are restricted to a shorter period each day when the thermals are strongest. The flight times of the smaller species migrating through Israel typically extend over 8-10 hours each day (beginning around 9 a.m.) and the larger ones over 6-7 hours (beginning around 10 a.m.). However, all species tend to make more rapid progress in the middle part of the day, when climbs are fastest and highest, and glides are longest (Figure 7.3). Particular species can travel across country twice as fast around noon than in the morning or evening. When a bird glides, it may partly fold its wings which reduces drag and thereby increases speed; but the reduced wing and tail area also provides less lift, so the bird sinks more rapidly.

Box 7.1 More about thermals

Thermal formation ultimately depends on heat from the sun, which is greater at mid-lower latitudes and passes through the atmosphere to heat the ground. The input of solar energy depends on whether the surface absorbs or reflects heat, and because the ground surface varies, heating is uneven. Undulating surfaces are more conducive to thermal formation than flat ones; land is more conducive than water, and bare earth is more conducive than snow and ice. Rock and dry sand heat more rapidly than damp soil. The bottom layer of air is heated locally by contact with the warm ground. This heated air then expands, reaching lower density than surrounding air, so rises as a thermal. Replacement air is sucked in at the base of the thermal, which in turn warms and rises, causing surrounding air to sink. As the ground heats through the day, thermals grow higher and faster. Condensation starts at a height which is determined by the temperature and water vapour content of the air, and gives rise to a cumulus cloud, which marks the position of the thermal. As the air temperature rises during the day, the cloud base rises too; and as heating declines during the evening, thermals gradually die out, until the next day. Particular thermals may last anything from about 20 minutes to several hours.

In light winds, thermals tilt in the direction of the wind, which causes passive drift in the birds using them. If necessary, this drift can be corrected in the next glide. Thermals sometimes form into lines (thermal streets) along the wind direction, and may even coalesce into roll vortices, giving continuous lines of lift. These are marked as 'cloud streets' which form above the condensation level. Soaring birds exploit cloud streets by flying along them above the line of thermals, but they are useful only when they run more or less along the path of migration, as at Panama. Strong winds can prevent thermal formation altogether, and even with winds in their migration direction, soaring birds are often grounded in such conditions.

Another condition in which rising air currents develop is when two air masses with different frontal characteristics converge, giving rise to a narrow zone of lift along the convergence. They include the sea breeze fronts that meet a heated land surface, so that coastlines are commonly used by gulls and other birds for soaring.

Figure 7.2 The relationships between wing load and average height band in thermals for four species of soaring migrants. Average climbing time in thermals (not shown) increased with increase in body weight. From Leshem & Yom-Tov (1996b).

Table 7.5 Average altitude (± SD) and daily progress velocity (± SD) of four species of soaring migrants over Israel, together with estimates of the daily flight time and distances travelled

Species

Number

Start

End

Height

Velocity

Mean

Mean

Estimated

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