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Birds KM

Kilometers

Figure 2.1 Map depicting bird migration over the United States within the altitude zone 196-2530 m on the nights of 10-11 May 2002. Arrows reflect the positions of weather surveillance radars, and show the directions and volumes of migration overhead. Map provided by S. Gauthreaux. For further details, see Gauthreaux et al. (2003).

In contrast to surveillance radar, a tracking radar emits a narrow 'pencil beam' by which individual birds or flocks can be tracked. When operated in automatic tracking mode, the radar locks onto a particular target bird (or flock) and records repeat measurements of distance, elevation and azimuth angles, from which the speed and direction of the tracked migrants can be calculated, and their flight trajectories plotted in three-dimensional space (Bruderer et al. 1995). Alternatively, the beam can be used in a conical scanning mode to provide information on the spatial distribution of migrants (although calculations of bird numbers from conical scanning present problems). Wind profiles can be obtained by using radar to track ascending weather balloons carrying aluminium foil for maximum reflectance. The heading and airspeed of the birds can then be calculated from the tracking data against the wind data. Another radar technique involves a vertical set designed to quantify the amount of migration taking place and the heights at which the birds are flying. This gives similar results to those obtained using a ceilometer, except that the radar can look through clouds and detect the birds at all heights.

Another method of remote detection involves the use of an infrared sensor to pick up the heat radiated from birds flying overhead. By pointing a thermal imaging device of 1.45° opening angle to the sky, migrating birds can be detected from 300 m up to 3000 m (Zehnder et al. 2001). Flight tracks are recorded on video, and targets are grouped into size classes to estimate flight altitudes. Infrared sensors work best at night under clear skies, so are not good for assessing weather effects on migration. If such an instrument is combined with a distance measure, quantitative information on migration can be obtained.

The most comprehensive picture of migration is obtained by a combination of radar and visual observations by ground-based observers in the same area. However, in their studies of migration of soaring birds in Israel, Leshem & Yom-Tov (1996a) also used a motorised glider in which they could actually accompany flocks of large soaring birds on part of their journeys. This enabled these researchers to record in detail the ups and downs of the birds' flight, as they climbed in each thermal and glided, losing height, to the next.

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