The use of radar for the systematic recording of bird migration began in the 1950s. A radar emits short pulses of radio-waves and records their echoes from targets, whether birds or aeroplanes. Because radio-waves travel at the constant speed of light, the distance between the radar and the target can be calculated from the time lapse between pulse emission and echo reception. The use of radar revolutionised the study of bird migration because it made observations almost independent of flight altitudes and weather, totally independent of light conditions, and hence fully comparable by day and night. It has taught us much about unseen migration and about the influence of weather on bird movements (Chapter 4). It has provided reliable information on the seasonal and diurnal timing of migration, and on the speed, direction and altitude of flight (for reviews, see Eastwood 1967, Bruderer 1997a, 1997b, Gauthreaux et al. 2003). Radar also swiftly disposed of the idea that migration occurred only in spring and autumn. Birds of one species or another could be seen migrating somewhere on earth at almost any time of year.
Individual birds can be followed by radar over enough of their journeys to reveal how they orientate during migration and react to different weather conditions, and hence how their flight behaviour is shaped by prevailing atmospheric conditions. The density of birds on a radar screen cannot be precisely related to the true number of birds flying over (because several birds flying close together may appear as a single echo-spot), but it provides a relative measure of abundance that can be used by day and night.
The most obvious disadvantage of radar work is the cost: the equipment itself is expensive, and trained personnel are needed to maintain and operate it. For the most part, it is available only at a limited number of fixed installations (although mobile units are also available). The main operational drawback is that the identities of the species are usually unknown, apart from broad categories distinguished by body size, flight speed, or wing-beat patterns. The radar echoes often show rhythmic fluctuations that can be recorded and used to estimate the wing-beat frequency. This procedure enables waders and waterfowl (continuous wing-beats) to be distinguished from passerines (wing-beats broken by pauses), and perhaps two size classes in each group. Other drawbacks are that birds flying close to the ground below the radar horizon are usually missed, and back-scatter from the ground can sometimes blur the image. Surveillance radars, like those used for traffic control at airports, have a fan-beam of wide vertical angle (10-30°) and narrow horizontal angle (up to 2°). By rotating the radar antenna, a wide swathe of sky can be scanned for echoes with a high horizontal resolution, but no altitude resolution. Spanning an area of more than 100 km across, surveillance radars are therefore good for studies of migration intensity, speed and general direction. On some modern radar sets, small songbirds can be detected to beyond 100 km, and larger birds to more than 500 km, providing they are high enough (Bruderer 1999). With most radar sets, the displays can be easily recorded on film for subsequent playback and analysis. A useful way of recording the slow-moving echoes of birds is with time-lapse photography, the radar screen with a clock beside it being photographed with a cine camera every 1-2 minutes. Projected at normal speed, a whole night's migration can then be viewed in a few minutes. The combination of records from many different surveillance radars at different locations has been used to provide a broad picture of bird migration on particular dates over large regions including much of North America (Figure 2.1; Lowery & Newman 1966, Gauthreaux et al. 2003). The latter study used a new system of WSR-88D weather surveillance radars. Nocturnal data on such a huge geographical scale could not have been obtained in any other way.
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