with a high horizontal resolution but no altitudinal resolution. Surveillance
radars are therefore used when studying migration intensity and general
migration direction. By using a continent-wide network of weather surveillance
radars, bird migration traffic can be quantified continent-wide and related to
weather conditions (Figure 7.3). This approach replaces the moon-watching
method, is less weather dependent, and requires no field observers (Gauthreaux
et al. 2003).
A tracking radar emits a narrow “pencil beam” (1.3–2) by which individual
birds or flocks can be tracked. When operated in automatic tracking mode the
radar records repeat measurements of distance, elevation, and azimuth angles to
the target, from which speed and direction can be calculated. Wind profiles are
obtained by using the radar to track ascending weather balloons carrying an alu-
minium foil for maximum reflectance. Heading and airspeed are then calculated
from the tracking data against the wind (Figure 7.2). The radar echo often shows
rhythmic fluctuations that can be recorded and used to estimate the wing-beat
frequency of the bird—a useful measure of size when the target could not be
identified. Tracking radar studies should be combined with visual observations
for specific identification of birds being tracked, although this is sometimes
impossible under cloud cover and at long distances.
168 |Migration
Fig. 7.3Radar-based map of nocturnal bird migration over the United States within
the altitudinal band of 430–1075 m for the period 26–30 April 2000. Arrows show
mean direction of migration and different pattern of arrows indicates the mean number
of birds (km^3 ). From Gauthreaux et al. (2003).
Birds per cubic km
0–25
26–50
51–75
76–100
101–125
126–150
> 150