signal from a small transmitter (e.g. 0.7 g) can be picked up at 20 km distance
from a departing and climbing bird. This method has shown that even though
most nocturnally flying birds initiate their flights shortly after sunset, birds may
depart at any time during the night ( Åkesson et al. 1996, 2001b).
The use of satellite telemetry has revolutionized the tracking of individual
birds. Positions of birds are obtained from the Argos satellite system, which is a
polar-orbiting-based system (Argos 1996). Argos provides position data includ-
ing accuracy classifications for each position obtained. One should be aware that
the precision of locations may be considerably lower than what manufacturer’s
data suggest. The life span of transmitters is mainly determined by the size of the
battery and hence the weight (minimum about 18 g), but the duration can be
extended using small solar panels. Thereby, the entire round-trip migration
including the wintering period of an individual can be tracked. The data obtained
are ideal for analyzing overall migration speed, stopover locations and duration,
ground speed of flight, orientation, and navigation abilities. The only drawback is
that the transmitters are still too heavy for use on small birds and the technique is
therefore restricted to medium sized and large birds, such as geese and eagles.
7.3.3Remote sensing: infrared device
Because birds are relatively warm objects in relation to the ambient air, a sensitive
infrared sensor can detect the radiation from birds flying overhead across the sky.
By pointing a thermal imaging device of 1.45opening angle to the sky, migrat-
ing birds can be detected from 300 m up to 3000 m (Zehnder et al. 2001). Flight
tracks are recorded on video and targets are classified into size classes to estimate
flight altitudes. Infrared sensors work best at night under clear skies. The method
detects birds at higher altitudes than ceilometer observations.
7.3.4Remote sensing: radar
Radars are the most powerful tools available for tracking migrating birds. A radar
emits short pulses of radio waves and records echoes of these from targets,
whether birds and aeroplanes. Since the radio waves travel by the (constant)
speed of light the distance between the radar and target is determined from the
time delay between emitting a pulse and receiving the echo. A great advantage is
that radars can be used in overcast conditions, and at any time of day and night.
Accounts of the principles and technical basis of radars are given by Alerstam
(1990) and Bruderer (1997).
Surveillance radars are mainly used for air traffic control at airports. They
have a fan-beam of wide vertical angle (10–30) and a narrow angle in the hori-
zontal plane ( 2 ). By rotating the radar antenna the sky is scanned for echoes
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