was a watch, the flight direction can be derived (Nisbet 1959). The apparent speed
is mainly determined by the distance from the observer. Moon watching has been
used on a continent wide scale (North America) to give a snapshot of the overall
migration intensity and flight directions (Lowery and Newman 1966). The draw-
backs are that it can only be used near full moon in clear weather and the observa-
tion cone has a relatively small angle (on average 0.52). Using a telescope with
40 magnification about 50% of the birds are detected at 1.5 km distance and
zero at 3.5 km (Liechti et al. 1995).
By pointing a ceilometer (a strong directed light) toward the sky and observing
birds blipping past the light using binoculars information can also be obtained on
flight direction (Gauthreaux 1969). Ceilometer observations are mainly useful for
studying low flying birds (up to 500 m), but can be used in overcast conditions.
The technique is also useful to study orientation in relation to local topography
(Åkesson 1993).
7.3.2 Tracking migrants
With an optical range finder (e.g. Leica Vector), the distance to a bird can be
measured, and furnishing the instrument with azimuth and elevation scales
provide polar coordinates to the bird, which can easily be converted to space
coordinates (x, y, z) (Hedenström and Alerstam 1996). Multiple registrations of
positions allow reconstruction of flight tracks and analysing the data in relation
to wind speed and direction at each altitude, the flight speed and direction in
relation to the air are obtained (Figure 7.2). The wind profile can be obtained by
tracking ascending helium-filled weather balloons using the range finder. If the
data are fed directly to a computer the instrument should be referred to as an
“ornithodolite” (Pennycuick 1982).
Using radio-transmitters with ground-based receivers (see Chapter 6), migra-
tory birds can be tracked during stopover and at departure, when timing of
flights and vanishing directions are obtained (Åkesson et al. 1996). The radio
166 |Migration
Fig. 7.2Triangle of velocities. The track vector (T) is the sum of the heading (H) and
wind (W) vectors. The angle between track and heading shows the amount of drift
with constant heading, while the angle shows the wind direction in relation to the
track direction.
HWTαβ