© Marcel Burkhardt
Schmid, B., S. Zaugg, S. Votier, J. Chapman, M. Boos & F. Liechti (2019)
Size matters in quantitative radar monitoring of animal migration: estimating monitored volume from wingbeat frequency
Ecography 42: 931–941
aeroecology, environmental impact assessment, radar
Quantitative radar studies are an important component of studying the movements of
birds. Whether a bird, at a certain distance from the radar, is detected or not depends
on its size. The volume monitored by the radar is therefore different for birds of different
sizes. Consequently, an accurate quantification of bird movements recorded by
small-scale radar requires an accurate determination of the monitored volume for the
objects in question, although this has tended to be ignored.
Here, we demonstrate the importance of sensitivity settings for echo detection on
the estimated movement intensities of birds of different sizes. The amount of energy
reflected from a bird and detected by the radar receiver (echo power) depends not only
on the bird’s size and on the distance from the radar antenna, but also on the beam
shape and the bird’s position within this beam. We propose a method to estimate the
size of a bird based on the wingbeat frequency, retrieved from the echo-signal, independent
of the absolute echo power. The estimated bird-size allows calculation of sizespecific
monitored volumes, allowing accurate quantification of movement intensities.
We further investigate the importance of applying size-specific monitored volumes to
quantify avian movements instead of using echo counts.
We also highlight the importance of accounting for size-specific monitored volume
of small scale radar systems, and the necessity of reporting technical information on
radar parameters. Applying this framework will increase the quality and validity of
quantitative radar monitoring.