Abstract
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 size‐specific 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.
Highlights
The lowest one to two kilometres of the atmosphere host huge quantities of animal movements, often invisible to the human eye (La Sorte et al 2015, Hu et al 2016, Chilson et al 2017, Bruderer et al. 2018)
Once corrected for the distance according to the radar equation, the echo size is expressed as the radar cross section (RCS [m2], see Lexicon), assuming objects have the same reflectivity properties
For a given object size, the RCS is maximal when the object passes through the beam axis, and 199 minimal at the detection threshold at the periphery of the beam
Summary
The lowest one to two kilometres of the atmosphere host huge quantities of animal movements, often invisible to the human eye (La Sorte et al 2015, Hu et al 2016, Chilson et al 2017, Bruderer et al. 2018). The significance and demand for quantitative radar studies on animal movements in the context of ecological or environmental impact assessment studies has increased considerably (Bridge et al 2011, Bauer and Hoye 2014). The monitored volume can be estimated using the radar equation (see Lexicon, (Eastwood 1967), requiring information on radar parameters and reflectivity properties of the objects. The RCS is the echo size corrected for the distance along the main axis, but the RCS is related to the object size only for objects that transit through the beam centre. Because the echo size decreases with increasing distance from the beam axis, objects illuminated in the periphery of the beam will appear smaller (have smaller echo size) than an object of the same size detected in the beam centre (Figure 1b). The RCS is a minimal measure of the object size
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