Size matters in quantitative radar monitoring of animal migration: estimating monitored volume from wingbeat frequency

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.

Cross‐calibration of different radar systems for monitoring nocturnal bird migration across Europe and the Near East

“…While migration monitored by the bird radar at the Fehmarn Belt consists exclusively of birds crossing the Baltic sea (coming from Scandinavia), the WR with their much larger surveyed range include a significant part of landbound migration coming from the northeast. This interpretation is in line with the results presented by Nilsson et al (), showing that the influx of migrants from the northeast makes up the major part of nocturnal migrants tracked across northern Germany.…”

“…A1. Extraction of bird profiles followed the protocol implemented by Dokter et al (), as available in the R‐package bioRad (Dokter et al ). To translate intensity of reflectivity into bird density we used a seasonal‐average radar cross‐section of 11 cm 2 .…”

“…The value corresponds to an average size in between an European robin Erithacus rubecula and a song thrush Turdus philomelos . Data were extracted only for nocturnal migration, because quantification of diurnal migration is still a major and not yet solved issue (Dokter et al ). The night was defined as the period of time between evening and morning civil twilights, which represents the time when the sun position is at least six degrees below the horizon.…”

“…To calculate MTR, the WR transect was kept perpendicular to the extracted ground speed direction at all times (Dokter et al ). For each study region we calculated mean MTR per night for all height ranges covered by an individual system, and additionally calculated a mean MTR for the overlapping height range of the different radar systems (Supplementary material Appendix 1 Table A1, Fig.…”

“…Another drawback is that these systems operate in different scanning modes ruled by the national met‐offices (Huuskonen et al ). It is therefore not surprising that most published studies on bird migration using multiple WR systems are based on the US‐WR network NEXRAD (Buler and Diehl , Horton et al 2016, La Sorte et al ); but see Nilsson et al ().…”