Bota G., Traba J., Sardà-Palomera F., Giralt D. & Pérez-Granados C. (2020) Acoustic monitoring of diurnally migrating European Bee-eaters agrees with data derived from citizen science.. ARDEA 108 (2): 139-149

The use of Autonomous Recording Units (ARUs) for wildlife monitoring has increased in recent years. Acoustic monitoring has been used for a wide range of research topics, but it has rarely been used for monitoring wildlife migrations. In this work we evaluate the use of acoustic monitoring to characterise the diurnal migration pattern of a bird species, the European Bee-eater Merops apiaster. We set up 3–4 acoustic monitoring stations daily from 11 August to 21 September 2017 in north-eastern Spain, during post-breeding migration of the species. We used the Vocal Activity Rate (VAR), defined as the number of calls per unit time, as an index of Bee-eater abundance to describe the daily and seasonal migration pattern of the species. We also assessed the relationship between daily mean VAR estimated by ARUs, with citizen science data uploaded to the platform Ornitho.cat over a large spatial scale. According to mean VAR, intensity of migration increased weekly until the last week of August when it peaked, with species abundance decreasing to the lowest values in the study area by late September. A significantly higher number of calls was detected in the first and last hours of the day. Our results agree with previously published seasonal and daily migration patterns described for the species. VAR was significantly, positively correlated with the percentage of citizen science records of Bee-eater uploaded to complete checklists, used as an independent source to compare migration timing. Overall, our results, validated through citizen science data, show that acoustic monitoring can effectively provide complementary data for monitoring the bird migration of vocally active species. The use of ARUs may help to improve our understanding of migratory behaviour and be useful for a wide range of purposes.