Abstract
Radar technology has emerged as a vital tool for studying bird migration, particularly due to the predominantly nocturnal and high-altitude nature of the phenomenon. Avian Vertical-looking Radars (AVLRs) offer detailed insights into migratory patterns, including flight behavior and altitude. Bird ringing, on the other hand, although useful for describing migratory phenomena at the species level, has limitations when used to quantify migration itself. In this study, radar data from the Bolle di Magadino wetland area in Switzerland were analyzed to describe and quantify the spring migration of passerine birds during 2021 and 2022. Migration activity was significantly higher during the night, with migratory peaks reached at night in mid-April in 2021 and in the second half of April in 2022. Passerine flight altitudes remained consistently higher at night than during the day. Also, the analysis of wing-flapping frequency (WFF) revealed both seasonal and diel variations, with WFF slowly increasing at night as the season went on, but remaining higher and stable during the day. A GLM did not reveal significant effects of rain and wind intensity on migratory traffic, but it did show a positive and strongly significant relationship with the number of ringed birds (beta = 0.55, p < 0.001). This confirms for the first time a positive proportionality between ground-based monitoring and radar-based monitoring in a stoHpover area, where theoretically we would have expected a weaker relationship between the number of ringed birds and MTR, compared to places where the migratory flow is uninterrupted and the birds have no means to stop.