The Red Kite (Milvus milvus) is a common raptor in suitable areas of Austria, the Czech Republic, and Slovakia during the nonbreeding season. However, the exact number of birds wintering in these countries remains unknown. Through an integrated approach using satellite telemetry, direct field surveys at nocturnal roosts, and citizen science databases, we estimated the wintering population of Red Kites in this area during the winter of 2020/2021. Based on three surveys taking place at the beginning (28 November 2020), middle (9 January 2021), and end (5 February 2021) of winter, respectively, we counted 566, 558, and 536 Red Kites. The observations from online citizen science databases mostly appeared to correspond with the area of communal roost sites. Using our combined approach, the estimates of wintering Red Kites were determined as minimum numbers in the study area. A comparison of historical data (from the second half of the 20th and beginning of the 21st centuries, respectively) and our own results suggests that the wintering population of Red Kites in the study area has increased substantially. This dynamic could be caused by changes in climatic conditions, a shift of the birds' breeding range in Europe, changes in migration behavior and/or food availability, and/or conservation efforts.

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