Patterns of Bird Migration Defined by a Weather Radar at Part of the East European Flyway (Via Pontica)

The results of the first comprehensive radar survey of the spring and autumn bird migration in Bulgaria in a key area of the migratory flyway Via Pontica are presented. The weather radar of Bulgarian Air Traffic Services Authority (BULATSA) located nearby the town of Aksakovo (Varna Region, North-Eastern Bulgaria) was used. The survey covered 1101 days and nights of four years (2014-2017). It was found that during the autumn migration the average Migration Traffic Rate (MTR) was 2.5 times higher than during the spring migration, and the intensity of nocturnal migration was on average 14 times higher than the diurnal one. The autumn nocturnal migration was the most intense, with an average of 17 times more birds than those during the day time in the same period. By contrast, in spring the nocturnal migration was only 11 times higher than the diurnal migration. Seventy four percent of birds were found in the height range of up to 900 In a.g.l. The evaluation of the quantitative characteristics of bird migration in North-Eastern Bulgaria presented in this study could serve as the basis for future monitoring of the important bird migration characteristics in a long-term period when the data of weather radar network will be available for larger territories in Europe.