Characteristics of soaring birds' spring migration over inland SE Bulgaria
We investigated the scale and characteristics of spring migration of soaring birds in Bourgas district, SE Bulgaria during three 5-day periods in March (27th-31st), April (20th-24th) and May (12th-16th) 2008. Simultaneous counts were carried out from six observation points in two separate areas (Karnobat and Sredets), located 24.5 km and 37.3 km inland from the Black Sea coast, respectively. In total, 31,049 soaring birds of at least 27 species were counted, by far the most numerous being the White Stork Ciconia ciconia (23,358 ind.), followed by the Lesser Spotted Eagle Aquila pomarina (3,112 ind.). In Karnobat area, situated farther from the Black Sea coast, the total number of soaring migrants (21,231) was more than two-fold higher than in Sredets area (9,818). Overall, there was a significant difference in flock size and flight altitude within separate area (i.e. among observation points) as well as between the two observation areas and the three 5-day periods. However, there was a substantial variance in factors significantly affecting the flock size and flight altitude among most numerous individual species. In general, birds passed at much higher altitude above Karnobat area (mean flight altitude 208.3 m ± 6.4) compared to Sredets area (mean flight altitude 130.5 m ± 4.4). In both observation areas, northern direction was the most commonly observed flight direction (46.3% of all recorded flight directions). The highest numbers of soaring migrants passed in the late afternoon, between 15.00 and 18.00 hrs - 11,554 individuals (43.0% of all birds counted). The study confirmed that the front of Western Black Sea migration route, or "Via Pontica" as part of the Mediterranean/Black Sea Flyway in SE Bulgaria, is rather wide, since the magnitude of inland migration in Bourgas district can be compared with that along the Black Sea coast, at least during spring migration. Both observation areas qualify as Important Bird Areas (IBA) of European importance according to the several BirdLife International criteria. We recommend that any new wind farms built should avoid the area of Hisar Hill south of Karnobat and the NW parts of Sredets Municipality.