Census of migrating raptors at Breginjski Stol (NW Slovenia) - the first confirmed bottleneck site in Slovenia

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Census of migrating raptors at Breginjski Stol (NW Slovenia) - the first confirmed bottleneck site in Slovenia

From 4 to 31 May 2010, raptor migration was monitored daily between 9.00 and 17.00 hrs CET at Breginjski Stol (NW Slovenia). In all, 2,385 raptor passes were counted, belonging to at least 17 species that were divided into resident and migratory birds. Residents (n = 875 passes) foraged, bred or daily migrated over the area. Among them, Griffon Vulture Gyps fulvus was the most frequent species (n = 575 passes) with the largest observed group of 35 individuals on 26 May. In the morning, Griffon Vultures were flying from west to east in search of food, whereas in the afternoon they were returning in the opposite direction to their colony in Forgaria nel Friuli (Italy). Their numbers increased after 15 May, when Croatian Griffons joined those from Italy. Altogether, 1,510 individuals of migratory raptors were counted, belonging to at least nine species. Among them, Honey Buzzard Pernis apivorus was the most common (1,368 ind., 90.6% of migratory raptors). The migration peak was reached on 14 May, with 552 individuals. Most raptors were seen migrating solitarily or in small flocks (2-4 ind.), whereas on five days (4, 7, 11, 13, 14 May) over 20% of all observed flocks were either medium-sized (5-15 ind.) or large (> 15 ind.). Raptors mostly migrated between 9.00 and 13.00 hrs. Taking into consideration several factors - short observation period and limited number of observation hours per day, overlooked raptors due to human- and topography-related causes, movements of observers between observation points, extremely bad weather and night migration of raptors - we estimate that the actual number of migratory raptors that passed Breginjski Stol in spring 2010 was 3,060-4,660 individuals. Thus, Breginjski Stol is the first confirmed bottleneck site of European importance for migratory raptors in Slovenia, as defined by BirdLife International IBA criterion B1iv, and a natural continuation of migratory pathways from northern Italy.

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