The Autumn Migration of Soaring Birds at Bourgas Bay, Bulgaria Tanyo Michev, Lyubomir Profirov, Konstantin Nyagolov and Milko Dimitrov Dan Powell
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The autumn migration of soaring birds at Bourgas Bay, Bulgaria Tanyo Michev, Lyubomir Profirov, Konstantin Nyagolov and Milko Dimitrov Dan Powell White Pelicans Pelecanus onocrotalus Abstract The results of a survey of the autumn migration of soaring birds via the western Black Sea migration route (‘Via Pontica’), at Bourgas Bay, Bulgaria, are presented here, for five species of waterbird and 33 species of raptor. Visual observations were carried out on 1,640 days in August, September and October between 1979 and 2003. The mean number of migrating waterbirds logged in autumn during the survey was 169,072 individuals (max. 250,623 in 1999), and the corresponding figure for migrant raptors was 38,534 (max. 65,065 in 1990). These data confirm that Bourgas Bay is one of the most important sites for monitoring the migration of soaring birds in Europe, and the most important for the autumn migration of several species, including White Pelican Pelecanus onocrotalus, Dalmatian Pelican P. crispus, White Stork Ciconia ciconia, harriers Circus, Levant Sparrowhawk Accipiter brevipes, Lesser Spotted Eagle Aquila pomarina and Red- footed Falcon Falco vespertinus. For a majority of the main species, the numbers recorded were either stable or increased over the study period, although eight species showed a declining trend. The data collected during 20 years of the Bourgas survey provide the opportunity to track long-term changes in the populations of migrant waterbirds and raptors in eastern Europe. 16 © British Birds 104 • January 2011 • 16–37 The autumn migration of soaring birds at Bourgas Bay, Bulgaria any of the raptors and large water- corridors of bird migration in the world birds that breed in the northern lat- (fig. 1). Mitudes of Europe migrate south to In Bulgaria, the Black Sea coast and spend the winter in warmer regions. A high nearby hills play an important role in the ori- proportion of the species involved migrate entation of these migrant soaring birds, and diurnally and complete the journey relatively the nature of the coastline near the town of quickly. In order to conserve energy they Bourgas creates a significant bottleneck. adopt passive – in other words soaring and From Cape Emine, the coast turns sharply gliding – rather than active flight, exploiting west for 15 km, along the northern edge of the thermals that develop over land masses. Bourgas Bay, then continues south and west Such birds are reluctant to cross large in a stepped fashion for a further 20 km. The stretches of open water, over which passive Black Sea reaches its westernmost extremity flight is difficult, and this leads to spectacular near the town of Bourgas and this is an ideal concentrations at strategic points. Soaring area for observing autumn migration. birds leaving Europe in autumn, en route to Beyond Bourgas the route diverges: some wintering grounds in Africa, use one of two birds follow the coast while others continue main migration routes: the Atlantic flyway inland. (which includes the bottlenecks at Falsterbo Reports on the migration of soaring in Sweden and Gibraltar, at the tip of the raptors using the western Black Sea flyway at Iberian Peninsula) and the western Black Sea the Bosporus were published in the late nine- flyway, also known as Via Pontica (Zalles & teenth century (Alleon & Vian 1869, 1870). Bildstein 2000). From the west coast of the Subsequently, observations covering the Black Sea, and the well-known bottleneck at whole migration period there were docu- the Bosporus in Turkey, the route continues mented by Porter & Willis (1968), Beaman et south across Israel and Jordan before al. (1975), Robel & Brauning (1992) and reaching Africa at the northern end of the Bossche & Lens (1994). In Bulgaria, Patev Red Sea, and is one of the most significant (1930, 1950) made an early attempt to find a similar watchpoint where soaring birds became concentrated (see also Baumgart 1971), while some details of the raptors migrating through the Bourgas area were given by Prostov (1964), Königs- tedt & Robel (1978), Laine (1978) and Donchev (1980). Kumerloeve (1957), Roberts (1979) and Michev & Simeonov (1981) made the first quantitative observations near Bourgas. Farther south, there have been a number of important studies of the autumn migration of soaring birds by ground-based observers in Israel, but the ten-year survey reported by Alon et al. (2004) pro- vides a convenient comparison for our data from Bourgas. Methods A complete autumn migration Fig. 1. The eastern part of the Balkan Peninsula (lighter colours) showing the location of the Danube Delta, western survey was carried out from an Black Sea coast including Bourgas Bay, the Bosporus and observation point at Lake Dardanelles. Atanasovsko, close to the town of British Birds 104 • January 2011 • 16–37 17 Michev et al. 42°33’32.9’’N 27°29’17.8’’E). When weather conditions were favourable, it was possible to monitor migration across an 8-km front of the flyway for the larger species. The observation post was manned daily by one or more observers, between 08.00 and 18.00 hrs local time. Each observer used binoculars and a binocular telescope (Carl Zeiss Jena 20–40 x 120). Species not identi- fied with certainty were logged as a species pair where possible (e.g. Pallid Circus macrourus and Montagu’s Harriers C. pygargus) or simply to genus. It was also difficult to separate migrants from some resi- dent or lingering birds which hunted in the area of the watchpoint, such as Marsh Harriers C. aeruginosus, Eurasian Sparrow - hawks Accipiter nisus and Common Kestrels Falco tinnunculus. Data collected included number of indi- viduals, time, distance and direction from the Fig. 2. Lake Atanasovsko watchpoint. watchpoint, direction, height and character of flight. Direction of flight, as well as an esti- Bourgas, from 1979 to 2003. Lake mate of distance from the watchpoint, was Atanasovsko forms part of the largest determined using particular landmarks at wetland complex in Bulgaria: a system of known distance/direction. The height of four lakes, with a total area of some 9,000 ha, flight was estimated visually. all of which are designated as Ramsar sites When possible, migrant flocks were and IBAs, and form part of the European counted in tens or hundreds, while estimates Natura 2000 Network (Kostadinova & Gra- were made of the larger flocks of storks and matikov 2007). pelicans. Whenever possible, repeat counts of Complete survey coverage, from 10th the same flock were made and the mean August to 31st October, was possible in most noted. We believe that count accuracy of the years from 1979 to 2003. However, there was estimates of larger flocks is within 10–15% of no coverage in 1994–95 and only partial cov- the actual numbers. erage in 1997, 2000 and 2002, and the results In 1982 and 1983, radar observations were from those years are omitted here. During undertaken, using the Plessey radio locators the survey, over 150 ornithologists, both pro- with four diapasons, situated at 15, 30, 60 fessional and amateur, participated in the and 120 km from the international airports observations. at Bourgas and Varna. The radar observa- Visual observations were made from the tions ran simultaneously with the visual same place throughout the survey, on the counts. With the help of observers at Varna eastern shore of Lake Atanasovsko (fig. 2; airport, Cape Emine and Bourgas Bay (fig. Table 1. Categorisation of species status based upon numbers of soaring birds counted during the autumn Bourgas surveys, 1973–2003. Status Numbers recorded per season Very rare Occurred in fewer than ten migration seasons Rare Occurred in more than ten migration seasons, 1–9 birds per season Uncommon 10–99 birds per season Fairly common 100–999 birds per season Common 1,000–9,999 birds per season Abundant More than 10,000 birds per season 18 British Birds 104 • January 2011 • 16–37 The autumn migration of soaring birds at Bourgas Bay, Bulgaria 6), it was possible to obtain precise data on Henrik Schou (HHS) outwith our survey the migration routes, speed, height and size years, are summarised here for key species. of some migrating flocks (Michev et al. 1987; Simeonov et al. 1990). Results and discussion The status of each species, determined by A total of 38 species of soaring migrants (five the number of individuals recorded per waterbird and 33 raptor species) were season, and based on the categorisation recorded during the Bourgas survey between adopted by BOU (1971), is outlined in table 1. 1979 and 2003, and the main results are sum- The main migration period, for those marised in tables 2 & 3. Table 2 shows that species considered to be uncommon to abun- the mean number of migrating waterbirds dant, was taken to be the period between the recorded per autumn during the survey two dates (one near the beginning of the period was 169,072, varying between 86,628 passage, one near the end) after/before which in 1982 and 250,623 in 1999. The correspon- the average migration count for three succes- ding seasonal mean number of migrating sive days was less than one individual. The raptors (table 3) was 38,534; the maximum peak migration period was defined as the of 65,605 was recorded in 1990. These figures period within which peak numbers were confirm that the Bourgas Bay area is second recorded in at least two years of the study only to the Bosporus in terms of the concen- period; and the overall peak date denotes the tration of migrant waterbirds and raptors in date with the highest mean count. For rare the whole European–African flyway system. and very rare species, the maximum and total numbers for the whole period of the survey Waterbirds (1979–2003) have been given instead.