Introduction
The numbers and species of soaring birds present at a migration site depend to a great extent on the migration routes of those species and the extent to which they depend on thermals. Soaring birds either avoid sea crossings completely or they cross at the narrowest straits available, thus creating migration bottlenecks (Bildstein Reference Bildstein2006). During autumn migration, the soaring birds of the East African-West Asian flyway either detour round the Gulf of Suez, passing near the city of Suez, or they move south down the Sinai Peninsula and cross the Gulf of Suez at its narrowest strait between El Tur and Gebel El Zeit or at the southern tip of Sinai. A third route leads down through the Arabian Peninsula and enters Africa at Bab-el-Mandeb (Welch and Welch Reference Welch and Welch1988).
The narrowest strait opposite the southern part of Sinai, at Zeit Bay, is recognized by ornithologists as an important migration site (Grieve Reference Grieve1996, Baha El Din Reference Baha El Din1999, Christensen and Jensen Reference Christensen and Jensen2002, Hilgerloh Reference Hilgerloh2008, Tammens Reference Tammens2008). Spring observations have shown that it is extremely important for migrants (Hilgerloh Reference Hilgerloh2009, Hilgerloh et al. Reference Hilgerloh, Weinbecker and Weiss2009) but, although it has long been acknowledged that the site is also of great importance during autumn migration (Baha El Din Reference Baha El Din1999), no systematic observations have previously been carried out. Birds often follow different routes in autumn and in spring (Newton Reference Newton2008). However, an autumn study is needed to establish the extent to which this is true of Zeit Bay. The area is designated as an Important Bird Area (IBA) but is under threat from proposed wind farm construction. Wind farms, if placed in areas with high densities of low-flying raptors can dramatically increase mortality (Langston and Pullan Reference Langston and Pullan2003, Hoetker et al. Reference Hoetker, Thomsen and Koester2004, Telleria Reference Telleria2009). In general, soaring birds lose height during a sea crossing because of the lack of thermals (Newton Reference Newton2008). If birds continue migration immediately after arrival at the coast of Zeit Bay, they will remain at a low altitude while flying over the coastal plains (Baha El Din Reference Baha El Din1999) and will therefore be under threat if the proposed wind farms are built. The data in this study were collected to assess the extent of this threat.
Methods
Study area
The study was carried out in an area of c.700 km2 situated in the coastal desert south of Ras Gharib and west and north-west of Zeit Bay (Figure 1; 27.73°N, 33.51°E). It was bordered on the west by the foothills of the Red Sea Mountains and in the east by plains stretching to the coast and to the foothills of the mountain chain of Gebel El Zeit. Except for a salt depression (sabkha) with some open water in the north, the study area consisted entirely of dry desert. Strong northerly winds prevailed during the entire study period.
Figure 1. Position of the observation sites M1-13 and S1-13 and of the IBAs “Gebel El Zeit”, “Hurghada Archipelago” and “El Qa Plain” (hatched) (produced by BirdLife International based on IBA information in the World Bird Database and information supplied by the author).
Daytime field observation
Two lines of 13 observation sites, 5 km apart, were aligned in an approximately NW-SE direction. The eastern line was close to the Suez-Hurghada main road and the second row was established 5 km west of it in the desert (Figure 1). Each site was defined by its coordinates measured by GPS. In spring, field observations were performed at the same observation sites (Hilgerloh Reference Hilgerloh2009). Given an observation radius from each observation point of 2.5 km, this created a double row of 13 observation circles succeeding one another without overlap. All observation sites were situated within the Gebel El Zeit IBA.
The observations were performed by ornithologists with more than 5 years of experience in bird identification. Two teams, each comprising two members, observed in shifts. One team worked from sunrise to noon, the other from noon to sunset, in order to ensure that all sites were visited at different times of the day. Travelling time between sites was approximately 20 minutes. Observation periods were of 60 minutes per site up to 16 September and of 40 minutes thereafter. The first team began observations at the southernmost desert site, moving successively northwards from site to site (Figure 1). The second team first visited the road site furthest north and advanced southwards. On each subsequent day, each team continued where it had left off the day before. The two teams worked alternately, thus avoiding double counts entirely, apart from a period at the beginning when the observation times of the teams overlapped; but these data were checked for potential double counts. Occasional deviations from this work plan were caused by logistic problems. For instance, the northern sites (site 9–13) were not accessible before 5 September, when we received notification that the mandatory mine clearing had been completed.
Estimations of the flight height of the birds were calibrated with the aid of laser binoculars (Geovid 10 x 40 and 7 x 40) and observers were trained using a 25kW Furuno ship radar. The procedure was as follows: after an individual bird was identified by sight and by radar, the observer’s estimate of the flight height was compared with the radar readings. The observers were instructed during systematic observations to estimate heights as closely as possible. In order to avoid a bias in the results, the observers were not informed of the evaluation categories later used to determine whether or not birds were endangered by wind turbines. Horizontal distances from the observer were calibrated wherever possible by reference to known distances to topographical features previously measured by means of GPS. Similarly, horizontal distances were estimated as closely as possible. In order not to bias the results, distance categories of the evaluation were not known to observers. During the first weeks, the composition of the teams was changed regularly in order to achieve standardised procedures and minimise differences between individual observers and between teams.
The following parameters were determined and documented: start and end times of observations at a site, species and number of birds, exact time of observation, flight direction, flight height above ground, distance and direction from the observer. Estimates were made when the birds were at the minimum distance from the observer. The observations were performed daily from 20 August to 29 October 2006. On 31 August, 2 September and 21 October no observations were performed. We did not interpolate numbers for these days but simply omitted them. The two teams performed the observations using 10 x 40 binoculars and 20x to 60x telescopes (for details see Hilgerloh Reference Hilgerloh2009). The study area was under observation for a total of 453.6 hours, an average of 17.45 hours per site.
Evaluation of daytime observations
The raw data were evaluated at three levels. The first evaluation ignored the distance at which birds were recorded. Then, in order to assign the birds to a specific area, subsequent evaluations took account only of birds recorded within 2.5 km of the observer (first at any height and second only birds at up to 200 m). According to Niemann (pers. comm.) the planned turbines were c.100m high.
Calculation of the mean direction of bird movements, the vector length and significance of uniformity of direction (Rayleigh test) were performed using the circular statistics program Oriana (Kovach Computing Services 2009). We used the number of independent sightings, which included both singles and groups of birds, rather then the total number of birds in analyses of flight directions. Only species with more than 15 records were included in the statistical analysis of migration direction and phenology.
In order to assess the global importance of the passage in this area, the percentage of the flyway population passing through the study area was extrapolated from the raw data; i.e. the total number of birds counted during the survey period. As observations were performed from sunrise to sunset during 70 days, the number of birds passing through the study area per season from sunrise to sunset (12 hours) during 70 days of migration was estimated. As the study area – c.10 x 70 km – extends from NW to SE, birds migrating to the south-south-west will cross the study area on a front broader than 10 km. As the northernmost part of the area is less frequented by migrants and as the start of observations in that area was delayed through mine-sweeping operations, for the extrapolations we assumed a migration front of 20 km.
Results
In total, 145,432 soaring birds including 134,599 storks and 9,376 raptors were observed during systematic observations (Table 1). The White Stork Ciconia ciconia (91.4%) was the most numerous species, followed by European Honey Buzzard Pernis apivorus (5.7%), White Pelican Pelecanus onocrotalus (0.8%) and Black Stork Ciconia nigra (0.9%) (Table 1). All other species were observed in numbers below 400 individuals. Significant observations included the ‘Near Threatened’ Pallid Harrier Circus macrourus. Small numbers of the ‘Endangered’ Egyptian Vulture Neophron percnopterus and ‘Vulnerable’ Lesser Kestrel Falco naumanni were observed.
Table 1. Number of soaring birds observed at any distance, at distances up to 2.5 km and at heights at or below 200 m within a radius of 2.5 km at Zeit bay from August 20 to October 29.
Within a radius of 2.5 km of the observers, 57,179 soaring birds were identified, including 50,238 storks and 6,413 raptors (Table 1). In general, birds were observed in greater numbers in the southern part of the study area than in the central and northern parts (Table 2; Figure 1). Lower numbers of White Storks were reported in the most northerly part and of raptors in the central part. However, many species, including Pallid Harrier, showed an even distribution throughout the study area.
Table 2. Comparison of numbers of soaring birds per observation hour (n/h) at the most southern sites, the central and the northern sites of the study area at Zeit Bay.
The number of birds observed during systematic counts resting or flying at heights of up to 200 m was 32,248, including 27,789 storks and 4,200 raptors (Table 1). This represents 55.3% of the storks and 65.5% of the raptors. Of this subset too, more birds were seen in the south of the study area than further north (Table 2), and the difference was much larger. At these heights, soaring birds were 3.6 times more frequent in the southern than in the central part and even 4.3 times more frequent in the southern than the northern part of the study area (Table 2).
Of the birds seen at or below 200 m 14,834 (46%) were resting on the ground (Table 1), of which 99% were storks. Nearly all of them were observed around the two southernmost sites along the road. In total 14,523 White Storks were seen resting in the southern part and 94 in the central and northern part of the study area (Table 2). Resting Greater Flamingos Phoenicopterus ruber, Eurasian Spoonbills Platalea leucorodia, Western Reef-egrets Egretta gularis and Grey Herons Ardea cinerea concentrated at the salt depression near site S13. Raptors were observed resting in small numbers (n = 29) scattered over the entire study area.
Black Storks actually observed accounted for 4.8% of the flyway population, White Storks for 33.3% and Great White Pelicans for 3.5%. The extrapolation of the number of birds passing during the migration season through the study area suggests that it was used by 92% of the White Stork, 12% of the Black Stork, 5% of the White Pelican, 4% of the European Honey Buzzard and 2% of the Pallid Harrier flyway population.
Flight directions showed little variation between species, all being oriented towards a south-south-west direction (Table 4). The flocks showed a high degree of uniformity of direction, but Black Kite Milvus migrans was the species in which the directions varied most (Table 4). The time of passage varied between the different species. Peak migration days in 2006 were between 28 August and 13 October. The two most numerous species were early migrants (Table 4; Figure 2). A total of 90% of the White Storks had passed by 10 September and 90% of the European Honey Buzzards by 20 September. Accordingly, in October, little migration was recorded. However, from 6 October onwards, small groups of storks (a total of 33 White Storks in four flocks and one flock of 23 Black Storks) were recorded migrating to the north.
Figure 2. Timing of soaring and gliding migrants at Zeit Bay between 20 August and 28 October 2006. The graphs are based on the daily number of birds per observation hour.
Discussion
This study documents the strong autumn migration of White Storks through the Zeit Bay IBA. The numbers observed represented 33% of the flyway population of White Storks and an extrapolated 92% of the flyway population (Table 3). The high percentage of the flyway population of White Storks observed crossing the study area during the autumn migration confirms previous studies according to which the main route of White Storks goes through Zeit Bay in both seasons (Creutz Reference Creutz1985, Schulz Reference Schulz1988, Leshem and Yom-Tov Reference Leshem and Yom-Tov1998, Berthold et al. Reference Berthold, van den Bossche, Fiedler, Gorney, Kaatz, Leshem, Nowak and Querner2001). Even White Storks that reached the southern tip of Sinai in autumn mostly turned back towards the NW, most probably to cross the Gulf of Suez at Zeit Bay (Celmins Reference Celmins1998 and pers. comm.). Although Zeit Bay is the main crossing point, some flocks crossed further south (pers. obs. during a visit to Tawila, situated 30 km south-east of Zeit Bay). Only exceptionally were storks observed at Suez and Bab-el-Mandeb (Bijlsma Reference Bijlsma1983, Welch and Welch Reference Welch and Welch1988).
Table 3. Number of observed birds and number of birds extrapolated for a migration season as percentage of the flyway population. Raptors, storks and pelicans observed in numbers exceeding 50 individuals are included in the table. (*Wetlands International 2006, **factsheet Birdlife International, for details see methods)
Table 4. Mean migration direction and vector length of the different species within a radius of 2.5 km from the observer. Date of median and peak migration day, numbers on peak day, percentage of all on peak day and total number. In White Stork and European Honey Buzzard the real median of passage may be earlier, as the passage of these species started before systematic observations began. Records refer to the number of independent sightings with identified flight direction. They include single flying or groups of birds. Only species with more than 15 records were included in the statistical analysis of migration directions and of phenology.
The most numerous raptor species was the European Honey Buzzard, accounting for an extrapolated 4% of the flyway population (Table 3). As this species was observed at neither Bab-el-Mandeb nor Suez in more than comparatively small numbers, it seems likely that Honey Buzzards cross the Gulf of Suez. Since this species does not greatly depend on thermals, it is conceivable that it crosses the Gulf of Suez on a broader front, as other observations have also indicated (Christensen and Jensen Reference Christensen and Jensen2002). Harriers were more evenly distributed over the study area than other species. This is consistent with the fact that these raptors, too, are not greatly dependant on thermals and are not narrow-front migrants. Although they were able to cross the Gulf of Suez anywhere, they nevertheless were more concentrated at Zeit Bay than at other migration observation sites (Bijlsma Reference Bijlsma1983, Welch and Welch Reference Welch and Welch1988, Celmins Reference Celmins1998). The ‘Near Threatened’ Pallid Harrier was present with the highest percentage of the flyway population of harriers.
Great White Pelicans are known to migrate through Israel in considerable numbers (Shirihai Reference Shirihai1996). At Zeit Bay we recorded only a part of the passage, as this species has a long migration period and we finished observations on 29 October, whereas in Israel the passage of the central 90% of migration did not end before 7 November (Leshem and Yom-Tov Reference Leshem and Yom-Tov1998). Nevertheless, we observed 5% of the flyway population at Zeit Bay whereas no Great White Pelicans were recorded at Suez or Bab-el-Mandeb. Therefore, we suspect that a significant proportion of the flyway population crosses the Gulf of Suez.
All the other large migrant species observed at Suez, Bab-el-Mandeb or Israel during autumn migration were seen in small numbers or not at all at Zeit Bay. Among them, the Steppe Eagle Aquila nipalensis with its loop migration is a well-studied species that enters the African continent either at Bab-el-Mandeb or at Suez and is consequently not expected at Zeit bay in autumn (Welch and Welch Reference Welch and Welch1991, Meyburg et al. Reference Meyburg, Paillat and Meyburg2003, Meyburg et al. Reference Meyburg, Paillat, Meyburg and Graszynski2005), whereas in spring it is a regular migrant (Hilgerloh Reference Hilgerloh2009). Egyptian Vultures seem to enter the African continent mainly at Suez and Bab-el-Mandeb and so only very small numbers were observed at Zeit Bay.
Levant Sparrowhawks Accipiter brevipes were observed in insignificant numbers at Zeit Bay. However they concentrated at the southern tip of Sinai migrating southwards (Celmins Reference Celmins1998).The Common Crane Grus grus was not observed in autumn at Zeit Bay. This was not surprising considering that this species was detected only in small numbers throughout Egypt (Goodman and Meininger Reference Goodman and Meininger1989). However, at the southern tip of Sinai some hundreds were observed within four days in the first half of October (Celmins Reference Celmins1998). This was interpreted as the beginning of an expected strong migration. As in spring, Common Cranes cross Zeit Bay in huge numbers (Hilgerloh Reference Hilgerloh2009), this species may have an autumn and spring route in this area similar to that of Levant Sparrowhawk.
With only White Pelicans recorded in any numbers, the passage in October was relatively light, the majority of early migrants, principally White Stork and European Honey Buzzard, having gone through by the end of September (Figure 2).
Unsurprisingly, after having lost height during a sea crossing, migrants prolong their low flight over the coastal plains or even land and stay over for a night. Consequently, the percentage of birds observed in the 0–200 m height band, was quite large as in spring (Hilgerloh Reference Hilgerloh2009). This means that not once but twice per year they would be threatened by the presence of huge wind farms in this area. Birds that try to avoid collisions with the rotors of the turbines have to revert to active flight, which is much more energy-consuming than soaring and gliding. This additional expenditure of energy could certainly lead to an increase in fatal casualties, not only at the wind farm site itself but also later on during migration. Conventions regarding the construction of wind farms clearly advise against any such developments in Important Bird Areas (Bern Convention 1979, Langston and Pullan Reference Langston and Pullan2003, Hoetker et al. Reference Hoetker, Thomsen and Koester2004, Drewitt and Langston Reference Drewitt and Langston2006, Langston Reference Langston2006). As already put forward on the basis of analogous spring data (Hilgerloh Reference Hilgerloh2009), the area should be kept free of all industrial development so as not to add to the already massive and acute dangers facing the birds along their migration route (Wilcove Reference Wilcove2008).
Acknowledgements
We wish to thank the following ornithologists who carried out field work with us: J. Weinbecker, B. Weinbecker, C. Völlm, G. Pegram, G. Nikolaus and K. Gauger. We are grateful to two anonymous referees for valuable comments. K. Wilson improved the language. A. Abdelmageed provided support in Egypt and E. Niemann during the entire study. The Deutsche Entwicklungsbank (KfW) financed the field work and the New and Renewable Energy Authority of Egypt (NREA) gave permission to publish these data.
