Many bird species call during migration, but call rates not necessarily reflect migration intensity. They rather seem to increase under deteriorating flight conditions. Often, nocturnal mass collisions at illuminated structures coincide with such conditions and are accompanied with high call rates of migrants. Thus, call rates could act as an indicator for situations with high collision risk for birds namely at offshore sites with hardly any alternatives for landing. In the face of increasing numbers of offshore wind farms knowledge about the environmental conditions in which maximum call rates occur, is needed for mitigation measures. In this first long‐term study at an offshore site in the southern North Sea we investigated the effect of weather on the frequency of flight calls of three thrush‐species at an illuminated platform. Flight calls were registered automatically during three autumn migration seasons. Besides generally higher call rates from 5 to 2 h before until 6 h after midnight, call rates increased with tailwinds, a change of the tailwind component during the first part of the night, offshore crosswinds and very high humidity. A monitoring programme is suggested that could help to reduce mass mortalities at illuminated structures.

SummaryThe study area at Zait Bay, Egypt (c. 700 km 2 ) is situated in the middle of the West Asian-East African migration flyway used by very large numbers of soaring migrants. At this site the corridor narrows into a bottleneck. There exist only very few bottlenecks of this magnitude in the world. Observations were performed at all hours between sunrise and sunset at 26 observation sites, situated 5 km apart. The northern part of the area under investigation (19 observation sites) was situated within the Gebel El Zeit IBA (criteria A1 and A4iv), while the southernmost part (8 observation sites) was outside. The overall evaluation has shown that 179,681 soaring birds including 122,454 storks and 36,976 raptors were observed in total. Within a radius of 2.5 km from each observation site 97,143 soaring birds including 59,308 storks and 30,489 raptors were observed during the 604 hours of observation. Eleven species were recorded in numbers that exceed 1% of their flyway populations. Of special concern regarding bird conservation are those birds resting or flying in the first 200 m of elevation. In total 6,624 soaring birds were seen resting (2,252 within a radius of 2.5 km). Thirty-three percent of the storks and 47% of the raptors were observed resting or flying at heights within the lowest 200 m. The median height of flying birds varied between 5 m (harriers) and 500 m (Common Crane Grus grus). According to the raw data, criteria for nomination of the area as an IBA (20,000 raptors and storks in one migration season, globally threatened species) were fully met outside the existing IBA. Also, to the south of the study area, very high numbers of migrants were confirmed by spontaneous, non-systematic observations. Accordingly, a change of the boundaries is suggested. The regional analysis, based on extrapolations, has to be regarded as a preliminary study. The analysis failed to show a geographical trend for any single species. The data from this study establish a high concentration of gliding and soaring birds in the study area, within and adjacent to the already designated IBA.

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Predation by herring gulls Larus argentatus and oystercatchers Haematopus ostralegus was evaluated on a newly established mussel A4ytilus edulis bed on tidal flats of the German Wadden Sea. The mussel bed covered an area of 2 ha and showed a decrease in biomass of 40 % in the most densely covered parts from August to January. Synchronously, the extent of the mussel bed was reduced, resulting in a decrease of average biomass of 98 % over the whole mussel bed. From the beginning of August 1994 to mid January 1995, the average size of mussels increased from 10.7 to 20.3 ram. The P/B-ratio was 0.68 in August and 0.18 between September and November. Herring gulls and oystercatchers were the most important mussel predators. On average, 266 herring gulls and 63 oystercatchers were present on the mussel bed during one low tide; 34 % of the herring gulls and 78 % of the oystercatchers were observed to be feeding. Herring gulls fed at a rate of 4.2 mussels per minute and oystercatchers at a rate of 1.3 mussels per minute. While herring gulls took the most common mussel sizes (mean: 20 mm), oystercatchers searched for the largest mussels available (mean: 25 ram). Herring gulls consumed 13 mussels/m 2 (0.3g AFDW) during one day and oystercatchers 1.7 mussels/m ~ (0.1 g AFDW). Predation by birds was compensated by 33 % of the production. The proportion removed by bird predation amounted to 10 % of abundance and to 16 % of biomass (including production). Oystercatchers were responsible for 1% of the reduction in abundance and for 3 % of biomass. Removal was highest in the most common size classes of mussels, mainly caused by herring gulls. However, the highest proportion of mussels was eaten in the largest size classes, mainly by oystercatchers.

Predation by birds on mussel beds was evaluated in the tidal flats of the East Frisian island Spiekeroog (Lower Saxony, Germany) based on measurements in 1991 and 1994. In May 1991,32 locations with blue mussels Mytilus edulis covered an area of 5.2 km2, 311 t AFDLV blue mussels being available in the whole study area. Areal extent decreased thereafter. In May 1994, there were 20 locations covering an area of only 2.3 km2, a total biomass of 48 t AFDW being available. After the strong spatfalls in the summers of both years, 1290 t biomass were available in the study area in 1991, and 550 t in 1994. Herring gulls Larus argentatus, oystercatchers Haematopus ostralegus and eiders Somateria rnollissjma are the main predators on blue mussels. During one year (1992), oystercatchers were present on 3.7 million bird days, eiders on 1.1 million bird days and herring gulls on 0.7 million bird days. The annual consumption of blue mussels by birds In the intertidal flats was est~mated at 165 t AFDW.The highest proportion of total consumption was by oystercatchers (54 'Yo)), while eiders consumed 39% and herring gulls 7 % . Consumption by all 3 species amounted to 32 g AFDW per m' of mussel bed in 1991 and to 71 g AFDW per m2 in 1994. Predation could be compensated by 12% of production in 1991 and by 29% in 1994. In order to evaluate the relevance of predation for the mussel stocks, the share of the total biomass eliminated annually by predation was assessed on the basis of standing stock and annual production. Predation was responsible for 7 % of total elimination in 1991 and for 15% in 1994.Thus, factors other than predation were responsible for 85 to 93 % of eliminat~on of mussel biomass