Nachtelijke trek en vlieghoogtes van steltlopers in het voorjaar over de noordelijke havendam van IJmuiden

During spring, many waders migrate at low altitudes and within relatively short distances from the seashore along the coast of The Netherlands on their way to their northern breeding grounds. Migration is particularly strong when birds are facing headwinds. In April and May 1995, we investigated whether waders continue their flight during the night at the same distance offshore and at the same altitude, by counting the calls of groups of waders passing the northern breakwater at Umuiden (figure I), and using a modified ship radar (Furuno FR 8250). Observations were conducted on three nights: 27/28 April (new moon, strong migration during the previous day), 6/7 May (half moon, moderate migration during the previous day) and II/I2 May (full moon, weak migration during the previous day). In the late evening of all three days, 0 j-, waders and terns were migrating at low altitudes (most groups at altitudes below 30 m) and within short distances from the seashore. During the evening of 27 April and 6 May, more groups passed than during the evening of U May. This situation was similar to that during the day. During the night. the same held for the number of wader groups passing on 6/7 May and 11/12 May, respectively. During the night of 27/28 April, however, the number fell below that during the night of 6/7 May. Species of waders most often heard were Bar-iailed Godwit, Grey Plover, Oystercatcher and Dunlin (table 1). Bar-tailed Godwits mainly passed in April, Grey Plovers, Oystercalchers and Dunlins in May, similar to the situation during the day. No terns were observed passing northwards during darkness on any of the three nights. Herring and Lesser Black-backed Gulls from nearby breeding colonies were roaming in the area throughout the night. Birds passed the northern breakwater at Umuiden at night at altitudes of less than 5 m and up to 105-135 rn (figures 2 and 3). Most bird groups flew at altitudes below 50 m (figures 2c, 3c and 4). Waders tended to pass at somewhat higher altitudes (mainly 20-50 m, maximum 90 m) than gulls (mainly below 20 m. maximum 50 m). The identity of most echoes, however, remains uncertain. Most birds were noted within 500 m from the radar post (700 m from the shoreline), but echoes were recorded up to 2000 m from the radar (500 m beyond the end of the breakwater). No differences were found between waders and gulls in this respect. Our results show that waders passing the coast of The Netherlands in spring. during the day, at short distances from the coast and at low altitudes. continue to do so in the late evening and at night. Terns, however, seem to stop their migration at dusk.