VLEK R (2002) Migration and night roosts of Gull-billed Tern Gelochelidon nilotica in the northern Ijsselmeer area. LIMOSA 75 (4): 147-158.

This paper analyses the migration of Gullbilled Tern through the Netherlands, especially with respect to the development of summer roosts in the northern lake IJsselmeer area. In the 1920s and early 1930s, Gull-billed Terns on their post-breeding migration from Denmark roosted at tidal beaches and banks on the southwestern coast of the province of Friesland. After the enclosure of lake IJsselmeer (May 1932) the terns gradually relocated their night roost to the Balgzand, a Wadden Sea saltmarsh in the north of the province of Noord-Holland. This roost was in use for almost 60 years (1937-1994), and attracted hundreds of Gull-billed Terns, especially from the late 1930s until the early 1950s. Parallel to the decrease of the NW-European population (table 2), the numbers in mid-August declined to below 100 from the second half of the 1950s until the early 1970s (table 3). In the 1970s and 1980s, the maximum numbers decreased further to less than 50 birds, while in the 1990s no more than some 30 birds used this communal roost. However, the total numbers passing through on their postnuptial migration and using the roost temporarily are difficult to estimate, since it is not possible to assess individual staging time and turnover. From 1995 onwards, Gull-billed Terns have used a new roost in the north of the province of Noord-Holland, while the roost at Balgzand lost its traditional function. This relocation was facilitated by a new practice to lower the impact of agricultural parasites by inundation of harvested bulb-fields during some weeks in summer. From the second half of the 1980s, inundated bulb-fields provided a roosting place nearer to the feeding area of Gull-billed Terns (figure 1). On this roost, it is possible for observers to identify and count adults and juveniles. Families with juveniles appear in Noord-Holland during the last week of July and in the first week of August, shortly after fledging (figure 2, table 1). In 1988-2002, an average of 0.96 young per pair was observed. In 5 out of 8 years since 1995 it has been less than one young per pair (table 4). Annual fluctuations in breeding numbers and breeding success in NW-Europe are considered to be responsible for the steady decrease in roosting Gull-billed Terns in Noord- Holland. Several questions remain to be answered for a full understanding of the passage of the species through the Netherlands, and for the necessary protection of the vulnerable NW-European population on its annual migration.

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