VAN DER WINDEN J, KELDER L, DE VRIES OL, SCHOBBEN HPM & POOT M (2022) The IJsselmeer area and Wadden Sea as post-nuptial staging site for Black Terns Chlidonias niger. LIMOSA 95 (3): 113-126.

Black Terns migrate after breeding to fresh and brackish wetlands to prepare for migration to West-African wintering areas. In Northwestern-Europe estuaries with substantial Smelt Osmerus eperlanus populations are crucial. The area of lake IJsselmeer with the adjacent western Dutch Wadden Sea is the most import post-nuptial staging site along the eastern Atlantic coast. However, the peak numbers declined from 50.000 - 100.000 in the eighties and nineties of the 20th century to less than 20.000 nowadays (Fig. 2). This is attributed to a decline in Smelt stocks. Five questions rose about the situation in this staging site. Firstly, we wanted to know if (daily) average numbers also declined in the period and secondly if feeding and roost areas changed since the decline in Smelt stocks. Thirdly, related to this, we investigate differences in seasonal timing. Finally, we wondered whether the percentage of juvenile Black Terns has decreased recently and whether Smelt is still the most important prey. Since 1980 volunteers have been collecting data on Black Tern numbers on night roosts and in feeding areas as well as data on juvenile percentages at roosting sites in the area of lake IJsselmeer and the Dutch Wadden Sea. The regular simultaneous roost counts were completed in intermediate periods by partial roost counts and estimates in feeding areas. All these estimates were interpolated to daily average numbers. It turned out that the average seasonal (1 July to 15 September) numbers correlated well with maximum numbers and the decline over the season is structural (Figs. 2 & 3). Juvenile percentages did not change substantially over the years (Fig. 9) so we have no indication of a lower reproductive success or age-related shifts towards other staging sites. We do see a substantial variation in yearly and within season numbers in the wetlands. Likely, the Wadden Sea has always been important, but numbers and staging duration still seem to increase in recent years, although here too terns are common at the start of the season in some years, while in other years they are most abundant late in the season. Despite this variation in feeding sites and fluctuations in numbers there is a clear correlation with Smelt stocks over the monitoring period (Fig. 10). Our field impressions indicated that Smelt is still the most important bulk prey although occasionally Black Terns can profit from peak emerging of chironomids and flying ants. We advise to improve the lake habitat for pelagic anadromous fish species like Smelt, Herring or Anchovy.

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