KUIPERS H & VAN ELS P (2022) Groningen as an important stopover in the annual cycle of the world's northernmost Gull-billed Terns Gelochelidon nilotica. LIMOSA 95 (2): 89-95.

The only Gull-billed Terns in Northwestern Europe breed on a salt marsh near Neufelderkoog, Elbe estuary, Germany (45 pairs in 2020). In 2010, measures were taken to protect this breeding colony against nest predation by ground predators and human disturbances. Since that year, Gull-billed Terns have annually appeared after the breeding season at an excavation lake near Alteveer, Groningen. Based on intensive colour-ring reading, it was estimated that in 2021 at least 77 individual Gull-billed Terns used this area as a stopover in summer. Since un-ringed birds cannot be identified as individuals, the actual number is likely to be higher. This means that at least half of the Northwestern European breeding population of Gull-billed Terns uses Alteveer and its surrounding area during autumn migration.
The accuracy of the daytime counts is low as adults are foraging in a larger area around the excavation lake. However, the nearby night-roosts appeared to be even more difficult to survey as the terns spread out over a large area of mudflat, where they roost in small flocks, partly out of sight due to high vegetation. Even during coordinated roost counts less than half of the number of birds seen during daytime were encountered at the night roost. Possibly some birds roost elsewhere. Thus, daytime counts at and around the excavation lake near Alteveer are so far the best method for inferring the number of staging birds.
Colour-ring readings confirmed exchanges of individuals with the other known post-breeding stopover area at Balgzand, Noord-Holland. As colour-ring reading is difficult in Noord-Holland, exchanges between these areas might be more common than currently thought. Hence, in order to get an idea about the total number of Gull-billed Terns in autumn simultaneous counts are required.

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