This video shows gull-billed terns in Spain.
Translated from the Dutch Wadden Sea Association:
In the Ems-Dollard region, in the Dollard salt marsh, after a long search, the sleeping place of gull-billed terns has been found. An important find, because these rare terns are monitored during their stay at these places. It took not less than four years before the sleeping place of gull-billed terns was discovered. During the day the birds stay in Nieuwe Pekela, but for a long time it was unclear where they spent the night. Despite the strong suspicion that gull-billed terns slept on the marshes along the Dollard, people did not succeed in actually seeing them.
The only place in North-Western Europe where gull-billed terns still nest is Neufelderkoog island in the German Wadden Sea. Of these birds about thirty percent migrate via stopovers in the Netherlands to the south. … In the Netherlands there are two of these places: the Van Ewijcksluisschor of Balgzand at Den Oever and the Dollard salt marsh.