This is one of three immature Willow Warblers that appeared on our cruise ship in the North Sea on our way from Scotland to Amsterdam. This one was the brightest with yellow color extending to the belly. Identification as Willow Warbler is confirmed by its long primary projection with only three outer primaries (p6-p8) emarginate. The birds were not present the previous day when there was dense fog. I first noted them in the morning of 4 August when the fog cleared leaving overcast skies and almost no wind. By the end of the day they were gone. I believe they all arrived on the ship, possibly attracted by lights at night. They were feasting on Silver Y moths (Autographa gamma) which also arrived on the ship the same morning presumably on some migration transit.