One would expect these snowy owls to have migrated from Arctic Eurasia to the Netherlands. However, the blog of the wardens of Vlieland island suggests that at least some of the owls may have a different origin.
A warden spoke to a passenger, Frans van der Esch, of the trans-Atlantic ship MSC Monterey. As the ship was going east to Europe, about fifty miles from Newfoundland, on 9 December 2013, in stormy weather, nine tired snowy owls landed on the ship. They were two males and seven females. They huddled together for some protection from the ocean spray.
As the ship came near European coasts, some owls left. The passenger says that on 15 December 2013, the ship was near Zeeland province in the Netherlands. Then, the last two snowy owls left.
This photo shows a snowy owl aboard the ship. A slide show with more photos is here.