This, however, has been observed near Heligoland (205/1.

Instances are recorded by Gatke in his "Heligoland as an Ornithological Observatory" (translated by Rudolph Rosenstock, Edinburgh, 1895) of land-birds, such as thrushes, buntings, finches, etc., resting for a short time on the surface of the water.

The author describes observations made by himself about two miles west of Heligoland (page 129).); and land-birds, after resting for a time on the tranquil sea, have been seen to rise and continue their flight.

I cannot give you the reference about Heligoland without much searching.

This alighting on the sea may aid you in your unexpected difficulty of the too-easy diffusion of land-molluscs by the agency of birds.