Travelers to Greenland would have been able to keep a course due west by observing the sky and sea currents. The instructions talk about visually observing the Faroes and, in good weather, Shetland, while other clues would let the sailors know that they were at the right distance from Iceland. Each kind of seabird flies only a certain distance from land, and whales like to stay in certain areas of the sea where they are able to find rich food supplies. By knowing the species of birds and whales and their idiosyncrasies, Viking Age sailors knew where they were, even when they could not see land. Additionally, cloud formations and even smell could tell experienced sailors where there was land beyond the horizon.