The northernmost island of the Orkney Islands, North Ronaldsay is further north than the southern tip of Norway although the Gulf Stream gives it an average temperature that is 10°F warmer. Old traditions, such as the custom of communal grazing of the unique North Ronaldsay sheep on the seashore, prevail here and Orcadian surnames predominate in this the remotest of the Orkney Islands. Owned for over 200 years by the Traill family, the island has an area of 690 ha (1705 acres) but its population has been in steady decline from 547 in 1881 to 161 (1961), 134 (1971), 109 (1981), 92 (1991), 70 (2001) and 72 (2011). Though only a small island, North Ronaldsay has a rich flora and a wide range of migrant birds. A 1.8m (6-foot) drystone wall around the entire island restricts the semi-feral sheep to the beach, where they survive on a diet of seaweed. At 33m (109 feet) the New Lighthouse built at Dennis Head in 1854 is the tallest land based lighthouse in the UK. Sites of archaeological interest include the Broch of Burrian, Tor Ness, the Brae of Stennabreck and Muckle Gersty dykes. The island has an airstrip and there is a weekly ferry service from South Bay to Mainland Orkney.