This Special Protection Area (SPA) borders onto the North Norfolk Coast Special Protection Area. To the north-west, the Wash extends to Gibraltar Point, another SPA. The partly confined nature of the Wash habitats, combined with ample tidal flows, allows shellfish to breed, especially shrimp, cockles and mussels. Some water birds such as oystercatchers feed on shellfish. It is also a breeding area for common tern, and a feeding area for marsh harriers. Migrating birds such as geese, duck and wading birds come to the Wash in large numbers to spend the winter, with an average total of around 400,000 birds present at any one time. It has been estimated that some two million birds a year use the Wash for feeding and roosting during their annual migrations. The Wash is recognised as being internationally important for 17 species of bird. They include pink-footed goose, dark-bellied brent goose, shelduck, pintail, oystercatcher, ringed plover, grey plover, golden plover, lapwing, knot, sanderling, dunlin, black-tailed godwit, bar-tailed godwit, curlew, redshank and turnstone. In Roman Britain, embankments were built around the Wash's margins to protect agricultural land from flooding. However, they fell into disrepair after the Roman withdrawal in AD 407. From 865 to sometime around 1066, the Wash was used by the Vikings as a major route to invade East Anglia and Middle England. Danes established themselves in Cambridge in 875. Before the 12th century, when drainage and embankment efforts led by monks began to separate the land from the estuarine mudflats, the Wash was a tidal part of The Fens that reached as far as Cambridge and Peterborough. Local people put up fierce resistance against the Normans for some time after the 1066 Conquest. The name Wash may have been derived from Old English "wāse" 'mud, slime, ooze'. The word "Wasche" is mentioned in the popular dictionary "Promptorium parvulorum" (about 1440) as a water or a ford ("vadum"). A chronicle tells us that King Edward VI passed "the Wasshes" as he visited the town of King's Lynn in 1548. By then, documents began to refer to the "Waashe" or "Wysche", but only for the tidal sands and shoals of the rivers Welland and Nene. 16th-century scholars identified the Wash as the "Æstuarium Metuonis" ("The Reaping/Mowing/Cutting-Off Estuary") mentioned by Ptolemy in Roman times.