MIGRATION ACROSS THE SOUTHERN NORTH SEA STUDIED BY RADAR PART 4. AUTUMN

Abstract

Summary. In autumn in Norfolk, the main nocturnal departures are (i) S.S.E., especially in September, of British summer residents; (ii) E., of unknown species; (iii, iv) S.S.W. and W., chiefly in October, the onward passage of birds, mainly thrushes, arriving in these directions from over the sea. Rarely, there are small northward movements. The main diurnal departures are (i) S.E., chiefly from mid‐September to mid‐October, of British summer residents; (ii) W., chiefly in October, the onward passage of diurnal migrants from Holland. The main immigrations are (i) S.S.W. from Norway, chiefly of chats, warblers, flycatchers and arctic waders in September, and of thrushes in October; (ii) S.S.E. from northern Britain, chiefly in September, of British summer resident night‐migrants drifted off‐shore by westerly winds; (iii) W. by both day and night from Holland, mainly of Lapwings and small night‐migrants in September and of Starlings, Chaffinches, thrushes and Lapwings in October; (iv) both nocturnal and diurnal migrants heading W.S.W. from Denmark, when drifted south by northerly winds, of the same species as in (iii). All 9 nocturnal and 4 diurnal movements analysed were far commoner and denser with following than opposed winds. They occurred with cross and opposed winds mainly when the wind was light. They tended to be commoner in clear than cloudy weather, and in anticyclonic than disturbed weather. After allowing for wind‐direction, they were equally common in warm and cold weather. Small passerine night‐migrants leaving England S.S.E. must change their heading to southwest somewhere around the south of France. Arctic waders arriving S.S.W. must have changed their heading in Scandinavia from one much further to the west. Norwegian thrushes change their heading from S.S.W. to S.S.E. when over the sea after dawn, provided that they have also been drifted by easterly winds, but they change back to S.S.W. over the land.