BUURMA LS (1987) Patterns of high bird migration over the North Sea area in October. LIMOSA 60 (2): 63-74.

Broad front migration over the North Sea area was studied by means of two long range surveillance radars, covering parts of S. Norway (Oslofjord region and Skagerrak) and the north of the Netherlands and adjacent sea (fig. I). The paper presents the average track directions and peak- echo-intensities of cohorts which could be recognized within the patterns of day and night movements above sea and land during October 1979 (both countries) and 1980 (Netherlands only) (tab. I, fig. 2-4). The main track directions above the sea both near Norway and the Netherlands appear to fall into a westerly and a southerly group, while normal SWoriented flights were detected above the Dutch mainland only. The strong underrepresentation of SW-migration over sea cannot be explained fully by missing birds due to low level flights against head winds. Both years offered enough possibilities with weak or (partial) tailwinds for migration at high altitudes in sw. direction (fig. 5). In stead of variation pointing at the existence of wind drift the distributions of the cohort directions near the Netherlands in 1979 and 1980 show much similarity. The angle between directions above sea and land suggests that the birds (have to) change navigational strategy in the coastal region (fig. 6). The radar observations and circumstantial evidence indicate that some Scandinavian migrants may arrive in the north of the Netherlands either after a direct flight SSSW from Norway, or by flying first WSW (cf. Myres 1964) over the northern North Sea at night followed by a reorientation towards SE-SSE at dawn. Last mentioned cohorts arrive in the Netherlands in the course ofthe day. Departures from the Netherlands into sea are usually between WSW and WNW, more or less perpendicular to the arrival directions. The sequence of the movements and scarce field observations (e.g. fig. 7) described so far suggest that the same short and medium distance migrants are involved, namely thrushes Turdus spec., Starlings Sturnus vulgaris, Skylarks Alauda arvensis, Meadow Pipits Anthus pratensis, Chaffinches Fringilla coelebs, Bramblings Fringilla montifringilla, Lapwings Vanellus vanellus, and probably others. However, it should be emphasized that direct identification of the birds producing radar echoes was not possible, while also the quantitative comparability of the (mass) movements was not assessed in enough detail. It is hypothesized that many Scandinavian October migrants possess an innate capacity to alternate between two preferred compass directions. When landmarks are available they are supposed to be able to fly intermediate directions which, in case of adults, are goal oriented (fig. 8). 73

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