BUURMA LS, LENSINK R & LINNARTZ LG (1986) Altitude of diurnal broad front migration over Twente; a comparison of radar and visual observations in October 1984. LIMOSA 59 (4): 169-182.

This study is aimed at assessing altitudinal aspects of broad front migration over East Netherlands (fig. 1) from 15 to 21 October 1984 during daytime as seen visually and by radar. October is the month of massive diurnal migration of short and medium distance migrants in western Europe. In this study two posts counted migrating birds visually from 30 minutes before sunrise to 14hOO while a third post went on until half an hour after sunset. Parallel to the visual observations we use a small mobile X-band tracking radar (type flycatcher) with a separate search and tracking system. Tracked birds could be viewed by a TV- telelens up to a distance of 2500 meter. During the study period normal weather conditions prevailed (fig. 2). Weak NO (tail) winds did not occur. Visual quantification of the migration was done by estimating the nearest passage distances projected over the ground in terms of within or outside 100 meter. The quality of these estimates were checked at post 1 by border observers at 100 meter NW and SE of the central post. Only 7% of all birds proved to be missed by the central post (horizontal plain). A comparison of echo densities within counting windows at the radar screen pictures and bird flocks per hour over a 200 meter front revealed that the visual observers missed c. 61% during the peak hour on 16 October. Due to lower flight levels in the course of the day the total proportion missed was much smaller (fig. 5). After 16 October, the field observers saw virtually all the passing birds. A direct comparison of visual altitude estimates and the tracking results shows that the radar was limited to bird detection over 40 ? 10 meter, while the visual observers started to miss (small) birds above 50 meter. Confirmation of this was found in the proportions of reversed movements (higher flights due to tailwinds, fig. 8). The height distributions caused us to suggest the existence of an undercurrent of landscape bound broad front migration. The birds concerned seem to seek for a compromise between flying low for contact with vegetation and conspecifics, and headwind avoidance, and rising for overview. The low level migration is extra low over bare country (post 2, fig. 6) while wind effects caused by roughness of the landscape may enable birds to fly higher on certain places (fig. 7). Larger birds seem to be hindered less by headwinds. TV-identifications confirmed the finding from the visual estimates that the proportion of birds over thrush size increased with altitude. Mallard Anas platyrhynchos Common Buzzard Buteo buteo Northern Lapwing Vanellus vanellus Black-headed Gull Chroicocephalus ridibundus = Larus ridibundus Stock Dove Columba oenas Common Wood Pigeon Columba palumbus Eurasian Skylark Alauda arvensis Meadow Pipit Anthus pratensis White Wagtail Motacilla alba Dunnock Prunella modularis Fieldfare Turdus pilaris Song Thrush Turdus philomelos Redwing Turdus iliacus Mistle Thrush Turdus viscivorus Great Tit Parus major Eurasian Jay Garrulus glandarius Western Jackdaw Coloeus monedula Rook Corvus frugilegus Carrion Crow Corvus corone Common Starling Sturnus vulgaris Eurasian Tree Sparrow Passer montanus Common Chaffinch Fringilla coelebs Brambling Fringilla montifringilla European Greenfinch Carduelis chloris Eurasian Siskin Carduelis spinus Common Linnet Carduelis cannabina Common Reed Bunting Emberiza schoeniclus

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