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Migration across the North Sea studied by radar: 2. The spring departure 1956-59
In: Ibis. British Ornithologists' Union/Wiley: London. ISSN 0019-1019; e-ISSN 1474-919X
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| Abstract |
1. Radar observations on the eastward emigration of winter visitors across the North Sea from East Anglia were analysed from 24 February to late April in four years.2. Bird-echoes were most numerous around 21.00 hours, with a second smaller peak an hour after dawn. Occasionally, emigration continued throughout the 24 hours.3. The volume of emigration was influenced by (a) the internal state of the bird, (b) temperature, (c) wind-speed, (d) rain and cloud; it was not influenced by barometric pressure, the general weather situation, or the direction of the wind as such.4. In very similar cold weather 6–13 April, there was much more emigration in 1958, after a long hold-up, than in 1957 after much previous emigration.5. There was more emigration in warm than cold weather in February and March, but not in April or May. The threshold temperature required was higher in February than March. A marked rise or fall in temperature had no immediate effect.6. Morning migration was visible from the ground with opposed but not with following winds.7. Most height; obtained by radar were below 5000 feet, but the highest at night were not infrequently at 13,000 feet.8. The track over the sea was normally the resultant between the bird's heading and the wind, the track nearly always being north of east with a southerly wind and south of east with a northerly wind. The bird maintains its sense of direction, but apparently not its sense of position, in flight over the North Ses.9. The migrants' mean heading varied on different days between N.E. and rather south of E.S.E., while occasionally two eastward emigrations proceeded simultaneously on mean headings differing by some 25.10. Under totally overcast conditions, migrants flew at random. This means that they would gradually drift with the wind.11. Observations on Starling flocks are described. |
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