DAWN ASCENT AND RE‐ORIENTATION OF SCANDINAVIAN THRUSHES (TURDUS spp.) MIGRATING AT NIGHT OVER THE NORTHEASTERN ATLANTIC OCEAN IN AUTUMN

• Published in: Ibis
• Main Author: Myres, M. T.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1964
• Subjects: Norwegian Sea
• Online Access: http://dx.doi.org/10.1111/j.1474-919x.1964.tb03679.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1474-919X.1964.tb03679.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1474-919X.1964.tb03679.x

Summary Observations were made with radar from the Shetland Isles in 1960. During the late autumn a regular broad‐front movement passed westwards over Shetland from Scandinavia about midnight. Its front evidently extended by dawn from the Outer Hebrides towards the Faeroe Islands. The movement consisted of fast‐moving bright radar responses. During the later hours of the night the responses from the overnight passage movement generally, though not invariably, almost totally disappeared. Their disappearance seems to have been due to the birds having descended below the radar beam. The radar display always filled up again with rather faint, evenly distributed, responses in a short period around dawn, over a wide area of the south Norwegian Sea. The evidence indicates that these responses were from the same species as those observed on overnight passage, and that the sudden reappearance of responses around dawn was due to a sudden gain of altitude by these birds. The responses then assumed directions of movement which were not randomly distributed around the compass. Continuation of flight southwestwards or westwards on the overnight heading was infrequent, even in southeasterly winds. Movement in directions between W. and N.N.W. was very rare after dawn. Off Shetland three main directions of movement were recorded, E.S.E.–S.E., N.–N.N.E. and S.–S.S.W. Ascents at dawn and re‐orientation behaviour have been established as taking place over both the northeastern Atlantic Ocean and the North Sea. Ascents with at least one direction of reorientation have been observed also off the Outer Hebrides, off eastern Scotland and north of East Anglia. Overnight passage was recorded from Shetland on 22 out of 48 nights and the dawn ascent was recorded on 33 mornings. Directions of movement after dawn were determined on 26 mornings, on only two of which were the directions random. On 15 of the 24 mornings when re‐orientation occurred the wind at 900 m. above Lerwick was not greater than 15 knots. In 1961 ascent, without ...