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

Abstract

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 re‐orientation, was recorded from Iceland Redwings and Wheatears off the Outer Hebrides by Lee (1963). The possibility that birds of Icelandic origin were responsible for any of the southeastward directions of movement observed around Shetland in 1960 could be excluded for a combination of reasons. The possibility that warblers, flycatchers, chats, or finches drifted to Fair Isle and Shetland could have been responsible for the dawn ascents and re‐orientation off Shetland was also excluded. The radar responses, the detailed analysis of the radar data, the correlation of radar observations with field observations on Fair Isle and North Rona and other circumstantial evidence have strongly suggested that thrushes of the genus Turdus (especially Turdus musicus and probably T. merula and T. pilaris) were responsible for the overnight passage movements, dawn ascents and re‐orientation behaviour after dawn. It is shown that Shetland falls within the regular migratory path of Scandinavian thrushes in the autumn. Neither da...

Full Issue

“…Dawn reorientation of migrating birds aloft toward the nearest shorelines is consistent with other studies that have documented dawn ascent flights (Myres 1964, Bowden et al 2015, Rathbun et al 2016. Additionally, as morning approaches, nocturnally migrating birds have been found to reorient toward land along the Atlantic coast of the United States and along the Mediterranean Sea (Bruderer and Liechti 1998).…”

Full Issue

“…Dawn reorientation of migrating birds aloft toward the nearest shorelines is consistent with other studies that have documented dawn ascent flights (Myres 1964, Bowden et al 2015, Rathbun et al 2016. Additionally, as morning approaches, nocturnally migrating birds have been found to reorient toward land along the Atlantic coast of the United States and along the Mediterranean Sea (Bruderer and Liechti 1998).…”

Radar quantifies migrant concentration and Dawn reorientation at a Great Lakes shoreline

“…Far less is known about the effect these factors may have on the migratory pathways of bats. Our observations of aerial activity support a broad and simple mechanistic explanation of one factor contributing to shoreline accumulation that is by no means new [ 24 , 27 ]: nocturnal migrants flying over water return to land at dawn. Meanwhile, migrants flying over land continue (for the time being) in the predominant migratory direction, and this results in a confluence of migrants along the shoreline.…”

“…Migrants use coastlines more frequently than inland areas for purposes of stopover and refuge, if only for the fact that more migrants terminate flights along shorelines to avoid flying over water during the day. These results suggest that the Great Lakes affect nocturnal migrants in a manner similar to large ecological barriers [ 24 , 46 ], and our findings of a dawn turn towards shore suggest migrants may need to cope with these spans of unhospitable environment unexpectedly. Additional work investigating how large water bodies affect migration, especially decision-making by migrants starting their journeys (potentially hundreds of kilometers from the shore), is still needed.…”

Migration and orientation of passerine night migrants in northeast England

“…One might then suppose that they would attempt to return on an intended track. In any event, it would seem that the Redwing presents such a complex case of migration that, in contrast to the suggestion by Myres (1964), it would be a most unsuitable species for orientation experiments.…”

Full Issue

“…Dawn reorientation of migrating birds aloft toward the nearest shorelines is consistent with other studies that have documented dawn ascent flights (Myres 1964, Bowden et al 2015, Rathbun et al 2016. Additionally, as morning approaches, nocturnally migrating birds have been found to reorient toward land along the Atlantic coast of the United States and along the Mediterranean Sea (Bruderer and Liechti 1998).…”

Radar quantifies migrant concentration and Dawn reorientation at a Great Lakes shoreline

“…Far less is known about the effect these factors may have on the migratory pathways of bats. Our observations of aerial activity support a broad and simple mechanistic explanation of one factor contributing to shoreline accumulation that is by no means new [ 24 , 27 ]: nocturnal migrants flying over water return to land at dawn. Meanwhile, migrants flying over land continue (for the time being) in the predominant migratory direction, and this results in a confluence of migrants along the shoreline.…”

“…Migrants use coastlines more frequently than inland areas for purposes of stopover and refuge, if only for the fact that more migrants terminate flights along shorelines to avoid flying over water during the day. These results suggest that the Great Lakes affect nocturnal migrants in a manner similar to large ecological barriers [ 24 , 46 ], and our findings of a dawn turn towards shore suggest migrants may need to cope with these spans of unhospitable environment unexpectedly. Additional work investigating how large water bodies affect migration, especially decision-making by migrants starting their journeys (potentially hundreds of kilometers from the shore), is still needed.…”

Migration and orientation of passerine night migrants in northeast England

“…One might then suppose that they would attempt to return on an intended track. In any event, it would seem that the Redwing presents such a complex case of migration that, in contrast to the suggestion by Myres (1964), it would be a most unsuitable species for orientation experiments.…”

Full Issue

“…Dawn reorientation of migrating birds aloft toward the nearest shorelines is consistent with other studies that have documented dawn ascent flights (Myres 1964, Bowden et al 2015, Rathbun et al 2016. Additionally, as morning approaches, nocturnally migrating birds have been found to reorient toward land along the Atlantic coast of the United States and along the Mediterranean Sea (Bruderer and Liechti 1998).…”

Radar quantifies migrant concentration and Dawn reorientation at a Great Lakes shoreline

“…Far less is known about the effect these factors may have on the migratory pathways of bats. Our observations of aerial activity support a broad and simple mechanistic explanation of one factor contributing to shoreline accumulation that is by no means new [ 24 , 27 ]: nocturnal migrants flying over water return to land at dawn. Meanwhile, migrants flying over land continue (for the time being) in the predominant migratory direction, and this results in a confluence of migrants along the shoreline.…”

“…Migrants use coastlines more frequently than inland areas for purposes of stopover and refuge, if only for the fact that more migrants terminate flights along shorelines to avoid flying over water during the day. These results suggest that the Great Lakes affect nocturnal migrants in a manner similar to large ecological barriers [ 24 , 46 ], and our findings of a dawn turn towards shore suggest migrants may need to cope with these spans of unhospitable environment unexpectedly. Additional work investigating how large water bodies affect migration, especially decision-making by migrants starting their journeys (potentially hundreds of kilometers from the shore), is still needed.…”

Migration and orientation of passerine night migrants in northeast England

“…One might then suppose that they would attempt to return on an intended track. In any event, it would seem that the Redwing presents such a complex case of migration that, in contrast to the suggestion by Myres (1964), it would be a most unsuitable species for orientation experiments.…”