DAWN ASCENT AND RE‐ORIENTATION OF SCANDINAVIAN THRUSHES (TURDUS spp.) MIGRATING AT NIGHT OVER THE NORTHEASTERN ATLANTIC OCEAN IN AUTUMN
Authors: Myres M. T. 
Published: April 2008

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 dawn ascents nor re‐orientation have yet been observed in Scandinavian thrushes in spring, but dawn ascent by Icelandic Redwings has been recorded in the spring on one occasion.

The occurrence of dawn ascents was a rather better indication of the occurrence of migratory passage than the actual detection of movement during the night. On 12 mornings out of 48 a dawn ascent occurred although no overnight passage had been detected. The frequency of arrival of thrushes off Shetland was less in both moderate and strong southeasterly winds than in light winds, and the density was only very slightly higher in moderately strong, compared with light, southeasterly winds.

While the immediate stimulus for the dawn ascent is undoubtedly sight of the sea beneath them, its purpose remains uncertain. The fact that it occurs not at dawn but as much as 11/2 hours before sunrise in the later part of the migration season is also unexplained.

Depending on the method of analysis re‐orientation took place around Shetland in 1960, E.S.E. on 11 out of 15 (or 16 out of 24) mornings, S.S.W. on 9 out of 15 (or 19 out of 24) mornings and N.N.E. on just over one‐third of the mornings. Re‐orientation in two or in three directions was nearly twice as common as re‐orientation in only a single direction. Marked re‐orientation by thrushes took place in the lightest of winds, of any direction. Re‐orientation does not occur only after more or less severe drifting in easterly winds.

The directions of movement after dawn were not simply down‐wind or into‐wind. While two‐thirds of the E.–S.E. movements were more or less into‐wind, most of the N.–N.E. movements, about half of those S.–S.W. and one‐third of the E.–S.E. movements were across the wind. N.–N.N.E. re‐orientation occurred only in light southeasterly winds and when the density of birds was high. In northeasterly winds movements were mainly down‐wind S.–S.W. Unfortunately westerly winds were very infrequent in 1960.

The directions of re‐orientation were more regular and exact in their occurrence than the observed variations in the aspect or the strength of the wind, but the direction of the wind may play some part in determining which direction is mainly adopted.

Although only one direction of re‐orientation, S.S.E., appeared to have been observed in the southern North Sea it is suggested that some of the responses travelling S.S.W., apparently on a continuation of the overnight heading there, were in fact on a re‐oriented S.S.W. course as off Shetland and the Outer Hebrides.

In northwestern Europe the southeastward re‐orientation in thrushes is evidently an adaptation enabling those of the population which find themselves out over the eastern Atlantic to regain the western seaboard of Europe. Comparison is made with the situation on the American Atlantic coast.

Migratory birds generally continue flying on the same course after dawn as before it. Scandinavian thrushes are the only birds known to make marked changes in direction after sunrise, but it is not very likely that thrushes could goal‐navigate after dawn while incapable of doing so during the night. It is argued that simple re‐orientation is the basis of the behaviour observed. This behaviour appears to be a simple “escape” response, not directed towards a localized goal, and on average it tends to bring birds which are out of sight of land at dawn to land in the minimum time.