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Journal of Ornithology
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Topography and wind moulding directions of autumn migration between Europe and the West African savannas

Authors: Bruno Bruderer; Dieter Peter;

Topography and wind moulding directions of autumn migration between Europe and the West African savannas

Abstract

Abstract This review on autumn migratory directions is thought as a complement to an earlier overview on the vertical distribution of bird migration between the Baltic Sea and the Sahara (Bruderer at al. 2018): (1) A broad synopsis shows that nocturnal migration is generally SW-oriented above the western half of the European mainland, bending gradually southward above the western Sahara and shifting SE at the Sahara-Sahel transition. Important deviations of the SW stream occur along mountain ranges and coastlines. (2) Taking the Alps as a case example of such a leading-line reveals that the effect of the Alpine arc on migratory directions increases from E to W, becoming most prominent in Switzerland where the northern border and the main ridges of the Alps bend from WSW towards SSW. Thus, the migratory stream gets increasingly aligned with the course of the mountain range and reaches highest concentrations in the Swiss Lowlands. (3) Simultaneously recorded tracking radar data on nocturnal migration above Southern Germany and above the Swiss Lowlands show similar distributions of head ings, but different tracks (flight directions over ground). (4) Generally, a large proportion of the tracks above the rather flat country N of the Rhine is shifted towards S or SE by frequent westerly winds. This contrasts with barely drifting birds facing south-westerly headwinds canalized along the Jura Mountains in the Swiss Lowlands. (5) Tracks and headings under varying wind conditions above Southern Germany visualise different reactions to following vs opposing winds as well as to side winds from the right and left. (6) Radar-tracked night migrants above three different sites in south western Switzerland show their reactions to different topographical conditions which vary from moderate leading effects of the Jura Mountains at a lowland site, to extreme funnelling at an Alpine pass, and wide scatter when a large Alpine valley perpendicular to the principal SW-direction of migration is crossed. (7) Distinguishing between three height zones reveals that (a) the proportion of SSW migration increases with height; this besides a few birds drifting across the Jura Mountains; (b) at the Alpine pass, forward migration is canalised as a narrow stream and complemented by notable reverse movements, while the highest level (above the crests) is characterised by wide directional scatter including moderate southward drift; (c) the proportion of movements along the SE-NW leading Rhone Valley decreases with altitude, while the proportion of SW migration increases, and the distribution approximates that at the pass in the highest zone. (8) This information leads to ideas for continuative studies, particularly on reverse movements, drift and compensation in the Alps and their northern approach areas. Zusammenfassung Topografie und Wind prägen die Richtungen des Vogelzugs zwischen Europa und den westafrikanischen Savannen. ieser Review über Zugrichtungen ist gedacht als Pendant zu einer früheren Übersicht über die Höhenverteilung des Vogelzugs zwischen Ostsee und Sahara (Bruderer et al. 2018): (1) Eine weit gefasste Synopsis zeigt, dass der nächtliche Vogelzug in der Westhälfte des europäischen Festlandes generell auf SW ausgerichtet ist, über der westlichen Sahara graduell südwärts dreht, und am SaharaSahel-Übergang südostwärts schwenkt. Bedeutende Abweichungen von der SW Richtung kommen vor entlang von Bergketten und Meeresküsten. (2) Die Alpen als Fallbeispiel eines solchen LeitlinienEffekts zeigen, dass der Einfluss des Alpenbogens von E nach W zunimmt, und besonders ausgeprägt wird, wo die Alpenketten in der Schweiz von WSW gegen SSW schwenken. Dort fällt der Zugverlauf zunehmend mit dem Verlauf der Gebirgsketten zusammen, was zu höchsten Zugkonzentrationen im Schweizerischen Mittelland führt. (3) Zeitgleich mit Zielfolgeradar erhobene Nachtzugdaten über Süddeutschland und dem Schweizerischen Mittelland zeigen ähnliche Eigenrichtungen, aber unterschiedliche Flugrichtungen (relativ zum Boden). (4) Generell werden über dem strukturarmen Gebiet nördlich des Rheins viele Vögel durch die häufigen westlichen Winde gegen S oder SE abgelenkt; dies im Gegensatz zu den Vögeln, die ohne wesentliche Drift gegen die entlang des JuraBogens kanalisierten südwestlichen Gegenwinde fliegen. (5) Flug- und Eigenrichtungen unter verschiedenen Windbedingungen über Süddeutschland veranschaulichen unterschiedliche Reaktionen auf Rücken- und Gegenwinde sowie auf Seitenwinde von links und rechts. (6) Mit Zielfolgeradar verfolgte Nachtzieher über drei topographisch verschiedenen Orten in der SW-Schweiz zeigen die Reaktionen von Zugvögeln auf unterschiedliche Bedingungen. Diese reichen von mässigem Leitlinieneinfluss der Jura-Ketten an einem Mittelland-Standort zu extremer Kanalisierung auf einem Alpenpass und breiter Richtungsstreuung beim Überqueren eines grossen, quer zur Hauptzugrichtung (SW) verlaufenden Tals. (7) Die Unterscheidung von drei Höhenbereichen zeigt (a) an der Tieflandstation mit der Höhe zunehmenden SSW-Zug neben wenigen über den Jura hinweg verdrifteten Vögeln; (b) auf dem Pass ist der Normalzug eng konzentriert; zusätzlich kommt erheblicher Umkehrzug vor; das höchste Intervall (über den Kämmen) ist charakterisiert durch erhöhte Richtungsstreuung, verbunden mit teilweiser südwärts Drift; (c) Flüge entlang des SE-NW orientierten Rhonetals nehmen mit der Höhe ab, während der SW-Zug zunimmt und die Richtungsverteilung im obersten Höhenbereich sich derjenigen auf dem Pass angleicht. (8) Diese Informationen führen zu Ideen für weiterführende Studien, insbesondere über Umkehrzug, Drift und Kompensation im Alpenraum und dem nördlichen Alpenvorland.

+ Publisher's Statement: This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature's AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s10336-022-01971-8

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Keywords

migratory directions, Southern Germany, tracking radar, topography and wind, leading-lines, tracks and headings, Western Palaearctic, Western Switzerland

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popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
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This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
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