Journal of Ornithology

, Volume 159, Issue 2, pp 315–336 | Cite as

Vertical distribution of bird migration between the Baltic Sea and the Sahara

  • Bruno Bruderer
  • Dieter Peter
  • Fränzi Korner-Nievergelt
Review

Abstract

This review covers 40 years of migration studies of the Swiss Ornithological Institute with the tracking radar “Superfledermaus”. Since 1968, this pencil-beam radar was—according to its (military) capacities of surveillance and tracking—applied for two different tasks: (I) To record the intensity of migration at various heights (either based on densities recorded by conical scanning, or frequencies of passage recorded with fixed beams at various elevation angles); densities (birds km−3) and frequencies (birds km−1 h−1) are averages per height intervals of 200 m and do not refer to the position of single targets; they are suited to show average height distributions. (II) Tracking individual birds or flocks provides information on the exact position and flight behaviour of single targets over time. Part I: 16 (among 22) study sites with data from 1991 onwards are chosen to visualize and explain the vertical distribution of nocturnal bird migration according to regionally shaped environmental conditions in the Western Palaearctic and in the trade-wind zone. Average distributions at sites devoid of important orographic or persistent meteorological distortions usually show 20–30% of nocturnal migration in the lowest 200-m interval, 50% below 700 m above ground level (agl), and the 90% quantile reaching heights between 1400 and 2100 m agl. The remaining 10% of migrants are usually scattered up to about 4000 m above sea level (asl). The lower parts of migration are forced upwards when crossing mountain ridges and often remain high after such crossings; at subsequent observation points, migration is often high above ground. Particularly high altitudes prevail where wind conditions are improving with altitude, as e.g. during spring migration in the trade-wind zone. Part II deals with the seasonal and diurnal variation in the spatial distribution of particular bird targets. Birds were on average higher up at night compared to daytime and avoided flights close to the ground in hot desert areas. Highest flights were recorded during diurnal migration above desert areas. The highest 0.3% of the tracked birds were found between 3500 m and 4900 m asl at most sites, but at 5000–6870 m in spring migration of the Mediterranean and the trade-wind zone. The most extreme tracks were at altitudes between 4500 and 6600 m asl above the European mainland, and between 6000 and nearly 9000 m above the Balearic Islands and in the trade-wind zones. A concluding discussion deals with the reasons for generally low flight levels and the particular conditions and requirements of high-altitude migration. Flight conditions in the lower atmosphere deteriorate with altitude. Cost of climbing may impose additional restrictions (specifically for large non-passerines). Decreasing oxygen density imposes limits to high-altitude flight, and compensatory ventilation may induce increased water loss, if no physiological countermeasures are available. High flights are compulsory when crossing high mountain ranges and favoured when wind support increases with altitude. This is particularly true when long non-stop flights lay ahead, when turbulences and/or high temperatures at lower levels can be avoided, and possibly when time for homeward flights in spring can be minimized.

Keywords

Quantification of bird migration Pencil-beam radar Fixed beam Conical scanning Vertical distribution Altitude variation with location Tracking radar Temporal variation of altitude Upper limit of migration 

Zusammenfassung

Höhenverteilung des Vogelzugs zwischen Ostsee und Sahara Dieser Review umfasst 40 Jahre Vogelzugstudien der Schweizerischen Vogelwarte Sempach mit dem Zielfolgeradar „Superfledermaus“. Seit 1968 wurde das für die (militärischen) Aufgaben Raumüberwachung und Zielverfolgung konzipierte Radargerät mit eng gebündeltem Strahl für zwei verschiedene Aufgaben eingesetzt: I) Um die Zugintensität auf verschiedenen Höhen zu registrieren (entweder Zugdichten, gemessen mit konisch rotierendem Strahl oder Zugfrequenzen, mit auf verschiedenen Elevationswinkeln fixiertem Radarstrahl); Dichten (Vögel km−1) und Frequenzen (Vögel km−1 h−1) sind Mittelwerte für Höhenintervalle von 200 m und sagen nichts aus über die Position einzelner Radarziele; sie sind deshalb geeignet für die Darstellung mittlerer Höhenverteilungen. II) Die Verfolgung von Einzelvögeln oder Schwärmen liefert Informationen über die exakte Position und das Flugverhalten einzelner Radarziele im zeitlichen Verlauf. Teil I: Unter insgesamt 22 Beobachtungsorten wurden 16 mit Daten ab 1991 ausgewählt, um die Höhenverteilungen des nächtlichen Vogelzugs unter regional geprägten Umweltbedingungen in der Westlichen Paläarktis und in der Passatwind-Zone aufzuzeigen und zu erklären. Durchschnittliche Höhenverteilungen an Orten ohne wesentliche orographische oder persistierende meteorologische Verzerrungen zeigen normalerweise 20-30% des Nachtzuges im untersten 200-m-Intervall und 50% unter 700 m über Boden; das 90% Quantil erreicht Höhen zwischen 1400 und 2100 m ü.B. Die restlichen 10% sind üblicherweise verstreut bis zu Höhen von 4000 m ü.M. Beim Überqueren von Gebirgszügen wird die Untergrenze des Zuges angehoben und bleibt nach dem Flug über einen Kamm oft auf der erreichten Höhe. Aussergewöhnlich hohe Flugniveaus ergeben sich, wenn die Windbedingungen mit der Höhe zunehmend günstiger werden, wie etwa beim Frühlingszug in den Passatwindzonen. Teil II behandelt die saisonale und tageszeitliche Variation in der räumlichen Verteilung von einzeln verfolgten Vogelechos. Die Vögel flogen im Mittel nachts höher als am Tag und in der Sahara stärker vom Boden abgehoben als in Mitteleuropa. Die grössten Flughöhen wurden tagsüber in Wüstengebieten aufgezeichnet. Die höchsten 0.3% der verfolgten Vögel wurden an den meisten Orten zwischen 3500 m und 4900 m ü.M. gefunden, aber auf 5000-6870 m im Frühlingszug des Mittelmeergebiets und der Passatwindzonen. Extremwerte der Flughöhen waren über dem europäischen Festland 4500-6600 m ü.M. und über den Balearen und in den Passatwindzonen zwischen 6000 und beinahe 9000 m. Abschliessend werden die Ursachen für generell tiefen Zug sowie die speziellen Bedingungen und Anforderungen von hohem Zug diskutiert. Verschiedene Flugbedingungen werden in der unteren Atmosphäre mit zunehmender Höhe schlechter. Die Kosten des Aufsteigens bewirken zusätzliche Einschränkungen (speziell für grosse Nicht-Singvögel). Abnehmende Sauerstoffdichte setzt dem Zug in grosser Höhe Grenzen, und kompensatorische Steigerung der Atmung kann zu erhöhtem Wasserverlust führen, sofern die Vögel nicht über physiologische Gegenmassnahmen verfügen. Grosse Flughöhen werden durch Gebirge erzwungen und durch mit der Höhe zunehmende Windunterstützung begünstigt, insbesondere wenn lange Nonstop-Flüge bevorstehen, wenn Turbulenzen und/oder hohe Temperaturen auf geringen Höhen vermeidbar sind, und allenfalls, wenn mit der Höhe abnehmender Luftwiderstand erhöhte Fluggeschwindigkeit und damit Zeitminimierung ermöglicht.

Notes

Acknowledgements

Our thanks go primarily to those who acted as leaders of radar stations at various sites and to all the numerous voluntary collaborators who helped to collect the data over 40 years. Members of the bird radar group at the Swiss Ornithological Institute were not only involved in the field work, but also in the analysis of the recorded data. We are grateful to the company Oerlikon-Contraves AG and to the Swiss Army for the loan and final donation of the radars. The development of the recording equipment started with Alfred Bertschi (Contraves) and Jürg Joss (Osservatorio Ticinese della Centrale Meteorologica Svizzera), continued with Raymond Bloch, and was led to perfection by Thomas Steuri from 1980 onwards. Many of the studies were financially supported by the Swiss National Science Foundation, some also by foundations such as MAVA for Nature Protection, Volkart, Vontobel, and Ernst Göhner. The Swiss Ornithological Institute was the stronghold of all the work throughout. The first applied project was financed by the Israeli Ministry of Communication; another applied project profited from cooperation with the Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU) in Germany; the Möggingen data were collected in cooperation with Wolfgang Fiedler from the Max-Planck Institute of Ornithology in Radolfzell. The data from the Baltic Sea and from southern Italy are due to environmental impact studies for the bridge projects “Fehmarnbelt” (BioConsult SH, Husum) and “Stretto di Messina” (Stretto di Messina S.p.A.), those from the St. Gotthard to a wind park project. Lukas Jenni and Felix Liechti provided advice to modify a first draft; Felix additionally contributed to the final discussion; two reviewers made important suggestions for improvement. Franz Bairlein encouraged us to present our long-term studies in a review article. The studies comply with the laws of the countries in which they were performed. The authors declare that they have no conflicts of interest.

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Copyright information

© Dt. Ornithologen-Gesellschaft e.V. 2017

Authors and Affiliations

  1. 1.Swiss Ornithological Institute/Schweizerische VogelwarteSempachSwitzerland

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