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Bruno Bruderer, Dieter Peter, Pius Korner

Flugverhalten von Nachtziehern über dem Schweizer Mittelland und den Alpen im Vergleich.

2022

119

212–231

Key words
(von 1994 bis 2006 vergeben)

Vogelzug, Durchzug, Langstreckenzieher, Zielfolgeradar, Vertikalgeschwindigkeit, Flugrichtung, Eigenrichtung, Richtungsverhalten, Topografie, Wind, Drift, Alpenpass

Schlagwort_Vogelart
(wissenschaftlich)

Schlagwort_Vogelart
(deutsch)

Schweiz, Bern, Waadt, Wallis, Jurabogen

deutsch

Abhandlung

Flight behaviour of nocturnal migrants above the Swiss Lowlands and the Alps compared. – Tracks and headings of nocturnal migrants recorded at three topographically different sites in the SW of Switzerland with tracking radar show the reactions of migrants to differential environmental conditions. Below the mountain crests of the Bernese Jura (near Kappelen), tracks and headings were influenced by the arc of the Jura-range, on a pass at the western edge of the Bernese Alps (Col de la Croix) they were extremely canalized along a high Alpine ridge, while in the Rhone Valley (leading across the principal direction of migration near Monthey) the birds’ directions were split according to three competing options. Above the mountain crests, the differences between the directional distributions at the three sites became reduced, SSW-components increased, most pronounced near Kappelen due to birds crossing the Jura mountains. Four sequences of comparable nights with largely undisturbed (anticyclonic) weather situation showed the following: (a) with winds blowing roughly in the direction of migration the birds tried to compensate side wind components; this led to broad scatter of headings (particularly at high altitudes and with strong winds); (b) weak opposing winds resulted in increased scatter of tracks despite partial drift-compensation with headings close to the basic direction; (c) notable drift with strong opposing winds. The analysis of different wind conditions on migratory directions at the two Alpine sites revealed for low-flying birds above Col de la Croix minimal variation of directions, but for birds flying higher than the crests of Diablerets considerable similarities with Monthey. The distributions of directions showed the following principal features: Under strong opposing winds, broad scatter of tracks with similar proportions of migration in normal direction and perpendicularly to both sides (above Monthey in both height bins, at the pass only above the crests); in contrast to migration above rather flat country (e.g. in southern Germany) no dominating SE-movements occurred. With NE-winds (following winds for normal migration) as well as with weak NW-winds notable proportions of low-flying birds moved towards SE (Rhone-upwards); reasons for and consequences of these deviating migratory movements are discussed. High above Col de la Croix drift-dependent deviations from the basic direction occurred with winds from the right, but less with winds from the left (thus confirming earlier observations on preferences for deviations towards left, i.e. towards S). In contrast to the pass of Hahnenmoos (30 km ENE) barely any SE-migration took place. Long-distance migrants in the first half of the season had somewhat more southerly directions than later and less far migrating birds. For Col de la Croix and Monthey the potential influence of prominent light sources for WNW-oriented components are discussed. Vertical speeds Vz above Monthey varied considerably: on average, only about 10–20 % of the birds flew horizontally (Vz < ± 1 m/s). The samples of birds flying lower than the neighbouring mountain crests comprised a higher proportion of climbing individuals in the evening; in the following time intervals average Vz declined slowly towards morning (analogous to autumn migration in the Arava-Valley, S Israel). The birds arriving above Monthey at the height of the passes and crests showed a conspicuously different behaviour: at these heights, positive and negative Vz were balanced in the first half of the night, while in the second half of the night, climbing birds prevailed. The birds flying below 1600 m asl upwards through the Rhone-Valley (under NW- and NE-winds) showed normal migratory behaviour, varying their Vz in the same manner as the mass of migrants with normal migratory directions. Overall, the results show important topography-related deviations of directions in the neighbourhood of mountain ranges and valleys, the degree of deviation varying according to wind and flight level. In the large valley of the Rhone River (which flows perpendicularly to the principal direction of migration), conflict situations require «decisions» that imply consequences for the subsequent course of migration. The fact that birds arriving above Monthey from Col de la Croix (at the height of the pass and the neighbouring crests) did not decrease, but rather increased their average Vz during the night may indicate pronounced long-distance migration, and support the idea that birds tend to maintain their flight level after crossing mountain ridges.