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Bruno Bruderer, Pius Korner, Dieter Peter

Einfluss des Windes auf nachts ziehende Vögel am Rand einer Gebirgskette.

2024

121

330–347

Key words
(von 1994 bis 2006 vergeben)

Vogelzug, Durchzug, Zielfolgeradar, Vertikalgeschwindigkeit, Flugrichtung, Eigenrichtung, Richtungsverhalten, Topografie, Wind, Drift, Pseudodrift

Schlagwort_Vogelart
(wissenschaftlich)

Schlagwort_Vogelart
(deutsch)

Kappelen bei Aarberg, Mittelland, Jurabogen

deutsch

Bericht

Impact of wind on nocturnally migrating birds at the edge of a mountain range. – The tracks of nocturnal migrants and pilot balloons recorded with a tracking radar were used to analyse the reactions of the birds to the direction and speed of winds measured at their individual flight altitudes. A radar site in the northern part of the canton of Berne, next to the mountain range of the Jura was chosen to compare the behaviour of the birds with migrants above an area in southern Germany, where topographical structures were much less pronounced (Bruderer et al. in press). To define a potential intermediate goal direction of the bird community, we calculated the basic direction (BD = average track direction of birds flying under negligible wind influence over a full season) as a reference. We modelled the deviations of tracks and headings from BD for increasing wind speeds from six sectors (two sectors of 45 ° for opposing and for following winds, 90 ° for the rare sidewinds, each from left and right). A comparison of air speeds Va with winds from four (90 °-) sectors confirmed that Va increased with increasing opposing winds; a similar increase occurred with winds from both sides. A minimal reduction of Va with increasing following winds was indicated at high flight levels. A predicted increase in average Va due to decreasing air density with increasing height was not observed; possible explanations (birds climbing to high altitudes in following, but not in strong opposing winds) are discussed. For the two height bins (below and above the crests of the neighbouring Jura ridges), track- and heading-directions relative to BD showed: The directional distributions of birds and winds tended to be aligned along the mountains (even above the crests). Besides the main direction there was often a small additional cohort of birds having crossed the Jura mountains. The sporadic and weak sidewinds from left and right induced important drift above and partial compensation below the crests. Most birds flew against the generally prevailing westerly and southwesterly winds. Opposing winds from the left (Gl) induced only minimal compensation, but wide splitting of tracks towards right (according to winds) and towards left (with birds tending southwards in spite of northward drift). Opposing winds from front-right (Gr) were accompanied by strong migration and full drift; with high wind speeds pronounced over-drift occurred, because many birds shifted their heading southwards. Following winds (Rr and Rl) required only minor corrections to remain close to BR; downwind flights were a particular feature. The scatter of directions was much smaller, and flights opposite to seasonal directions less frequent than in southern Germany. Similar at both sites was an astonishingly high prevalence of drift over partial compensation.