NOCTURNAL AUTUMN MIGRATION OVER SOUTHERN GERMANY - WINDDRIFT AND COMPENSATION

Authors
Citation
F. Liechti, NOCTURNAL AUTUMN MIGRATION OVER SOUTHERN GERMANY - WINDDRIFT AND COMPENSATION, Journal fur Ornithologie, 134(4), 1993, pp. 373-404
Citations number
47
Language
TEDESCO
art.tipo
Article
Categorie Soggetti
Ornithology
Journal title
ISSN journal
0021-8375
Volume
134
Issue
4
Year of publication
1993
Pages
373 - 404
Database
ISI
SICI code
0021-8375(1993)134:4<373:NAMOSG>2.0.ZU;2-7
Abstract
This study analyses the influence of wind on the directional behaviour of nocturnal migrants. Flight paths of more than 10000 birds were rec orded at a site near Nuremberg (southern Germany) with a tracking rada r (3 cm wavelength) during the migration season of autumn 1987 (30 Jul y to 30 October). Wind measurements were taken every 4h up to 4000 m a bove ground level (AGL). A model is introduced to describe the relatio n between the directional behaviour and wind. An increasing proportion of southerly flight directions with increasing height is mainly cause d by the wind and to a lesser extent by more southerly headings of bir ds above 1000 m AGL. Favourable winds (north-easterly tailwinds) were very rare during the observation period (Fig. 4). Nocturnal migrants c ompensate wind drift in relation to wind direction and not according t o wind speed. The amount of compensation corresponds to an average win d speed of 2-3 m/s (7.2 to 10.8 km/h; Fig. 8). If wind speed is higher , the birds are drifted off their preferred direction (SW 230-degrees) . High flying birds (>1000 m AGL) maintain a heading compensating for the wind below 1000 m AGL. Moreover later in the night the headings ar e related to the winds at low levels 4h previously (Fig. 9). No compen satory reactions to winds in preceding nights could be found. Differen ces in compensation for wind from the left (SE-wind compared to NW-win ds) could be explained by the instability of SE-winds which usually oc cur in connection with inversion layers. Under strong headwind conditi ons (SW > 10 m/s) many birds show more southerly headings which result in tracks towards SE or even E. This seems not to be a result of pseu dodrift. The small amount of SE-migration over southern Germany showed no correlation with north-westerly winds. A classification of the mig rating birds according to wing-beat pattern (echo signatures) reveals only small differences in the directional behaviour of five classes. L arge and small songbirds as well as swifts show similar behaviour rela tive to the wind. The directional behaviour of waders and waterfowl di ffers by being less correlated with the model. This could be a consequ ence. of the heterogeneity of this sample (small sample size compared to the number of species). Compared to other studies we found a large influence of the wind on the flight directions. It is suggested that t he weak influence of the wind in several studies based on surveillance radar is caused by two effects: a) the detection capacity of such rad ars being better when the migration is at higher levels ; b) high bird migration occurring mainly with tailwinds.