Ardea
Official journal of the Netherlands Ornithologists' Union

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Piersma T., Zwarts L. & Bruggemann J.H. (1990) Behavioural aspects of the departure of waders before long-distance flights flocking vocalizations flight paths and diurnal timing. ARDEA 78 (1-2): 157-184
The departure behaviour of waders was studied on the Banc d'Arguin in Mauritania in spring, and in spring, and in The Netherlands in both spring and autumn. Most flocks consisted of 10-60 individuals, with the larger species occurring in larger flocks. When conspecifics were best 'available' (during peak migration and at high tides) starting flocks were largest. During departure flocks sometimes joined others or split up, 'stabilizing' between 30-60 birds. Starting flocks usually assembled from clusters into V-formations or echelons. About 4% of the flocks contained more than one species. In more than 90% of the departing flocks birds vocalized intensely. With an optical range-finder we were able to follow flocks leaving the Banc d'Arguin to heights of 1.5 km, with no signs of flocks levelling off. During this ascent birds usually headed into the wind with climb rates of 0.55-0.69 m s-1. Except under the rare condition of tail winds, departing flocks showed no compensation for wind drift. A fixed orientation would make it easier for the birds to calculate wind drift effects between successive geographical fixes on which to base subsequent orientational adjustments. All species headed towards NNW, instead of NNE where the next stopover areas are found. This westerly orientation might represent an adaptation to correct for a continuous easterly drift at high altitude. Most flocks left before sunset, the departures showing a variable relationship to tide height. In the Dutch Wadden Sea, where cloud conditions vary, all 277 witnessed departures occurred with a visible sun. The diurnal time timing of departures and the patterns of flocking can be understood as a result of the interactions between and energetic advantages.


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