Ardea
Official journal of the Netherlands Ornithologists' Union

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Zwarts L. (1976) Density-related processes in feeding dispersion and feeding activity of Teal (Anas crecca). ARDEA 64 (3-4): 192-209
The central question is: can feeding birds, which are distributed over a feeding area, limit their occurrence by means of their dispersion behaviour. The data on the dispersion and feeding activity of Teal in tidal habitat suggest that the distribution as well as the feeding proportion were dependent on the population density. The observations were carried out on tidal mudflats in the former estuary of river Rhine and Meuse (Fig. I). On the Ventjager, Teal were common in late summer (Fig. 2). The dispersion and activity of Teal was determined by variation in water level. The Teal roosted at high water. They fed during ebb and low water on the exposed mudflats, whilst they abandoned the higher parts. During incoming tide the Teal were concentrated in the zone of shallow water near the water-line. The high water roosts on the Ventjager were situated near the reed vegetation on feeding area A (Fig. 1); during ebb they spread out to occupy the emergent flats on A and B. Not more than about 9,000 remained on A at low water, the 'overflow' went to B (Fig. 4). On average, 51% of the Teal on intertidal flats fed during day-time in late summer and in autumn. Feeding mostly occurred at incoming and receding tide (Fig. 5). During low water the Teal only foraged in areas where the substrate of the intertidal flats was very muddy (Table I). Multiple regression analyses were performed on the low water activity counts on the Ventjager (Table 2-5). The feeding proportion at low water decreased in the course of the day and appeared to be higher at low temperature. The feeding activity on B, but not on A was negatively density-dependent. On the Ventjager about 4,000-6,000 Teal fed at average low water independently of the total number of Teal present (Fig. 6). In feeding Teal the distance to the nearest neighbouring Teal was 25 cm - 3 m (Fig. 7). In the incoming tide-line Teal fed in higher densities than on the mudflats. It is concluded that A was preferred as a feeding area to+ B, because the high water roost was situated on A and also because on A the feeding efficiency was probably higher. The number present on B was determined during ebb when the Teal competed for the emergent mudflats. Presumably the feeding density of Teal within a group - other things being equal- was more or less independent of the number present: if more Teal were present there occupied feeding area was larger (Fig. 4) and relatively more birds did not feed (Fig. 6). It is suggested that the competition for spaces an important factor in the ecology of shorebirds.


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