THE WINTER FEEDING ECOLOGY OF AVOCETS RECURVIROSTRA-AVOSETTA ON INTERTIDAL AREAS .1. FEEDING STRATEGIES

The behaviour of Avocets Recurvirostra avosetta feeding on emerged int ertidal areas in a major wintering area, the Tagus estuary (Portugal), was studied during 2 years. Avocets used four different feeding strat egies. By far the most common was the Normal Feeding Strategy, in whic h the median sweeping rate was 28 sweeps per min and the percentage of swallowing movements (ingestions) was very high (median = 90%). No pr ey were visible in the bill while birds were using this strategy. Anot her less common feeding strategy was the Worm Feeding Strategy, in whi ch the sweeping rate was higher (median = 46 sweeps/min) and the perce ntage of swallowing movements was much lower (median = 6%). While usin g the Worm Feeding Strategy, Avocets preyed on ragworms Nereis diversi color. The Mixed Feeding Strategy and Scrobicularia plana Siphons Feed ing Strategy were rarely observed. In a detailed study on the use of t he mudflats by Avocets, it was found that the density of birds on the upper shore was much higher than on the lower shore. There were differ ences in feeding behaviour between these two areas, although the same feeding strategy was used. On the lower shore, where there was a lower biomass of prey available, sweeping rates were higher and aggressive behaviour did not occur. On the upper shore, sweeping rates were lower and aggressive behaviour was frequent, with some birds defending terr itories. Nevertheless, estimated food intake rates were 1.5 times high er on the upper shore.