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.