Feeding traces and bioturbation by birds on a tidal flat, Dutch Wadden Sea

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Abstract
Gulls (mainly Larus ridibundus) and Shelducks (Tadorna tadorna) make characteristic feeding traces on the Dutch tidal flat studied. Troughs up to 3 m long, ca. 15 cm wide, and ca. 3 cm deep made by L. ridibundus were most frequent from July to October. Craters made by Shelducks, ca. 10 cm deep and up to 60 cm in diameter, were most common in summer. The larger craters compare well with feeding traces reported for Eider ducks (Somateria mollissima) and rays (Hypotremata); the smaller craters compare with those of Flounder (Platichtys flesus) and Bream (Abramis brama). All illustrate uniformity in origin: benthic feeding by predators. Annually ca. 30% of the area studied was reworked by gulls and ca. 15% was reworked by Shelducks. The annual sediment reworking rate by these birds equals a sediment layer 2.5 cm thick. This volume is about one order of magnitude less than the ∼35 cm reworked annually by the most important deposit feeders of this tidal flat, collectively: the polychaetes Arenicola marina and Heteromastus filiformis and the bivalve Macoma balthica.