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During low tide, up to 50% of Mya arenaria values were observed in the unstable concave-up position on a tidal flat of the Dutch Wadden Sea. These shells were overturned by foraging birds (turnstones, Arenaria interpret L.). During high tide most of these shells are overturned again by the flood current. Turnstones overturned more left than right Mya arenaria values. This cannot be explained by selectivity of the bird, but left Mya values have a projecting chondrophore that acts as an anchor in the concave-down position. Therefore, more left than right values remain on the tidal flat. This, together with turnstone activity, can also explain high left/right ratios in floating and eolian transported Mya shells