Birds as producers of shell fragmentsin the Wadden Sea, in particular the role of the Herring gull

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Herring gulls ( Larus argentatus) are common birds in the Dutch Wadden Sea the year round. Their diet is varied but consists of ca 70% molluscs, largely bivalves (mussels and cockles). Large shells are crushed by dropping them from the air. Bivalves < ca 35 mm are ingested whole and crushed internally. Larger shell fragments (with a peak in the 2–4 mm fraction) are regurgitated in pellets, smaller fragments are defecated (peak in the 1–2 mm fraction). From average numbers present ( ca 100,000), daily consumption (0.067 kg), % molluscs in diet (70%) and an average shell/meat ratio (10), annual shell fragment production can be estimated at ca 17,000. 10 3 kg. This is half of the production of shell fragments by the Eiderduck ( Somateria mollissima) population; also the angular fragments produced are very similar. The Oystercatchers ( Haematopus ostralegus) in the Dutch Wadden Sea consume fewer bivalves and they crush only part of the shells, producing ca 10,000. 10 3 kg shell fragments. The role of other shell-consuming bird populations is less [ e. g. Shelduck ( Tadorna tadorna) 1,800. 10 3 kg, Knot ( Calidris canutus) 1.6. 10 3 kg]. All birds together fragment 30–35% of the annual shell carbonate production. The other predators (shorecrab, Carcinus maenas; fish) probably produce an equal amount. This indicates that shell-crushing predators account for most of the shell fragments present in Wadden Sea sediments, leaving little room for physical factors. Palaeoecologists have to be aware of the role of shell-crushing by predators in addition to and sometimes even surpassing physical processes in shell fragmentation in ancient environments.