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

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. 103 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. 103 kg shell fragments. The role of other shell-consuming bird populations is less [e. g. Shelduck (Tadorna tadorna) 1,800. 103 kg, Knot (Calidris canutus) 1.6. 103 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.