Herring gulls (Larus argentatus) learned to exploit a new food source in the Wadden Sea near Texel, the introduced Pacific oyster Crassostrea gigas. The first 'wild' specimens of this oyster were observed in 1983. Due to a series of warm summers in the 1990s, the invader success- fully reproduced and rapidly spread over the Dutch Wadden Sea. Herring gulls break large bivalves by shell-dropping; they lift oysters (and adhering attached objects) of up to 93 grams in weight. They were less successful in breaking the shells of oysters than of mussels Mytilus edulis; only about one third of the oysters were broken by shell-dropping, in mussels this was nearly 100%. Experiments indicated mussels to break easier during shell dropping. Future research will indicate whether herring gulls will leam to improve their breaking technique.
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