The status of the swallowtail butterfly in Britain

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Abstract
Since the early 1950s, when the British swallowtail butterfly (Papilio machaon britannicus) became extinct at Wicken Fen, this subspecies has been confined to marshes around the Norfolk Broads. Here too, however, its abundance appears to be declining. The ecology of the butterfly has been studied in Norfolk and at Wicken (by small‐scale introductions) and the main factors determining its abundance are described. A morphometric study of museum specimens has shown that populations of the butterfly in Norfolk and at Wicken differed in a number of size and shape characters. These differences were most marked between 1890 and 1920, but subsequently were almost lost. Studies of the status and performance of the butterfly's food plant, Peucedanum palustre have shown that it is smaller, produces less seed, and is shorter‐lived at Wicken than in Norfolk. In the light of these findings, the possibility of successfully re‐establishing the butterfly at Wicken, and the possible causes of the decline of the species in Norfolk are discussed.