Rates of Change in the Numbers of Dunlin, Calidris alpina, Wintering in British Estuaries in Relation to the Spread of Spartina anglica

Abstract

(1) The number of dunlin, Calidris alpina alpina (L.), wintering in the British Isles has decreased by almost half since 1973-74, much of the decrease occurring before 1977-78. A similar decrease seems to have occurred in the other main wintering areas in Holland and France between 1973-74 and 1977-78. But since then, numbers in these two countries have not decreased whereas in Britain numbers declined by a further 77,000 by 1985-86. (2) The rate of decline since 1977-78 has varied between British estuaries, many showing no change in numbers. Dunlin numbers declined at the greatest rates in estuaries where cord grass, Spartina anglica C. E. Hubbard, has spread most over the intertidal flats where the birds feed. In estuaries where the extent of Spartina has not changed, numbers of dunlin have not declined significantly, except in some small estuaries in south Devon and in north Scotland. (3) The results suggest that most birds displaced from estuaries where Spartina has spread have not yet been able to re-establish themselves elsewhere in Britain or in some other countries within the wintring range of Calidris alpina. Nor have dunlin numbers increased n estuaries where Spartina has declined through natural die-back. The reasons for this, and the implications of the results as a whole for the conservation of dunlin, are discussed.