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Published online by Cambridge University Press: 05 December 2011
Relationships between feeding ecology, population dynamics and conservation of estuarine shorebirds are becoming better understood mainly as a result of detailed long-term studies of birds in the northwest European estuaries most vulnerable to industrial and agricultural developments. Until five years ago the tidal flats of the Clyde Estuary held internationally and nationally important populations of ducks (Anatinae) and waders (Charadrii). To understand the reasons for changes in bird distribution it is necessary to know more about the factors which determine distribution. In this paper we examine the pattern of association between the Clyde's birds and their prey by evaluating the extent to which the distribution of birds is related to that of benthic invertebrates, heterospecific birds, season and tidal state.
Stepwise multiple regression analysis indicated that heterospecifics had effects on bird distributions over-riding those of prey. In summer, the significant independent variables explained 41–61% variation in bird density (number km−2) and 33% variation in bird feeding hours (bird-hours km−2); in winter they explained 17–35% and 29–32% respectively. The amount of variation explained was greater during flow than ebb tides, and the number of species for which some of the variation was explained was greater in winter than in summer. Three associations, each consisting of two bird species, are suggested: redshank with lapwing, dunlin with mallard, and shelduck with gulls. We provide explanations for some of the mechanisms underlying the above patterns and indicate areas for more detailed observational and experimental work. The integrity of the avian component of the estuarine ecosystem appears to be dependent on associations and interactions between birds as well as between birds and their prey. The associations are labile, and in many species stem from the effect of tidal movement on prey availability.