Towards Predicting the Effect of Loss of Intertidal Feeding Areas on Overwintering Shorebirds (Charadrii) and Shelduck (Tadorna tadorna): Refinements and Tests of a Model Developed for the Wash, East England

Abstract

1. The limitations of a model developed in an earlier study to predict the effect of salt-marsh reclamation on shelduck (Tadorna tadorna) and seven wader species (Charadrii) feeding on the Wash, east England, are addressed. The model related the numbers of each species feeding down-shore of the marsh in a 1-km broad transect to the area of mud or sand present, and thus to the width of the shore itself. 2. Three censuses of the intertidal feeding grounds were made during the winters of 1989-90. 1990-91 and 1991-92, in addition to one made in 1985-87 on which the earlier model was based, to incorporate annual variations in bird numbers into the model and to re-affirm the constancy in bird distribution. Sediment distribution was determined by satellite image analysis. The model was reformulated to include quadratic relationships. 3. The model was tested both within the Wash and on the Essex coast of east England. Within the Wash, its predictions improved as each year's census was incorporated and as predictions were made for areas of increasing size. Outside the Wash, the model predicted the numbers of all species with reasonable precision, suggesting it can be applied over a wider geographical area. 4. It was concluded that the model gives satisfactory predictions of how, in general, bird numbers would change in response to changes in shore width and sediment composition. By converting numbers to percentages, its predictions can be applied to particular areas for which the pre-change bird numbers are known. The model may also be applicable to predicting the effect of other changes that directly or indirectly reduce the shore width; for example, barrage construction, sea-level rise and disturbance.