Sediment Characteristics, Invertebrate Densities and Shorebird Densities on the Inner Banks of the Wash

1. This paper tests the possibility that, primarily through their effect on invertebrate prey densities, sediment characteristics can be used to predict the densities of shorebirds (Charadrii), on the inner banks of the Wash, east England. 192 quadrats were established with, on most shores, a transect of nine quadrats from the marsh edge to the Low Water Mark (LWM). Shorebirds were counted in winter in each quadrat. The densities of the main invertebrate prey species were sampled in early winter/late autumn at the mid-point of each quadrat, together with the sediment particle size and organic content. The time for which each quadrat was inundated over high water spring tides was also measured. 2. Regression analyses revealed that the densities of each of the eight shorebird species were related to the densities of several of their known, or suspected, prey species. A similar analysis showed that the densities of most prey species were, in turn, related to the proportion of one or more of the four sediment particle size categories in the substrate; coarse sand, fine sand, silt and clay. The densities of most invertebrates were also related to inundation time, with most relationships being strongly non-linear. 3. In most cases, the effect of prey density on bird density remained significant when the sediment characteristics and inundation time, together with a measure of a quadrat's distance from the LWM, were added to the regression analysis of variations in bird densities. In a few cases, one component of the sediment particle size distribution additionally affected bird density. On the west and south-west sides of the Wash in particular, the densities of several shorebirds were also markedly higher in quadrats close to the LWM, even with the effect of both prey density and sediment particle size taken into account. The results were largely consistent, however, with the hypothesis that the main factors determining spatial variations in bird densities were variations in prey densities. 4. It was concluded that, because of the strong influence of the sediment on the densities of most prey species, and because the model was based on transects which spanned the whole width of the shore so that the tidal inundation time was comparable in each, the sediment particle size distribution could be used to predict shorebird densities directly. It was not necessary first to predict the densities of the invertebrates on which they feed.