SEDIMENT CHARACTERISTICS, INVERTEBRATE DENSITIES AND SHOREBIRD DENSITIES ON THE INNER BANKS OF THE WASH

1. This paper tests the possibility that, primarily through their effe ct on invertebrate prey densities, sediment characteristics can be use d to predict the densities of shorebirds (Charadrii), on the inner ban ks of the Wash, east England. 192 quadrats were established with, on m ost 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 ea rly winter/late autumn at the mid-point of each quadrat, together with the sediment particle size and organic content. The time for which ea ch quadrat was inundated over high water spring tides was also measure d. 2. Regression analyses revealed that the densities of each of the e ight shorebird species were related to the densities of several of the ir known, or suspected, prey species. A similar analysis showed that t he densities of most prey species were, in turn, related to the propor tion of one or more of the four sediment particle size categories in t he substrate; coarse sand, fine sand; silt and clay. The densities of most invertebrates were also related to inundation time, with most rel ationships being strongly non-linear. 3. In most cases, the effect of prey density on bird density remained significant when the sediment ch aracteristics and inundation time, together with a measure of a quadra t's distance from the LWM, were added to the regression analysis of va riations in bird densities. In a few cases, one component of the sedim ent 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 partic le size taken into account. The results were largely consistent, howev er, with the hypothesis that the main factors determining spatial vari ations 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 tr ansects which spanned the whole width of the shore so that the tidal i nundation time was comparable in each, the sediment particle size dist ribution could be used to predict shorebird densities directly. It was not necessary first to predict the densities of the invertebrates on which they feed.