Effects of prey density and site character on estuary usage by overwintering waders (Charadrii)

Abstract

The usage by wading birds of 14 sites on the Forth Estuary in winter was examined in relation to the density of their invertebrate prey and the area, configuration and exposure sequence of the intertidal sites. Six common waders were studied over the whole estuary and three species in greater detail at the largest single site. Significant associations were shown in five species for feeding-hours km −2 and numbers km −2 with the density of at least one of their main prey; including oystercatchers Haematopus ostralegus (L.) with Mytilus edulis (L.); curlew Numenius arquata (L.) with Nereis diversicolor (L.); redshank Tringa totanus (L.) with Nereis diversicolor; knot Calidris canutus (L.) with Cerastoderma edule (L.) and dunlin Calidris alpina (L.) with Nereis diversicolor. For four species site characteristics also apparently affected feeding patterns [bar-tailed godwit Limosa lapponica (L.), area; redshank, exposure sequence; knot, area, configuration and exposure sequence; dunlin, area]. Overall, significant variables explained 43–96% of the variation in wader feeding-hours km −2 on the estuary and 41–87% of numbers km −2. It is emphasised however, that the observed variation in feeding patterns on the Forth Estuary represents only a part of the total variation in usage of estuaries by feeding waders and further studies are necessary to explain the extent and causes of this additional variability.