Aspects of the breeding biology of wading birds (charadrii) on a saltmarsh

Aspects of the breeding biology of Lapwing, Oyster- catcher and Redshank were studied on a dry, cattle-grazed saltmarsh, Rockcliffe Marsh, in Cumbria. The vegetation of the marsh was predominantly graminoid, due to the influence of grazing and trampling by cattle. There was a halosere from the landward Lolio-cynosuretum to the seaward Puccinellietum. Invertebrate abundance and biomass declined across the halosere, as did grazing intensity, which was indicated by cowpat density. Cowpat density was positively correlated with the abundance and biomass of Diptera and total invertebrates. The proportion of dung-associated invertebrates varied across the halosere, but over 80% of Diptera in each vegetation type were dung-associated. The proportion of eggs plus chicks of each species which was trampled was positively correlated with cowpat density, indicating that cowpat density was a valid measure of grazing intensity. Each wader species nested at a higher than average cowpat density where the mean cowpat density was low, to maximise food availability, and at a lower than average cowpat density where the mean cowpat density was high, to minimise the risk of trampling. Lapwing nest density was positively correlated with cowpat density (proximate factor) and total invertebrate biomass (ultimate factor). The main prey of adult and chick waders were dung-associated invertebrates. The proximate factors involved in breeding area and nest-site selection by the fore-mentioned wader species and Dunlin and Ringed Plover were elucidated by a multivariate comparison of nest and non-nest samples. The proximate factors were typically related to those features associated with a grazed habitat, e.g. tussock abundance, cowpat density, and with the avoidance of inundation, e.g. distance to nearest creek and plateau edges. The proximate and ultimate factors were discussed with reference to their implications for breeding wader habitat management.