Nonbreeding shorebirds in a coastal agricultural landscape: Winter habitat use and dietary sources

The Fraser Delta's intertidal mudflats currently support approximately 44,000 shorebirds during the winter. Adjacent agricultural fields also provide roosting and feeding habitat, but land-use changes are reducing the availability of open-soil farmland. A multi-faceted research program was designed to quantify farmland use by Dunlin (Calidris alpina pacifica), Black-bellied Plover (Pluvialis squatarola), and Killdeer (Charadrius vociferus) during three non-breeding seasons (October–April) from 1997–2000.; Habitat surveys and farmer interviews revealed disproportionately greater use of fields close to shore for Dunlin and Plover, and smaller fields for Killdeer. Dunlin made disproportionately greater use of bare, cover crop, winter vegetable, and grass fields; Plover mainly used bare and cover crop fields, and Killdeer used all crop types equally. Agricultural practices appearing to enhance field usage were multi-year applications of manure, inorganic fertilizer, laser levelling, and a longer time with the same crop. All species occurred more frequently in fields previously used by conspecifics and where other shorebird species were present.; The frequency of occurrence in fields was greater by night for Dunlin and by day for Plover and Killdeer. Environmental predictors of field use differed between day and night, and among species. Shorebirds used fields less frequently during increased nocturnal moon illumination, consistent with the hypothesis that perceived predation risk is an important modulator of field use.; The proportional contribution of diet from terrestrial fields versus marine mudflats was quantified via stable isotope analysis (13C and 15N) of Dunlin blood samples. Isotopic turnover rates and tissue fractionation factors for these isotopes were also measured experimentally for Dunlin in captivity. Based on these results, mudflats were the main source of invertebrate prey for Dunlin, contributing approximately 70% of their diet. However, extreme inter-individual variation was found, with farmland dietary contribution ranging from 0–87%. Juveniles had consistently more terrestrial diets than adults in all years. The proportion of diet from fields varied with body shape, but not with overall skeletal size or sex. Culmen lengths were shorter relative to wing and tarsus lengths for Dunlin with more terrestrial-based diets. The results of this research will facilitate targeted conservation and management strategies for non-breeding shorebirds.