Food Sources And Diet Composition Of Calidris Shore Birds At Chongming Dongtan

Habitat loss has become the leading reason for worldwide decline of shorebirds. Feeding ecology therefore is of great significance for evaluating habitat quality and developing strategies to protect shorebirds. Previous studies showed that shorebirds mainly consumed the macrobenthic invertebrates such as bivalves, crustaceans, polychaetes and gastropods in non-breeding seasons, including stopover and wintering periods. However, temporal dynamic change of shorebird diet has been poorly understood. Recent research has assumed that at some stopover sites, biofilm has been an important food source of shorebirds, especially of small sandpipers. Such results challenged food composition and also the conventional trophic position of shorebirds.Spartina altemiflora is a perennial rhizomatous grass native to the east coast of North America. It was introduced intentionally to Shanghai in2001. Then it spread rapidly throughout the intertidal area at Chongming Dongan and has currently become one of the dominant plants in the estuarine wetlands. Potential impacts caused by Spartina invasion, such as food source change of shorebirds and the evaluation of new habitats on the East Asian-Australasian Flyway, are urgent issues to research.The present study was focused on the diet of sandpipers in non-breeding seasons at Chongming Dongtan. First we reconstructed the diet composition of Great Knot (Calidris tenuirostris) during spring stopover, using dropping analysis combined with the investigation of macrobenthic invertebrates. Foraging strategies of Great Knot were discussed as well. After that, we used stable isotope analysis to study the seasonal variation of diet composition of dunlins(Calidris alpina), and to compare the diet composition of Great Knot, Red Knot (Calidris canutus), Red-necked Stint (Calidris ruficollis), Dunlins, Sharp-tailed Sandpiper (Calidris acuminata) and Long-toed Stint (Calidris subminuta). We also discussed the contribution of biofilm and the food source contribution of Spartina alterniflora to the six sandpipers at Chongming Dongtan.The main conclusions were as follows:1. Our results indicated that prey proportions in the diet of Great Knots were not significantly different from their abundance in the feeding area, implying that diet composition of Great Knots varied with the fluctuation of benthic fauna. In other words, Great Knots adopt an opportunistic foraging strategy during the spring stopover at Chongming Dongtan. As shorebirds typically migrate across vast landscapes where wetlands are temporally and spatially dynamic, it might be costly to discriminate between profitable prey and unprofitable prey. Consequently, the flexibility of an opportunistic foraging strategy allows mem to increase their probability of being able to replenish energy and nutrient reserves for continuing their migration to breeding and wintering grounds as well as arriving on the breeding grounds in good condition.2. The results of stable isotope analysis indicated that:(1) The discrimination of δ15N between biofilm and the sandpipers exceeded the empirical fractionation factor. Thus biofilm might not be assimilated by the sandpipers, but was likely to be swallowed randomly. Biofilm made little contribution to the sandpipers during their stopover at Chongming Dongtan. The components of sediments could explain that.(2) There was significant seasonal variation in the diet composition of dunlins. Bivalves made up the most of the diet during both spring and autumn stopovers. The contribution of crustaceans increased significantly in autumn, while gastropods and polychaetes contributed less. The proportions of macrobenthic invertebrates in the diet of dunlins were similar in winter. Temporal and spatial dynamics of macrobenthos biomass may be the primary reason of such variation.(3) In the food source contribution from three types of primary producers, there was also significant seasonal variation. The food source of dunlins was mainly from C3plants in spring stopover and C4plants in autumn. The contribution from C3plants, C1plants and microalgae to dunlins was similar in winter. Seasonal variation of diet composition was probably the reason for such difference. Spartina alterniflora made up the most of food source to dunlins at Chongming Dongtan, especially during south migration when the contribution of Spartina was over60%.(4) The food composition of six sandpiper species was different. Heterogeneity of macrobenthic invertebrates at feeding areas may account for such food divergence between species.(5) Spartina alterniflora and microalgae were the main food sources of Great Knot, Red Knot, Red-necked Stint, Dunlins, Sharp-tailed Sandpiper and Long-toed Stint. Spartina contributed almost as much as the native C3plants did to the sandpipers.