The coastal grasslands of eastern Buenos Aires Province (Argentina) are used by Rufous-chested Dotterel (Charadrius modestus), Tawny-throated Dotterel (Oreopholus ruficollis), American Golden Plover (Pluvialis dominica) and Buff-breasted Sandpiper (Tryngites subruficollis). These species differ in size but all use visual pecking when feeding. The diet of each species was studied, measuring temporal variation in the abundance of invertebrates and dietary preferences of invertebrates. From March 1996 to January 1999, feces were collected of shorebirds on Medaland Ranch grasslands (Villa Gesell District, Buenos Aires Province, Argentina). Pitfall traps were used to capture surface-active invertebrates (from June 1996 until March 1998). All four bird species ate animal and plant items. In all cases, plant fragments rather than seeds dominated the plant fraction. Beetles comprised the dominant item in all four species of shorebirds. Earthworms were important for Patagonian shorebirds, but they were less frequent in the diets of Nearctic shorebirds. Relative use (proportion in the diet) of items differed by site and date. Numbers of captures were high in spring-summer and low in autumn-winter. All shorebird species preferred adult beetles. Rufous-chested Dotterel and Nearctic shorebirds also showed a preference for beetle larvae. In contrast, spiders and ants were avoided by all shorebird species. Diet information from Nearctic shorebirds at other sites in the Northern Hemisphere and in South America, confirmed that co-occurring species within a given region typically had similar diets, but that the same species in different regions showed low similarity. Dietary flexibility allows exploitation of variable resources and, as a consequence, is highly advantageous to shorebirds that migrate over long distances and use a variety of habitats.
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Vol. 28 • No. 2