Temporal variation in abundance and the population status of non-breeding Nearctic and Patagonian shorebirds in the flooding pampa grasslands of Argentina

Abstract

This study describes annual variation in abundance and population status of non-breeding migratory shorebirds in flooding pampa grasslands of Argentina. We studied two Nearctic species, the American Golden-Plover (Pluvialis dominica) and the Buff-breasted Sandpiper (Tryngites subruficollis), and two Patagonian species, the Tawny-throated Dotterel (Oreopholus ruficollis) and the Rufous-chested Dotterel (Charadrius modestus), at Medaland Ranch in the Villa Gesell District of Buenos Aires Province, Argentina. Counts were made every 15 d between March 1996 and January 2000. Numbers of Rufous-chested Dotterels were generally highest toward the end of winter (July–September) indicating that the species uses this area as a stopover site. Tawny-throated Dotterels were present throughout the entire winter period (April–September). The Nearctic species spent the austral spring–summer months at Medaland (September–February) before returning to their breeding grounds in North America. Comparison of our numbers to historic counts indicates declines of Buff-breasted Sandpipers (especially during the past 25 yr) and American Golden-Plovers throughout the past century. Medaland Ranch is clearly an important wintering site for these four migratory shorebirds. The departure of Patagonian species and the arrival of Nearctic species always occurred within a short period of time (days), but the time factor between departure of Nearctic and arrival of Patagonian species was measured in months. Our observations tend to support the hypothesis that the migratory pattern of Patagonian shorebirds results from avoiding competition with Nearctic shorebirds.