Migration of endangered Red Knots Calidris canutus rufa in Uruguay: important sites, phenology, migratory connectivity and a mass mortality event

摘要

Southern populations of Red Knots Calidris canutus rufa have suffered a dramatic decline since the year 2000. Although knots are one of the best known shorebird species in the Western Hemisphere, little is known about them in Uruguay. However, in 2007 the discovery of at least 312 dead knots at La Coronilla on the northern Atlantic coast of Uruguay attracted international attention to this part of the flyway. Here we present historical information gathered from museum collections, local ornithologists and literature to identify high-priority sites, as well as abundance and dates of occurrence of Red Knots along the Uruguayan coast. Information is also presented regarding the mortality event in 2007. To establish the current role of Uruguay in the Red Knot flyway, we also investigated phenology, minimum length of stay, connectivity with nearby Argentinian and Brazilian sites, and habitat use at a main Uruguayan site during northward migration from 2009 to 2011 and in the austral summer in 2007. We identified 96 historical records from 10 localities where Red Knots were detected at least once in the period 1951-2008. The number of birds per record ranged from 1 to >2,000, but most observations (ca. 80%) ranged from a few to 100 birds. The sites with more records of Red Knots and the highest counts were relatively flat sandy beaches of the dissipative morphodynamic type, which have a higher abundance of potential prey for Red Knots (especially Wedge Clams Donax hanleyanus), for instance Barra del Chuy beach, near the Brazilian border. Most historical observations occurred during northward migration in late austral summer and fall. The maximum count at Barra del Chuy was 1,191 birds in April 2010. However, very few birds were recorded in 2011. Median minimum length of stay of individually color-marked birds that were seen on at least two days was estimated as 5 days (range: 2-26 days). Several individual birds were observed on both sides of the Brazil-Uruguay border, suggesting that Rio Grande do Sul (Brazil) and Barra del Chuy (Uruguay) should be treated as a single staging area. Resightings of Red Knots banded at Rio Grande (Tierra del Fuego non-breeding site) and San Antonio Oeste (a Patagonian stopover area) in Barra del Chuy, suggest that Rio Grande do Sul-Barra del Chuy and San Antonio Oeste function as