| Summary: | The Marismas Nacionales wetland system (Nayarit, Mexico) is a Site of International Importance as designated by the Western Hemispheric Shorebird Reserve Network. The American Avocet (Recurvirostra americana) is one of the most numerous shorebirds in the area; nevertheless, there is little information about its overwintering stay at Marismas Nacionales, despite part of the site being decreed as a Biosphere Reserve. In order to clarify some aspects of American Avocet winter ecology, we used transects and scanning sampling method during the winter: to determine distribution and abundance in the Marismas Nacionales (2010-2011), as well as density and population structure in one of the sectors of the Biosphere Reserve (2011-2013). The American Avocet wintering population in the Biosphere Reserve was calculated to be about 26,000 individuals (5.7% of the world’s population), but approximately 36% of avocet wintering population in Marismas Nacionales was outside of the reserve, also there was a certain preference for some wetlands. Evidence suggests that spring migration starts during the first weeks of March. We observed some spatial segregation between sexes, but our results, although valid, are inconclusive because of difficulties in sexing individuals in the field. In spite of local changes in these wetlands, American Avocet winter population has apparently been relatively stable over the last twenty years. Therefore, Marismas Nacionales remains a key wetland for American Avocet in North America.
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