Introduction

The Sanderling (Calidris alba) is a widespread shorebird that breeds in the high arctic tundra, and winters in coastal areas of North America and Europe (^{Macwhirter et al. 2020}). In Mexico, the Sanderling is a transient and winter visitor on both coasts, being rare along the Gulf of Mexico. Punctual inland records with small numbers have been reported during summer in southern Coahuila, Guanajuato and Jalisco, in aquatic habitats of ponds and reservoirs (^{Howell and Webb 1995}). However, no records of this shorebird have previously been reported in the Cuatro Ciénegas Valley.

This intermontane valley, located at the eastern edge of the Chihuahuan desert in the Mexican state of Coahuila, is characterized by extensive areas of wetlands that provide a variety of habitats for migratory and resident bird species (^{Contreras-Balderas 1984}). These wetlands with high endemism of its aquatic biota (^{Minckley 1969}, ^{Contreras-Balderas 1990}) have been designated a RAMSAR site of international importance for the conservation of biodiversity (^{CONANP 2002}). At least eight shorebird species (Charadriidae and Scolopacidae) have been recorded for this region (^{Contreras-Balderas 1984}, ^{Contreras-Balderas and Ruiz-Campos 2022}).

Observation

Our observations were conducted in the Cuatro Ciénegas Valley, a protected natural area of 84,347.47 ha (^{CONANP 2002}) that maintains aquatic biotopes (ponds, springs, streams, rivers, swamps) characterized by its lightly alkaline pH and high conductivity due to the presence of sedimentary rocks of karstic nature (^{García-Ramírez 2005}). This protected area is considered as RAMSAR site 734 that is characterized by springs connected by a channel system that keep the water circulating constantly (^{CONANP 2002}). Emergent plants of Typha domingensis and Phragmites sp. dominate the riparian habitat of the banks (^{Pinkava 1984}), while floating macrophytes as Nymphaea ampla and submerged macrophytes as Chara spp. are representative of the littoral biotope (^{García-Ramírez 2005}).

On 14 September 2022, we recorded two Sanderlings with juvenile plumage at a small shallow pond adjacent to Río Mezquites in the protected area of Cuatro Ciénegas (26°55'13" N, 102°06'21" W, altitude 725 m, Fig. 1). We identified the two juvenile Sanderlings based on their peculiar plumage characterized by the mottled dorsum with black feathers edged with white, the crown streaked with black, and a distinctive dark shoulder patch on the bend of the wing (Fig. 2). We photographed the two juveniles while feeding along with two Western Sandpipers (Calidris mauri) and ten Least Sandpipers (Calidris minutilla).

Figure 1 Sighting of the Sanderling (Calidris alba) in the Área de Protección de Flora y Fauna de Cuatro Ciénegas, Coahuila, Mexico, showing the new record (+) and the previous nearest records (2014-2021) in northeastern Mexico. The interior box depicts the distribution range of the Sanderling in the Americas. 

Figure 2 Photographic records of juvenile Sanderlings (Calidris alba) recorded in the Área de Protección de Flora y Fauna de Cuatro Ciénegas, Coahuila, Mexico, on 14 September 2022. Photographs by Gorgonio Ruiz-Campos. 

The closest previous inland record of Sanderling in the state of Coahuila was a juvenile, recorded on 13 September 2014 in El Tulillo reservoir, General Zepeda, at 156 km SE of our record in 2022 (Fig. 1), but with no photographs. Other records for northeastern Mexico occur in Nuevo Leon state, and include: three individuals with winter plumage in La Juventud reservoir, Marín, at 235 km SE on 27 September 2015, no photographs; two females with breeding plumage in Cerro Prieto reservoir, Linares, at 345 km SE on 5 September 2015, with photographs; and four individuals with winter plumage in Rodrigo Gómez reservoir, Santiago, at 256 km on 24 September 2021, with photographs (Fig. 1).

Discussion

Our observation is the first report of the Sanderling for the Cuatro Ciénegas valley, and the first photographic documentation for the State of Coahuila. Inland records of Sanderlings in northeastern Mexico have been scarce and of few individuals (^{Howell and Webb 1995}). Furthermore, all these inland records in northeastern Mexico (including ours) were obtained in September for individuals resting and/or feeding during the fall migration (^{eBird 2024}).

The presence of winter shorebirds in inland waters has been reported in previous studies on the regional avifauna of northeastern Mexico (^{Urban 1959}, ^{Contreras-Balderas 1984}, ^{Contreras-Balderas et al. 2008}). However, there has been no specific monitoring to evaluate the population dynamics and use of habitat by wintering shorebirds. Therefore, a high spatial and temporal sampling effort is required to evaluate the importance of inland water bodies as stopovers for aquatic birds during migration.