Coastal ecosystems provide important feeding opportunities for shorebirds, depending on their prey availability, hydrodynamic conditions and human pressure. We aimed to evaluate short-term responses (occurrence and minimum approach distance) of shorebirds to urbanization and natural drivers on an extensive beach arc adjacent to the largest hypersaline coastal lagoon of South America. The presence of the migrant Sanderling Calidris alba and resident Collared Plover Charadrius collaris was geo-coordinated and for each record and an equivalent number of random points we measured the distance from urban settlements, swash width and invertebrate abundance (food availability) in a snapshot sampling. The distance at which each shorebird flock escaped from humans was determined (minimum approach distance). The occurrence of shorebirds was predicted by food availability, and higher crustacean density was found in areas of shorebird occurrence (37 ± 19 individuals/m2) compared to random points (10 ± 10 individuals/m2). This result highlights the importance of the beach as a feeding area and the need for conservation of shorebirds' prey. The resident Collared Plover delayed their escape from humans in areas closer to urban areas, suggesting a higher tolerance to humans on disturbed beaches, where they can prioritize the food intake rather than vigilance. The larger flocks let the researcher get closer, corroborating the risk-dilution theory stating that flocking behaviour during foraging provides protection of birds from predators. In conclusion, our results showed that monitoring of sandy beaches based on shorebirds' presence and behaviour can be a reliable tool, especially close to coastal lagoons that constitute foraging sites for these charismatic species.
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Vol. 56 • No. 2