Effects of intertidal habitat availability on the use of anthropogenic habitats as foraging grounds by shorebirds: a case study on semi-intensive shrimp farms

Abstract

Loss of wetlands may restrict foraging opportunities for many species of migratory shorebirds that depend on such habitats during the non-breeding periods. In addition, tidal cycles, both daily (high and low tides) and lunar (spring and neap tides), periodically modulate the availability of foraging areas within coastal wetlands, which can limit area and foraging time for shorebirds. We tested whether densities of shorebirds (marbled godwit, willet, western sandpiper, black-necked stilt, American avocet, two dowitcher species, and whimbrel) observed at a shrimp farm varied with predictable tidal cycles on the coast of northwestern Mexico. We found that most species occurred in higher densities during high- and neap-tides, when nearby intertidal areas were flooded and thus unavailable for foraging shorebirds. The majority of shorebirds at the shrimp farm were actively feeding, indicating that the shrimp farm can provide an alternate foraging habitat. As an exception, western sandpipers were found in lower densities during neap tides, and we suggest that predation risk from peregrine falcons at the shrimp farm may displace this species to other nearby safe habitats. Understanding the spatio-temporal variability on the use of shrimp farms will increase the general knowledge about the function of alternative anthropogenic habitats for migratory shorebirds.