Macroalgal Mats in a Eutrophic Estuary Obscure Visual Foraging Cues and Increase Variability in Prey Availability for Some Shorebirds

Conservation of habitat for flagship species, such as birds, aids in the protection of biodiversity in critical ecosystems. Extensive mats of macroalgae stimulated by nutrient input to estuaries threaten critical successes in conservation made by legislation that reduces habitat destruction. Macroalgae can cover intertidal mudflats that are key foraging grounds for obligate visual foragers such as black-bellied plovers (Pluvialis squatarola), as well as foragers that routinely switch from visual to tactile foraging such as least sandpipers (Calidris minutilla), western sandpipers (Calidris mauri), and willets (Tringa semipalmata), and predominately tactile foragers such as marbled godwits (Limosa fedoa). We hypothesized that macroalgae would affect shorebirds directly by altering foraging behavior and indirectly by reducing prey availability. Mats reduced visual foraging (pecking) for sandpipers and marbled godwits and caused them to probe more often. Willets spent overall less time foraging than sandpipers and marbled godwits and did not alter their foraging strategy due to macroalgae. While foraging, black-bellied plovers, marbled godwits, and willets avoided macroalgae while sandpipers did not select between habitat types. This suggests macroalgae may have negative effects on plovers, marbled godwits, and willets by reducing foraging area but that sandpipers may utilize both macroalgae and bare sediments. Macroalgal mats indirectly influenced shorebirds by enhancing variability in prey availability across the mudflat landscape. By quantifying avoidance of or preference for mats, foraging behavior, and variation in prey availability, we showed that macroalgae have differential effects across shorebird foraging guilds.