Chironomus salinarius larvae are the dominant benthic organisms in the salt pans of the Odiel marshes, southwest Spain, an area internationally important for migratory shorebirds. Predator exclosures were used to examine the effects of shorebirds on chironomid larvae for periods of 32 to 46 d during spring migration. During the 1^st experimental period, shorebird numbers were relatively low, peaking at 4 feeding birds/ha. During the 2^nd period, shorebird numbers were higher, peaking at 32 feeding birds/ha. The changes in larval density and biomass from the beginning to the end of each period were significantly greater in exclosures than in controls. Predation decreased larval density by 35% and 32% during periods 1 and 2, respectively. Predation decreased larval biomass by 37% and 49% during periods 1 and 2, respectively. Predation also changed prey size distributions, reducing the proportion of larger larvae. These effects did not vary significantly between periods, suggesting that increased predation in period 2 was compensated for by higher larval growth or recruitment in period 2. Variation in predation effects was not related to variation in the initial larval density or biomass in a given plot.