A general decrease in populations of waders on the Ythan estuary, Aberdeenshire, has coincided with an increased biomass of green algae on the intertidal mudflats. It was hypothesised that the reduction of bird numbers was due to the impact of macroalgal mats. The main objective of this study was to test the above hypothesis by investigating the impact of macroalgal mats on aspects of the behavioural ecology of waders, principally on their dispersion, diet and foraging behaviour. Redshank (Tringa totanus L.), curlew (Numenius arquata L.), oystercatcher (Haematopus ostralegus L.), bar-tailed godwit (Limosa lapponica L.) and dunlin (Calidris alpina L.; for study of dispersion only) were chosen for study. To test this hypothesis, wader counts on different estuary sections, pellet and dropping analysis, direct observations on foraging, visual and photographic estimations of weed cover, and invertebrate data were assessed in the context of the energetics of waders and their prey described in the literature. Foraging behaviour was examined on weed-covered and weed-free sediments in natural sites and in field-manipulated plots. Weed cover below 25% (biomass <1 kg fresh weight/ m²) did not adversely influence the foraging of redshank, curlew, oystercatcher and bar-tailed godwit. However, weed cover >50% (biomass > 3 kg fresh weight/ m²) negatively affected the foraging of redshank, curlew and bar-tailed godwit but did not consistently affect oystercatcher. Redshank that fed on weed-covered areas had a reduced numerical food intake rate when compared to the birds on weed-free areas. During peak weed cover on the estuary, redshank on weeded areas had a 35.8% lower gross energy intake rate than those on weed-free areas.