Invertebrate prey activity varies along the tidal cycle and depends on sediment drainage: Consequences for the foraging behaviour of waders

The influence of prey activity on the use of intertidal flats by foraging waders is largely unknown. Due to differences in sediment drainage some flats remain covered by a layer of water during much of the exposed part of the tidal cycle, whereas others drain soon after the passage of the tidal line. Invertebrate prey may respond to the presence of water over the sediment, so their activity is likely to vary throughout exposure and among differently drained sites. Because active prey tend to be more conspicuous to predators, sediment drainage may also influence the foraging behaviour and intertidal flat use by waders. Using video recordings and controlled drainage experiments, we studied how the activity of the two most important wader prey in the Tagus estuary, Hediste diversicolor Malmgren and Scrobicularia plana (da Costa), is influenced by sediment drainage, and how it varies throughout the exposed part of the tidal cycle. H. diversicolor was far more active in well drained sediments and at later phases of the exposure period. In contrast, activity of S. plana peaked in early phases of exposure but declined quickly as the sediment drained, except when poor drainage maintained a thin water layer. In concordance with these results, wader species that feed extensively on S. plana in the Tagus tend to follow the tidal line and concentrate on poorly drained areas, while those that prefer H. diversicolor are not tide-followers and remain on well drained areas. This agreement suggests that sediment drainage and associated prey rhythms greatly influence wader foraging patterns on sediment flats.