The tidal movements of flocks of oystercatchers foraging on mudflats at low tide and roosting inland behind a dike at high tide were studied and the effects of day-to-day variations in the time of mudflat exposure by ebb analysed. High mean water levels and short low tides led to reduced intake during low water due to increased bird densities in addition to temporal constraints. Increased feeding around the roost apparently compensated for some of the reduced intake although accurate intake measurements could be made for foraging on the tidal flats only. It is argued that optimal timing of foraging flights to coincide with exposure of the mussel banks would contribute to exploitation of this tidal food source. The median departure time from the roosts relative to the time of mudflat exposure was early on days when the tide went out late and late when the tide was early. Daily variations in departure time were predicted by the daily variations in tabulated high water times, but not by variations in mudflat exposure or coverage. The conclusion is drawn that the birds employ a timing mechanism not directly associated with the tidal water movements. In some pilot experiments in caged oystercatchers, feeding schedules elicitated feeding attempts in anticipation of expected food. The anticipatory patterns were different for fixed end tidally shifting daily food schedules, and moreover differed between the two feeding times per day. Five possible mechanisms for tidal anticipation are discussed, making use either of unknown exogenous cues, or of - likewise unknown - endogenous timers of hourglass type or rhythmic with circatidal, circalunadian or circadian period. Experimental tests for these possibilities are outlined.