Hulscher J.B. (1976) Localisation of cockles (Cardium edule L.) by the Oystercatcher (Haematopus ostralegus L.) in darkness and daylight. ARDEA 64 (3-4): 292-310

This article describes feeding behaviour of an Oystercatcher with clipped wings by day and by night on cockles buried just beneath the substrate surface. In daylight the bird generally makes pecking movements, probably directed visually at cockle surface marks. In darkness the bird places the bill at an angle of about 70 degrees 1-2 cm into the substrate and walks in this posture in a straight line while the bill makes quick 'sewing' movements (little ups and downs) without retracting the bill from the mud. When the bill touches a cockle sewing stops abruptly and the Oystercatcher tries to pierce its bill as quickly as possible between the two valves at the 'persistent gape'. In daytime at low densities (13 and 40 cockles/m2) both the pecking and sewing technique are applied. The sewing technique suggested a chance encounter with cockles by the foraging bird (random-searching hypothesis). This was tested experimentally by observing the foraging bird on cockle beds at different densities. The number of cockles found was compared with the number calculated that could be found when random searching. Calculations are possible if the distance the bill tip travels through the mud is known, as well as the density and size of the cockles (Fig. 2). The distance the bill tip travelled through the substrate during sewing was calculated in two ways: by counting the number of steps taken (step length 8.2 cm) and by determining the total time spent in sewing (displacement rate of the bill tip 13.9/sec). In daylight, the number of cockles localised at densities 13 and 40/m2 using the sewing technique were in agreement with the random-searching hypothesis. At night at a density of 40 cockles/m2 just too few cockles were found, at a density of 150 cockles/m2 quite a lot too few were found according to the random-searching hypothesis (Fig. 4). It is suggested the bird actually did localise all cockles it theoretically ought to have encountered, but it made the decision of opening or not opening a cockle instantaneously after localisation, so that the observer was prevented from distinguishing between the fraction of the moment of localisation and continued sewing. Overall food intake in daylight and in darkness under comparable conditions is identical: 3.40 grams flesh (total-dry-weight) per hour (Table 2). Procentually more cockles localised with pecking were opened successfully than with sewing (Table I). It is assumed that in daylight the bird is better informed on the orientation of cockles in the substrate via surface marks than in darkness. The bird thus has a greater chance of thrusting its bill precisely between the two valves of the cockle while attacking, in order to sever the adductor muscles. This fits in with the fact that in daytime less time is spent in opening and eating a cockle than in darkness (Fig. 9).