Substrate Preference and Substrate Related Foraging Behaviour in Three Calidris Species

In Calidris species (sandpipers) for whom probing in the substrate is a normal foraging technique, bill morphology shows many constructional features meeting the demands set by probing. A comparative study is made of relations between foraging activities and substrate type for three species: Calidris alpina, C. alba and C. maritima. During the wintering period these species are more or less restricted to foraging on mudflats, sandy beaches and rocky shores, respectively. Under aviary conditions different sorts of substrates, comprising identical prey and equal prey densities, were offered to adult individuals of the three species. The influence of mud, sand and of small and large pebbles on the foraging behaviour was registered, using a time-true and detailed event-recording technique. This allowed a direct quantitative analysis of all behavioural subpatterns and foraging performances, on the level required for a functional morphological explanation. In general mud and sand are preferred to pebbles as substrates to forage on in spite of equal prey densities. The three species showed on all substrates two types of hunting : touch-hunting and eye-hunting. The ratio touch-huntingleyehunting is higher on mud than on sand or pebbles. The one exception is that C. alba has equal ratio's on mud and sand. A comparison between species reveals that each species has its characteristic foraging behaviour pattern for each of the substrates offered. Calidris alpina is by far the most dedicated touch-hunter of the three species. Calidris alba inclines more to eye-hunting but is rather similar in other aspects, while C. maritima is found to be both an extremely successful eye-hunter and touch-hunter, at least under exprerimental conditions. Calidris alpina prefers mud, which corresponds with its natural habitat, but this coincidence is not observed in the other species. Notwithstanding the substrates they encounter in nature neither C. alba nor C. maritima prefers sand respectively pebbles under experimental conditions. A hypothesis about "default" and "current" hunting techniques is formulated to explain short term suboptimal foraging activities. The "default" hunting technique is used when the bird has no immediately available information (default of information) to decide what technique is most appropriate under operative conditions. The "current" hunting technique describes conservative application of techniques while the foraging situation is changing. From the literature and this study two rules are formulated describing the relation between the sandpiper foraging behaviour and the substrate. The first rule states that, in the process known as habitat segregation, the relatively long-billed species and individuals will select softer substrates than the short-billed counterparts. The second rule states that Calidris individuals show a shift towards eye-hunting in the firmer substrates. The quantified foraging behaviour is one of the variables needed (ZWEERS, 1985a) to determine the functional demands used in the explanation of bill construction (DULLEMEIJER 1974). This study serves to understand the relations between foraging behaviour and substrate.