HOLLINGS FUNCTIONAL-RESPONSE MODEL AS A TOOL TO LINK THE FOOD-FINDINGMECHANISM OF A PROBING SHOREBIRD WITH ITS SPATIAL-DISTRIBUTION

1. Knots Calidris canutus are high-arctic breeding shorebirds which sp end the nonbreeding season in intertidal areas where they feed on smal l buried molluscs which are swallowed whole. We tested whether their i ntake rate can be adequately described by a functional response model (the disc equation of Holling) assuming prey detection by direct touch . 2. Knots were fed two bivalve species (Macoma balthica and Cerastode rma edule) of different size and at different density and in Macoma at variable depth on small areas of soft Sediment. Five different indivi duals were used in feeding trials during which six prey were allowed d etection and ingestion. 3. Knots perfectly obeyed the two basic assump tions of Holling's disc equation: neither the instantaneous area of di scovery nor the handling time changed with variation in prey density. 4. However, two predictions for prey perception by direct touch (that the instantaneous area of discovery is a positive function of shell si ze and a negative one of prey depth) were rejected. The instantaneous area of discovery was usually also higher than predicted from the area touched by probing knots. 5. Knots may, therefore, use a 'remote sens e' to locate buried hard-shelled prey. Gerritsen, van Heezik & Swennen (1983) showed that knots do not use taste to discover bivalves at a d istance. We argue that the detection involves a self-induced pressure mechanism rather than a passive sense for vibrations, since it is unli kely that static bivalve prey emit such signals just after experimenta l handling. 6. Whatever the exact prey detection mechanism, the record ed (high) value for the instantaneous area of discovery of deep-living bivalve prey is in accord with the observation that knots use large a reas of intertidal flat in the western Dutch Wadden Sea where bivalve stocks are too low to support knots according to a previous functional response model invoking direct touch.