A NEW PRESSURE SENSORY MECHANISM FOR PREY DETECTION IN BIRDS - THE USE OF PRINCIPLES OF SEABED DYNAMICS

We demonstrate a novel mechanism for prey detection in birds. Red knot s (Calidris canutus), sandpipers that occur worldwide in coastal inter tidal areas, are able to detect their favourite hard-shelled prey even when buried in sand beyond the reach of their bills. In operant condi tioning experiments designed to find out whether the birds could tell buckets containing only wet sand from buckets containing hard objects in wet sand, we show that they detect the presence not only of deeply buried live bivalves but also of stones. The latter finding virtually excludes, under experimental conditions, prey-detection mechanisms bas ed on vision, acoustics, smell, taste, vibrational signals emitted by prey, temperature gradients and electromagnetic fields. A failure to d iscriminate between food and non-food trays with dry sand indicates th at pore water is involved. Based on the presence of large arrays of He rbst corpuscles (sensory organs that can measure the acceleration due to changes in pressure), the specifics of foraging technique and the c haracteristics of sediments on which red knots feed, we deduce that th e sensory mechanism involves the perception of pressure gradients that are formed when bills probe in soft sediments in which inanimate obje cts block pore water flow. To our knowledge, this mechanism has not be en described before. It is argued that repeated probing in soft, wet s ediments allows red knots to induce a residual pressure buildup of suf ficient strength to detect the pressure disturbance caused by a nearby object. The cyclic process of shaking loosely packed sand grains foll owed by gravitational settling into a closer packing, leads, owing to insufficient drainage of the sediment, to a locally increased pressure disturbance that is 'pumped up' at each shake.