Coastal Birds "Feel" Their Prey Under The Sand

The locational ability of the knots was demonstrated in experiments in which the researchers hid little stones in the sand. That the birds can detect stones is remarkable because stones –obviously enough– do not send out any signals. They do not move, their temperature does not differ from that of the sand, and they have no smell or electro-magnetic field; they can therefore not be observed with the birds' normal sense organs. The birds' ability to detect them nevertheless must therefore be based on their beaks' being sensitive to differences in currents in the water in wet sand between the individual grains, stones, or shells. Tame knots used in the experiments were unable to find hidden stones in dry sand.

Under the horny layer of the end of their beak, knots have clusters of 10 to 20 corpuscles of Herbst in the bone and these are sensitive to differences in pressure. When the bird sticks its sensitive beak into the sand at low tide, it produces a pressure wave because of the inertia of the water in the interstices between the particles. The pattern thus created betrays the presence of objects which are larger than the grains of sand. Knots are able to read the disturbances in the pressure field and perhaps even amplify it to some extent. The rapid up-and-down movements of the bird's beak loosen the grains of sand, which then become packed together more tightly, displace the interstitial water and cause the residual pressure around the object concerned to increase.

The birds' ability to detect food also explains why knots can find enough to eat on intertidal flats where there are only few shellfish. It also explains for the first time why these wading birds only forage on wet sand; in dry sand, the sensor in their beak does not work. As soon as the tide has gone out far enough for the shallows to dry out, the birds therefore fly to areas which are still wet. The nature of their localising ability means that knots are unable to distinguish between stones and shellfish in the sand. It is therefore no wonder that they never look for food in areas where the sand contains stones, no matter how much shellfish could be found there.