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Coastal Birds "Feel" Their Prey Under The Sand

Knots (a kind of sandpiper) can locate their favourite food -shellfish- under wet sand by inserting their beak half a centimetre into the sand for a few seconds. Scientists at the NWO's Netherlands Institute for Sea Research (NIOZ) and the universities of Groningen and Leiden have carried out experiments to demonstrate this. The birds' ability to do so seems to be based on a hydro-dynamic principle.

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 inte
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Contact: Dr. Theunis Piersma
theunis@nioz.nl
31-222-369300
Netherlands Organization for Scientific Research
9-Feb-1999


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