Fancy a spot of dinner?

By Jon Copley SHORE birds usually find prey in the sand by probing until they strike a clam or a crab. Some can even feel vibrations from buried animals with their bills. But now researchers in the Netherlands have found one species which takes advantage of the properties of wet sand to find deeply buried, motionless molluscs. Redknot sandpipers sense variations in the patterns of pressure in the sediment created by the probing of their bills. Theunis Piersma of the Netherlands Institute for Sea Research on the island of Texel noticed that redknot sandpipers, Calidris canutus, catch more buried molluscs than they should if they were just probing the sediment at random. But these molluscs do not move very much and so are unlikely to produce vibrations the birds can detect. To find out how the birds found their prey, Piersma and his colleagues trained several redknots to distinguish between buckets containing objects buried in wet sand and those containing only wet sand. If the bucket contained a clam, the bird could collect food pellets from a feeder on the right. If a bucket only contained sand, the bird would have to go to the left-hand feeder to get the reward. After training the birds, the researchers could tell whether a bird had detected a buried object by which feeder it visited first. The researchers placed the birds in a dark room containing a bucket of wet sand either with or without a clam just under the surface, then illuminated the bucket for 15 seconds. When the light over the sand went out, a second light came on over two feeding stations, distracting the bird from the bucket, as well as the clam if they found one. The redknots could easily distinguish buckets with shallow clams from buckets without clams. But when Piersma buried the clam beyond the reach of a bird’s bill, it could still detect it and work out which feeder had food. The birds could also recognise buckets containing clam-shaped stones instead of actual clams, which means that they did not need to rely on vibrations or chemical cues from the clams (Proceedings of the Royal Society Series B , vol 265, p 1377). A redknot’s bill contains sensory structures known as Herbst corpuscles, which may act as tiny pressure gauges. As a bill penetrates wet sand, it generates pressure in the trapped water. Any solid object buried nearby produces a slight back pressure against the bill. Piersma proposes that the Herbst corpuscles detect these tiny pressure build-ups. The birds may also amplify the effect by “pumping” their bills into the sediment several times per second. This shakes the sand grains down so that they pack more densely, says Piersma,
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