Seashore Hunting

Watch sea gulls, plovers, and other birds hunt in the tidal zone, and you may notice them stepping over and over in the same spot. This is part of bird’s hunting strategy. Each footfall compresses the wet sand and drives water out. Mechanically, this is the same thing that happens when a human walks on wet sand; you’ll see the sand go from a glossy appearance to a matte one as the local water level falls. Once the weight is removed, the water will seep back and the sand appears glossy again.

For the birds, the flood of returning water loosens and dilutes the sand. That makes prey easier to expose and reach without the additional effort of digging. (Image credits: bird – C. Davis, illustration – P. Fischer; via Physics Today)