By radio-tracking and recording the movements of flocks, the distribution of feeding Red Knots (Calidris canutus rufa) was studied day and night at a migration stopover site near San Antonio Oeste, Río Negro, Argentina in March and April 1998. By day, the birds fed in dense flocks of 500-4000 on an area of restinga or rock platform where there were beds of the small mussel Brachidontes rodriguezi. By night, this site was deserted, and the birds were found widely scattered over nearby sandflats. It was evident that the birds were feeding at night because variation in the signal strength of the radio-transmitters indicated that the birds were active. Also fresh knot droppings were found in an area which only became exposed by the tide after dark. The reason for the change in feeding distribution may be that the restinga is close to terrestrial habitats that harbor night-adapted predators. Therefore it is avoided at night. By day, it may be safer because better visibility means that predators can be identified more readily. Alternatively, it could be that feeding opportunities become available at night that are better than on the restinga, and this is why the birds feed elsewhere.