SUMMARYSight-dependent predators may tend to form" searching images "for common varieties of a polymorphic prey species. Selection would then be frequencydependent and could maintain the polymorphism. This hypothesis has been tested in 13 experiments by presenting populations of green and brown lardand-flour "baits" to wild passerine birds in their normal surroundings. In each experiment groups of birds were " trained" on greens or browns and were then exposed to populations with the two colours in equal proportions. In every case there was a highly significant tendency for the familiar colour to be taken in excess. In three of the experiments the birds were given a second period of training, this time with the unfamiliar colour. Their overall preferences were reversed and the effect of the training seemed to decrease with time.