Apostatic selection by song thrushes (Turdus philomelos) feeding on the snail Cepaea hortensis

There is no direct evidence that predators exert apostatic selection (the systematic overpredation of commoner forms) on live, naturally polymorphic prey. This study tested whether captive song thrushes ( Turdus philomelos ) select apostatically when presented with dimorphic populations of yellow five-banded and yellow unhanded morphs of the snail Cepaea hortensis. Four thrushes were used. Two were presented with a 9: 1 ratio of five-bandeds to unbandeds and two were presented with 1:9 ratios. Each thrush was given four trials in succession. In each trial 30 snails were presented and the trial was stopped when 15 had been eaten. There were no differences in shell size between morphs or between eaten and uneaten snails of each morph. Three thrushes selected apostatically and one thrush exerted virtually no selection. Overall, there was statistically significant apostatic selection.