Wild birds prefer the familiar morph when feeding on pastry-filled shells of the landsnail Cepaea hortensis (Müll.)

Abstract

Colour polymorphisms in prey could be maintained if predators concentrate on common morphs and confer a selective advantage on rare morphs. We describe experiments to test whether wild birds feed on pastry-stuffed shells of Cepaea hortensis in a manner that might lead to such apostatic selection. The birds were first given a ‘pre-training’ choice test of a shell population with equal numbers of yellow unbandeds and yellow five-bandeds; they were then trained on one morph alone, given a second choice test, trained on the other morph and, finally, given a third choice test. The birds preferred five-bandeds in five of the six pre-training tests. In all six experiments the first training session increased the birds' preferences for the morph that was familiar. The results were less clear-cut when selection during pre-training was compared with selection after the second training session. However, a comparison between selection after each of the two training sessions showed that in all six experiments the results were in the direction predicted from the hypothesis that familiar morphs are preferred. This set of experiments is one of the few in which behaviour which could lead to apostatic selection has been tested with morphs that differ in pattern. The findings support the idea that polymorphism in Cepaea could be maintained by apostatic selection.