Further evidence for apostatic selection by wild passerine birds—9:1 experiments

Summary

Apostatic selection occurs when predators concentrate disproportionately on the common varieties of a non-mimetic polymorphic prey species. This has been tested in 14 experiments by presenting populations of green and brown lard-and-flour “baits ” to inexperienced wild passerine birds in their normal surroundings. In seven experiments a 9 green : 1 brown population was presented for a number of days, followed by a 1 green : 9 brown population for a similar period. In the remaining seven experiments the populations were presented in the reverse order. The birds often had strong “natural” colour preferences (for example, blackbirds and songthrushes preferred browns) which were not caused by the relative conspicuousness of the two colours. The data within most of the experiments were very heterogeneous, but in every experiment there was good evidence that the birds tended to concentrate on the common colour. The consistency of the replicated experiments gives strong reason to believe that apostatic selection is a widespread phenomenon among avian predators, and provides an explanation for many of the non-mimetic colour and pattern polymorphisms found among their prey.