Analysis of a Selective Predation Experiment

Abstract

A new index of selection, α, is presented. It measures the degree to which a predator is more likely to take one kind of prey rather than another, and thus provides a direct estimation of predator behavior. It is unaffected by sampling in a finite population, this being the advantage over other methods of estimating selectivity. If selection is independent of the frequency of prey types, α is constant. Under frequency-dependent predation α changes with frequency. Measures of the frequency-independent and the frequency-dependent components may be obtained when both kinds of selection act together. The method of analysis has been applied to data on the behavior of Japanese quail, Coturnix coturnix japonica, presented with red and blue pastry food under controlled conditions. Variability in response between individual pairs of birds is large. A strong red preference is nevertheless exhibited, and also a distinct tendency for the birds to eat more than a proportional amount of whichever color was the most common over a range of frequencies from 10% to 80% red. Behavior of this kind would favor a stable equilibrium in a polymorphic animal prey species, and uniformity of color in edible fruits if the bird acts as a beneficial agent of dispersal.