Countershading enhances crypsis with some bird species but not others

Although the theory of self-shadow concealing countershading is over a century old, there are very few direct empirical tests to substantiate the prediction that prey that are dorsally darkened and ventrally lightened (generally termed countershaded) suffer lower rates of attack than other prey. In this paper, we report experiments designed to determine whether artificial, countershaded prey are chosen by predators less often than those that are all light, all dark, or reverse shaded (i.e., dorsally lightened and ventrally darkened). Artificial prey were presented in gardens and parks to free-living birds, either on white backgrounds or on backgrounds with some degrees of color matching. In one experiment, birds were unmarked, and in the other, they were individually identifiable. We found that in three experimental trials, countershaded baits were attacked at a rate not significantly different from that of uniformly dark baits. In one experimental trial, countershaded baits were at some advantage. When we examined the data set for this trial more closely, it was apparent that blackbirds were taking countershaded baits least often, but blue tits and robins conferred no special advantage to countershaded baits. Hence, the efficacy of countershading may vary with species of predator.