Are Conspicuous Birds Unprofitable Prey? Field Experiments with Hawks and Stuffed Prey Species

Abstract

-The unprofitable-prey hypothesis predicts that birds of prey, when given a choice, should more often attack a cryptic than a conspicuous bird if conspicuousness is a signal of unprofitability. We tested this prediction by exposing pairs of stuffed birds to migrating birds of prey. In experiment 1, we paired the conspicuous White Wagtail (Motacilla alba) with the cryptic Meadow Pipit (Anthus pratensis). In experiment 2, we paired the conspicuous female Great Spotted Woodpecker (Dendrocopos major) with the cryptic female Blackbird (Turdus merula). Automatic cameras documented attacks by birds of prey, mostly accipiters, on the mounts. The numbers of attacks on wagtails and pipits were similar, whereas Blackbirds were attacked much more often than woodpeckers. Experiment 2 thus supports the prediction of the unprofitable-prey hypothesis. Using photographs of the experimental situation, we examined detectability of mounts by human observers. Wagtails and pipits were equally easy to detect from a distance, but woodpeckers usually were easier to detect than Blackbirds. Thus, wagtails and pipits did not differ in long-distance conspicuousness, which may explain the result of experiment 1. To analyze reasons for unprofitability, we presented flesh from woodpeckers and Blackbirds to captive falcons, but found no evidence that woodpeckers are distasteful. Other explanations for why woodpeckers were rarely attacked are discussed. Received 26 April 1993, accepted 17 August 1993. THE PLUMAGE of diurnal birds exhibits striking variation in color patterns, from dull, cryptic patterns in some species to bright red, yellow, or black-and-white in others. The evolution of conspicuous colors in birds has been debated since Darwin (1871) proposed his theory of sexual selection to explain the bright colors of many male birds. Darwin's idea of selection through female choice of bright males was discussed by Hingston (1933), who argued that contest competition among males primarily selected for conspicuous colors (review in Butcher and Rohwer 1989). Several studies have supported both the idea of female choice (e.g. Hoglund et al. 1990, Norris 1990, Hill 1991) and male-male competion (Roskaft and Rohwer 1987, Rohwer and Roskaft 1989). Avian coloration is also related to risk of predation. Cryptic plumages in birds such as nightjars and female ducks suggest an important role for predation. Conspicuous plumages may be antipredator adaptations as well. Certain brightly colored birds are distasteful to hornets, cats, and people and, therefore, may be aposematic (Cott 1947, Cott and Benson 1970, Gotmark 1994a). Recently, two or three apparently