In "Conspicuous Plumage Does Not Increase Predation Risk: A Continent-Wide Test Using Model Songbirds," published in the American Naturalist, Kristal E. Cain examines the factors that drive the predation levels of Australia's fairy wrens. After measuring attack rates on both conspicuously and dull colored 3D fairy wren models in various habitats, Cain found that bright or "conspicuous" plumage is not associated with an increase in predation.
"These findings do not support the long-standing hypothesis that conspicuous plumage, in isolation, is costly due to increased attraction from predators," Cain writes. "Our results indicate that conspicuousness interacts with other factors in driving the evolution of plumage coloration."
The forces shaping plumage color of female birds -- who are sometimes brightly colored like their male counterparts and other times much more dull -- is a long-debated topic of evolutionary biology that remains unresolved. Fairy wrens, who vary greatly in both female coloration and the habitats in which they live, are an excellent group for investigating the evolutionary forces shaping female plumage, Cain writes.
Cain and her co-authors produced 60 3D fairy wren models at the University of Melbourne School of Engineering. The models had three types: conspicuously-colored male purple-backed fairy wren, dull-colored female purple-backed fairy wren, and conspicuously-colored female lovely fairy wren. Field experiments took place at eight different fairy wren habitats across Australia that ranged from open savannah to dense forest. Mimicking actual fairy wrens' foraging habits, models were placed approximately 5 meters apart on bare ground or in short vegetation, attached to metal stakes with magnets. Cameras were used at some locations to determine that the models were knocked off their perches by attack or by accident, and in the case of attacks, to identify the predator.
Cain found that, contrary to their predictions, there was not a significant difference between attack rates on the conspicuous models and the dull models. Attack rates did vary, however, depending on habitat and latitude: predator pressure was stronger at sites that were open savannahs, as well as at sites that were further from the equator. These increases in predation in open habitats occurred more dramatically for female models, both dull and conspicuous, than for the conspicuous male models. Females also saw a greater decrease in predation pressure in habitats that fell on the opposite end of the spectrum: dense forest habitats or habitats closer to the equator.
"Our data suggest that adult birds living in open Australian habitats experience higher predation pressure than those in closed habitats, though it is unclear whether this pattern is due to differences in detectability, predator density, or both," Cain writes.
The lack of differences between attack rates between conspicuous and dull models imply that coloration alone doesn't predict how often a bird will be attacked. This doesn't mean, however, that conspicuousness is not an important factor, Cain writes. For instance, it is possible that a combination of predator behavior, sex differences in behavior, and conspicuousness against particular backgrounds may all play a role in the evolution of plumage colors.
As for the female models experiencing a wider range of predation across sites, Cain writes that this finding joins a substantial body of empirical evidence suggesting that predators may avoid male birds, or that they preferentially attack females or cryptic bird species. There are many theories as for why this may be the case, including some studies that have found that birds that are conspicuously colored, male, or both, are more vigilant against predators.
Further research is needed to determine the mechanisms that are at play with fairy-wrens, Cain writes. "These conflicting patterns suggest that this relationship may be less straightforward than is often assumed and that explicit tests of the relationship between color and predation risk are required. "