Prey concealment : visual background complexity and prey contrast distribution

A prey may achieve camouflage through background matching and through disruptive coloration. Background matching is based on visual similarity between the prey and its background, whereas disruptive coloration emphasizes the use of highly contrasting pattern elements at the body outline to break up the body shape of the prey. Another factor that may influence prey detection, but has been little studied, is the appearance of the visual characteristics of the background. We taught blue tits (Cyanistes caeruleus) to search for artificial prey and manipulated the appearance of the prey and the background. We studied the effect of diversity of shapes in the background on prey detection time. We also studied the differing predictions from background matching and disruptive coloration with respect to contrast level and location of high-contrast elements in prey patterning. We found that visual background complexity did indeed increase prey detection time. We did not find differences in detection time among prey types. Hence, detection time was not affected by contrast within prey patterning, or whether the prey patterning matched only a sub-sample or all the shades present in the background. Also, we found no effect of the spatial distribution of shades (highest contrast placed marginally or centrally) on detection times. We conclude that background complexity is important for the evolution of prey coloration. We suggest that it may facilitate concealment and favor the evolution of camouflage over warning coloration. Preference for visually complex backgrounds might provide prey with a so far untested means to decrease predation risk