Hide and seek: properties of prey and background patterns affect prey detection by blue tits

We studied the effects of visual appearance of background and similarity between background and prey patterning on prey detection and camouflage. Although increased similarity with background (background matching) is known to impede prey detection, the relative importance of different aspects of visual similarity has received little interest. We used blue tits (Cyanistes caeruleus) as predators and trained them to search for artificial prey items presented on printed background plates. We particularly investigated the effect of the density and the shape of the elements constituting the background and the prey patterning. Our experiment shows that increase in the density of elements in the background caused an increase in search times of all prey types. We also found that compared with fully background-matching prey, prey patterning that sported a mismatching element shape and, interestingly, also prey patterning that mismatched the element density of the background decrease prey search time and, hence, deteriorated camouflage. There was no difference in search time between the shape- and the density-mismatching prey categories. We conclude that element-dense backgrounds are more protective both for background-matching prey and background-mismatching prey than backgrounds with low element density. Further, our results suggest that even if prey patterning consists of elements that closely match the visual elements in the background, high-level crypsis through background matching only arises if the density of the elements is also similar between the prey patterning and the background. These findings are important when considering prey habitat choice and the evolution and limitations of background matching and signaling coloration.