Why do orb-weaving spiders (Cyclosa ginnaga) decorate their webs with silk spirals and plant detritus?

To improve foraging success or to enhance survival, animals may give out false signals to mislead prey or predators. Web decorations are purported to be structures that some orb-weaving spiders build to transmit deceptive signals to their advantage. Cyclosa ginnaga incorporates a variety of silk (silk decorations) and plant materials (plant detritus decorations) into its webs as decorations. It has been hypothesized that web decorations are deceptive signals that either attract prey, provide protection through camouflage or act as a warning signal. Nevertheless, despite numerous studies on web decorations, their function has remained elusive because of conflicting results. Using a combination of field video recording and the quantification of colour contrasts in the visual systems of hymenopteran prey and passerine bird predators, we tested two functional hypotheses (prey attraction and predator defence) of two forms of web decorations (silk spiral and plant detritus) built by C. ginnaga. Silk-decorated, and to a lesser extent plant-decorated, webs intercepted more prey than undecorated webs, thus improving foraging success. This supports the prey attraction hypothesis; however, we could not rule out the predator defence hypothesis because there was no predation on spiders observed for any web with or without decorations.