Decoration theft in the Great Bowerbird: preferences for colour, size and placement at the bower

Male bowerbirds build and decorate stick structures (bowers) to attract and mate with females. Like other animals that use non-bodily ornaments in courtship, males must acquire, arrange, and replenish decorations to create an effective signal. Males also steal decorations from other males, so they may need to weigh the costs of stealing particular types of decorations relative to their benefits to the overall visual signal, which could influence what the signal conveys to females. We examined whether male Great Bowerbirds (Ptilonorhynchus [= Chlamydera] nuchalis) prefer to steal certain colours, types or sizes of decorations; and how stolen decorations are sourced from, and placed on, bowers. Males were more likely to steal green and red decorations than grey ones. Among 44 decoration types, males stole only red wire and green plastic more frequently than expected by chance. Stolen red wire was shorter and lighter than non-stolen red wire. For both red wire and green plastic, stolen objects had higher chroma than non-stolen objects. Males were more likely to place stolen decorations in the avenue or hanging on the bower walls than on the courts, sides, or periphery, and they were more likely to steal decorations from within the avenue. Through theft of colourful decorations, some of which are lighter and smaller, thieves may increase the visual contrast or attractiveness at their bowers while reducing the costs of transport. Placement of decorations in specific locations such as the avenue may help females quickly assess the competitive ability of the male.