Bowers are display structures built by male bowerbirds (Ptilonorhynchidae) to court females. Avenue bowers consist of two parallel walls of upright sticks. Visiting females enter the avenue between the walls to watch the male displaying on a platform at either end of the bower. According to the "Optimal Illumination" hypothesis, bowers are oriented to optimise the amount of sunlight falling on the displaying male and the decorations on the platform. We studied bower orientation and platform utilisation at bowers of the western race of the Great Bowerbird, Chlamydera nuchalis nuchalis, near Darwin, Northern Territory, Australia. Most bowers were oriented in a NNE-SSW direction, in contrast to the prevailing NNW-SSE pattern observed in the eastern race, C. n. orientalis, in north Queensland. The disparity between these study populations in bower orientation appears to be related to their geographic latitudes and, consequently, azimuth of the sun during the peak morning display period. Males displayed on the platform that received the most sunlight. Moreover, mating success at east-facing bowers was higher than at west-facing bowers. These findings support the "Optimal Illumination hypothesis". This is the first report demonstrating that bower orientation influences mating success based on quantitative data in avenue bower building species.