Bower location and orientation in Satin Bowerbirds: optimising the conspicuousness of male display?

Abstract

Male Satin Bowerbirds, Ptilonorhynchus violaceus, have been reported to construct their bowers with the decorated portion of the bower platform oriented northward. Given the regularity of this orientation, it has been suggested that the intensity and quality of incoming solar radiation may enhance male displays. However, previous reports of bower orientation originate from studies conducted in open-canopy, sclerophyll forest, where diel variation in the intensity of light striking the bower is mostly influenced by the altitude and azimuth of the sun. We studied 12 Satin Bowerbird bowers in the mountainous rainforest of the Atherton Tablelands, Australia, and found no significant tendency for bower platforms to face north. Instead, bowers were invariably constructed near canopy gaps, and there was a significant tendency for bowers to face away from these gaps. We suggest that this pattern of orientation allows displaying males to be illuminated predominantly from the front when viewed by females from within the bower avenue. There was also a significant positive association between bower orientation and the orientation of the slope of the hill on which the bower was constructed, such that bowers were oriented upslope, further enhancing frontal illumination. We suggest that Satin Bowerbirds may use a variety of means, including bower orientation, to achieve this objective.