Experimental Study of Bower Decoration by the Bowerbird Amblyornis inornatus, Using Colored Poker Chips

Abstract

Selection of natural objects as bower decorations varies among species, populations, and individuals of bowerbirds. For quantitative analysis based on standardized objects of known availability, I tested poker chips of seven different colors on two bowerbird (Amblyornis inornatus) populations known to differ radically in their use of natural objects. Experiments were of four types: placing chips in the bower or on the mat and noting which eventually appeared as decorations in the bower or on the mat (harvesting); watching the temporal sequence in which birds harvested chips; and monitoring the sequence in which numbered chips were stolen from a neighboring bower. In the southern Kumawa Mountains, [Indonesia], where colored natural decorations are not used, no individual harvested a poker chip, and most adults weeded chips of any color. Wandamen birds, which do use colored natural decorations, selectively weeded and harvested poker chips. The preference sequence averaged over all Wandamen individuals was blue > purple > orange > red > lavender > yellow > white, but individual birds differed in their rankings of the five intermediately preferred colors from purple to yellow. All four tests yielded similar preference sequences: the color harvested most often was harvested first, stolen first, and weeded least often. Among harvested colors, more-preferred colors were placed inside the bower hut, less-preferred ones outside the hut on the bower mat. Chips of the same color tended to be grouped near each other and and near similarly colored natural objects. Chips of different colors were occasionally stacked according to color preference. Use of chips by birds involved much testing of different colors or much testing of the same chip at different positions in the bower. There were numerous parallels between the uses of poker chips and of natural decorations. Finally, I reexamine six questions: how male bowerbirds became emancipated from care of the young; how the focus of displays became transferred from bodily to non-bodily ornaments; why bowers provide females with a comprehensive test of male quality; whether bower-design rules are arbitrary; whether the rules are transmitted genetically or culturally; and why bower building is unique to bowerbirds.