Experimental blocking of UV reflectance does not influence use of off-body display elements by satin bowerbirds

Abstract

UV reflectance of plumage display is important in mate choice of many avian species, but its role in off-body display has received little consideration. Male satin bowerbirds (Ptilonorhynchus violaceus) collect objects for use as bower decorations. Here, I test the hypothesis that UV and non-UV colors are important in choice of satin bowerbird bower decorations (colored squares and blue parrot feathers) in an experiment using UV blocking agents at the bowers of 31 males. I found a highly significant preference for blue decorations but no evidence of a preference for UV-reflecting decorations. UV-blocked blue objects were used to the same extent as identical unblocked decorations, and UV-reflecting decorations that were not blue were not attractive. The evolution of off-body decoration displays may have contributed to a reduced role for UV-reflecting decoration displays in satin bowerbirds in 3 ways: 1) use of UV-reflecting display objects that are not made by the bird does not directly signal their owners' condition as would the birds own plumage, 2) because of the relatively low level of ambient UV light available to illuminate decorations displayed on a court the forest floor, UV may be less effective than other wavelengths for these displays, and 3) decorations that are both blue and UV reflective may be too rare under natural conditions to be effective signals of male quality. Off-body displays may provide very different kinds of information from plumage displays, and these differences may have contributed to the evolution of complex multicomponent displays.