Figure 1.

Left axis and solid lines, transmission spectra of the two filter types (UV+, UV−) used in the experiment. Spectra are the mean of five randomly located measurements on each filter taken with a Unicam Prism spectrophotometer. The two filter types were equalized for transmitted quantal flux (<1% difference in flux 300–700 nm) as measured for the spectral irradiance on a female viewing empty stimulus cages, with lights as described earlier (6). In this way, any preference for UV+ or UV− is unlikely to be a preference for higher or lower quantal flux. A hyper-Graeco Latin square design (10) was used to allocate individual filters to stimulus cages, stimulus cages to positions in the room, and males to individual filters; order of treatment was randomized in a balanced way. Right axis and dotted lines, reflectance spectra of iridescent feathers on two body regions (throat and wing coverts) of male starlings. Spectra are the mean of the 32 bird means taken across 10 randomly located measurements within each body region and are plotted at 20-nm intervals. Measurement details in legend of Fig. 3.