Figure 10. Illustration of the effect of background variation on the visibility of a window marking.

A 30% blue-absorbing grid was digitally superimposed on the image. The reduction in human blue cone stimulation is comparable to what the 50% treatment in this study should have on the UV cones of a UVS bird. Against a blue rich and homogenous background (the sky) the grid becomes clearly visible but the introduced chromatic contrast is marginalised against a heterogenous and longwave dominated clutter (the apple tree). Photo by Olle Håstad.