The week's PNAS has a nice short article bringing up something I guess should have been obvious: birds can see differences among themselves that are invisible to the human eye. Birds have special cones which can see into the UV range, and several species of passerine birds, which are listed as not having sex differences, actually do have flares or tufts of ultraviolet bling-bling which distinguish males from females.
This picture shows three males of different species. They appear very similar in visible light (left) but the uv shows differences.
The point is that when you have a difference, you can get a mating preference, and sexual evolutionary presssure is likely to be acting on these "drab-colored" birds just as much as on peacocks. Also, these patches were so common that maybe the focus should be on the converse case: explaining evolutionary pressure on the rare birds without any sexual dimorphism.