In Northern Flickers, the male moustache is essential for sex recognition--mates will not recognize each other without it. In Red-winged Blackbirds red shoulders are needed for driving off territorial intruders. In Harris's Sparrows immatures have white throats while adults have black. Adults take optimal feeding sites--bleached adults loose status in the flock. In Mallards' heads and speculums function in sex recognition--dyed males will not find mates. In Yellow Warblers, males are territorial in winter in Central America but will tolerate females in territory. Immature males retain female plumage all winter, thereby deceiving adult males.
One last thought--Convergent Evolution in plumage coloration. This is when two gene pools converge on same strategy for survival. Specific selection (whatever that may be) pressures must be producing same results. African Longclaws and Eastern Meadowlarks look identical. Can you think of other examples?
Adult and Young Harris's Sparrow