The epaulette is a patch of red and yellow feathers on the lesser and median wing coverts, and its visibility is under muscular control. A displaying male on a cattail stem spreads the shoulder wide, leans forward and sings, making the patch as large as possible. The same bird foraging off territory or moving through a neighbour's ground compresses the patch until it nearly disappears.
The function was tested by manipulation. Males whose epaulettes were dyed black lost territories to rivals at much higher rates than controls whose patches were left visible, indicating the patch signals ownership and fighting capacity to other males rather than being simply ornamental for females.
The mating system is often described too simply. Males are polygynous, with several females nesting within one male's territory, but genetic parentage work shows a substantial share of nestlings are fathered by males other than the territory holder. Holding the ground does not guarantee the paternity that goes with it.