There's nothing peace, love and mung beans about dusky moorhens (Gallinula tenebrosa). They squabble over everything. Disputes are settled by flapping, pecking and shirt-fronting but physical confrontations aren't always the best way to sort things out. Making contact brings the risk of injury. Is it worth it? There must be a better way of solving quarrels.
Like other species of moorhen, dusky moorhens bear a bright red shield on the forehead. A combatant with a large, vivid one is more likely to win a clash against an opponent with a small, drab one.
Although there is some relationship between shield size and body size — bigger birds tend to have bigger shields — that is not a sufficient explanation for the difference in success. That might be due to higher testosterone levels, reflected in the hi-vis facial bling.
In effect, the shield acts as an indicator of prowess. So effective is this signal that moorhens presented with a model that has a large shield will turn tail rather than risk a belting.
So when you're betting your quatloos on moorhens, you know how to pick the winner.
Crowley, C.E. & Magrath, M.D. (2004) Shields of offence: signalling competitive ability in the dusky moorhen, Gallinula tenebrosa. Australian Journal of Zoology 52; 463 – 474.