Shields of offence: signalling competitive ability in the dusky moorhen, Gallinula tenebrosa

The 'status-signalling' hypothesis states that individuals might avoid unprofitable fights by signalling their competitive ability and assessing the signals of others. The frontal shields of rails (Aves: Rallidae) are fleshy structures that are displayed prominently during agonistic interactions. Shield maintenance probably requires elevated levels of testosterone and deposition of carotenoid pigments, which implies that shield size and colour could be honest signals of the ability or willingness to fight. We used field observations and a model experiment to study the signalling function of the red shield of the dusky moorhen, Gallinula tenebrosa. Birds with relatively larger shields were more likely to win aggressive encounters, and the outcome was apparently unaffected, or less strongly affected, by body size, weight, condition and sex. Shield size increased with body size and weight, but neither explained more than 26% of the variation in shield size, even after controlling for age and date. Shield size was not affected by body condition, as estimated from residuals of a regression of weight on body size. Moorhens confronted with a model fitted with a large shield retreated more quickly than they did when presented with the same model bearing a small shield, supporting the results from natural interactions. We conclude that the frontal shield functions as a signal during agonistic interactions, and possibly signals the ability or willingness to fight.