Contests allow animals to monopolize indivisible resources such as mates and food . Contests also come with costs, however, including energy expenditure and risk of injury or death (e.g., Briffa & Sneddon, 2007;Riechert, 1988). To resolve these important contests while minimizing costs, many animals use signals that communicate their quality or ability to win. Understanding the evolution of signaling systems requires testing how variation in the form of signals used in contests (also known as "aggressive signals") predicts contest behaviors and outcomes (Searcy & Beecher, 2009; Searcy & Nowicki, 2005). Red-shouldered widowbirds (Euplectes axillaris), for example, use red-colored epaulets as signals of competitive ability. Males with larger and redder epaulets are more likely to hold territories (Pryke & Andersson, 2003a), and males with epaulets manipulated