Condition-dependent resource value affects male–male competition in the blue–black grassquit

Male--male competition frequently can be resolved without overt aggression through the use of behavioral and phenotypic traits that signal body condition, dominance status, and fighting ability. In this study, we used male blue--black grassquits (Volatinia jacarina) to examine the relationship between male dominance status and behavioral, body condition, and ornamental traits in intrasexual agonistic encounters over a food resource. We found an association between body condition and winning, where winners were lighter than losers. This pattern was explained by low amounts of aggression exhibited by losers when they were heavy. In addition to being lighter, winners were, on average, up to 5 times more aggressive than the heavier losers. There were no associations between ornamental characteristics and dominance status, which suggests that male blue--black grassquits do not exhibit a badge signaling body condition and fighting ability, and we propose that such ornamental characteristics may be more functional in mate-choice contexts. However, the amount of male nuptial plumage of winners predicted the aggressiveness of their opponents, suggesting a social cost for sustaining this ornament. Copyright 2009, Oxford University Press.