Ultraviolet crown colouration affects contest outcomes among male blue tits, but only in the absence of prior encounters

Summary 1.  Static animal colour patches may function among competitors to minimize conflict escalation over resources, by serving as a signal of resource holding potential or aggressiveness. Empirical evidence for the use of colour patches in conflict resolution is largely restricted to pigment‐based colours (melanins and carotenoids) and rarely defines the context in which the signals are used. 2.  Here we test whether structural‐based ultraviolet (UV) crown colouration functions in conflict resolution among dyads of first‐year male blue tits Cyanistes caeruleus in captivity, in a context that is known to favour aggressive individuals. 3.  We found that on first encounter in a pairwise context, experimentally UV‐reduced males were significantly more likely to lose to control‐treated opponents than expected by chance. However, this disparity was less pronounced when conflicts were settled with physical fighting or when the opponent was considerably smaller in size. 4.  When the same dyads were tested again several weeks later, but with the UV treatment reversed among opponents, none of the effects remained significant, but instead the winner was most likely to be the individual that won at their first encounter. 5.  Our results suggest that structural‐based UV colouration can affect the outcome of an interaction, but that size differences and the outcome of initial interactions between opponents can override the influence of this signal in conflict resolution. Whether there is a functional basis to maintain a link between aggressiveness and colouration may thus be highly dependent on the general context under which individuals compete in the wild.