It has been suggested that the cohesion of mixed species flocks relies heavily on conspicuous species that act as an attractive nucleus for other birds. I evaluated the role of the White-flanked Antwren as a nuclear species in mixed-species flocks of gallery forests of eastern Colombia, and tested experimentally the hypothesis that the conspicuous wing-flashes of males contribute to interspecific cohesion. Although most of the species present in my study site showed a high propensity to flock heteroespecifically, interspecific associations of the White-flanked Antwren tended to be short. The white flanks of male White-flanked Antwrens were not critical for maintaining a particular number of species inside the flocks, for spacing out the individuals in the interspecific groups, or for prolonging the duration of the interspecific associations. The wing-flashes of the White-flanked Antwren provide an example of how assumptions about nuclear roles in species with conspicuous traits may lead to wrong conclusions about the dynamics of mixed-species flocking. My results suggest that although this antwren may be important for recruiting new members in forest flocks, it does not promote the maintenance of those associations.