If understood as a way to forage socially without incurring intra-specific competition for mates or other resources, mixed-species foraging flocks are predicted to be composed of functionally similar species. In the most intensively studied mixed-species foraging system, understory forest birds, relevant functional traits are however extremely difficult to measure and best replaced by phylogenetic relatedness. A multicontinental analysis of flock phylogenetic structure revealed departures from the expected phylogenetic clustering. Long-lasting associations (> one day) were phylogenetically overdispersed, indicating that these associations are affected by competitive exclusion or by mutualistic interactions. However, where kleptoparasites occurred, this effect disappeared completely, as expected if the dilution of kleptoparasitism risk compensated competition between related species. Mixed-species flocks should not be analyzed as a homogeneous phenomenon.