Collective behaviors displaying a variety of fascinating movement patterns are thought to be products of complex interplay among individuals. Previous studies have proposed the hierarchical leadership networks and the coexistence of compromise and leadership in pigeon flocks, but these conclusions have not been confirmed by theoretical or modeling studies. Here, based on the same datasets, using a more reasonable research route, we found a more concise leadership structure in pigeon flocks. i.e., the tree structure, which was verified by our modeling studies. We showed that each individual may follow its only pilot (leader) during collective flights of pigeon flocks, and the only top leader of a certain flock determines the flight direction of the whole flock. Our results confirmed the leadership hypothesis, denying the illusion of compromise between individuals at the same level. The findings shed light on the hierarchical leadership structure in pigeon flocks and have implications for artificial collective systems, e.g., autonomous formation control of multiple unmanned aerial vehicles and unmanned surface vehicles.