<p>Starling murmurations are puzzling: night after night, thousands of birds engage in prolonged, erratic, yet tightly coordinated flights above their roosting sites. This behaviour is largely unexplained; in particular, current hypotheses do not explain satisfactorily why birds participate. This review shows that murmurations are closely linked to reedbed roosting in starlings and across bird species. Collective winter roosts are hierarchically structured: dominants roost in the centre while subdominant juveniles, particularly females, are relegated to the periphery, and suffer a disproportionately high mortality. A new hypothesis is proposed: murmurations emerge as competitive interactions in which birds vie for safer central roosting spots via a leading position in the flock. Under this hypothesis, the erratic flight patterns result from strategies employed by subgroups (<i>caucuses</i>) in the worst positions that trigger collective predator escape behaviours to improve their positions. Rather than producing shared benefits, murmurations impose collective costs: they are prolonged and energy-consuming negotiations about relative roosting positions, a fixed resource. Participation is still beneficial as the cost of not participating, roosting solitarily or in the periphery, is greater than that of participating. Dawn swarms, prolonged pre-roosting flights near the subsequent roost observed in many cavity-dwelling bats in temperate climates, could have evolved likewise. This hypothesis delineates individual-level traits consistent with how starlings coordinate. It explains why, where, when, and in which species murmurations occur.</p>