The mechanisms by which self-organized systems like an ant colony reach collective decisions are poorly understood. Models explaining how trail-laying ants select the best food source and adapt to changes in their foraging conditions are often based on mechanisms that are either unrealistic or overly simplified. We studied the individual-level behaviour of the mass-recruiting Argentine ant, Linepithema humile, to determine the mechanisms that underlie the colony's ability to choose collectively the better of two food sources of different quality. Ants returning from a high-quality food source were more likely to U-turn back to it than ants returning from a lower quality food source. Those U-turning ants were also more likely to lay pheromone, and deposited pheromone at a higher rate than nonturning ants. We show that U-turning is pivotal to the rapid establishment of a strong pheromone trail to high-quality food sources, and that the trail is constructed from food to nest, not from nest to food. The pheromone-boosting U-turns were performed only during the initial period when the trail was being established. Once a trail was established, the trail was maintained by nonturning ants. U-turners appear vital in allowing a mass-recruiting colony to allocate workers rapidly to the food source of highest quality.