Ant colonies are self-organized systems, meaning that complex collective behavior emerges from local interactions among colony members without any central control. Self-organized systems are sensitive to initial conditions, whereby small random effects are amplified through positive feedback and have a large influence on collective outcomes. This sensitivity has been well demonstrated in collective decision-making by ants that use mass recruitment via trail pheromones, where it is attributed to the highly nonlinear relationship between the amount of pheromone on a trail and its effectiveness at attracting recruits. This feature is absent in many species, such as the rock ant Temnothorax rugatulus (Emery) whose tandem run recruitment shows a linear relationship between effort and effectiveness. Thus, these ants may have other behavioral responses that amplify initial differences during collective choices. We investigated this by testing whether nest site selection is influenced by small differences in the amount of brood at competing sites. Our results show that T. rugatulus colonies prefer a nest containing brood items to an empty nest, even when the brood-containing nest has only one brood item. When both nests have brood, colonies prefer the nest that contains more. However, as the numbers of brood items becomes more similar, this preference becomes weaker. Moreover, the smaller the difference in brood number, the more likely are colonies to split between sites. We discuss potential behavioral mechanisms for the observed effect, as well as its implications for number sense in ants.