In species that defend all-purpose breeding territories, territory size is usually inversely related to food abundance, but it is unclear whether this relationship results from reduced need to defend a large territory if food is abundant or from greater competitive pressure from conspecifics. Although studies of both breeding and winter territories suggest that competitive pressure is the proximate determinant of territory size, with food abundance acting indirectly by increasing the density of competitors, results are often equivocal because competitor density and food abundance tend to be positively correlated. In the Tree Swallow (Tachycineta bicolor), an aerial insectivore that defends only nest sites, defense of a food supply is not a determinant of territory size. I describe two series of experiments involving manipulation of nest-box spacing and measurements of insect abundance at four sites at and near Long Point, Ontario, Canada. Rates of occupancy of nest boxes spaced 24 m apart (beyond the normal range of territorial defense) were high at all sites (75–100%) and positively correlated with insect abundance. Occupancy of nest boxes placed 3 m apart (within the range Tree Swallows defend) was 28% to 100%, and the effect of close spacing on occupancy (attributable to territorial behavior) was also positively correlated with insect abundance. I conclude that the latter relationship must result from variation in competitive pressure because the food resource is not defended. Food abundance acts indirectly on occupancy and territory size by influencing the level of competitive pressure for nest boxes.