Bird species around the world are threatened with extinction. In North America, aerial insectivores are experiencing particularly severe population declines. To conserve these species, we need to know which life stages have the largest influence on population growth. We monitored a box-nesting population of Tree Swallows (Tachycineta bicolor) from 1975 to 2017. From this long-term dataset, we derived estimates of 9 vital rates: clutch size, reproductive attempts, and overwinter return for 2 age classes of adult females, and hatching, fledging, and juvenile recruitment rates. We conducted a life-stage simulation analysis on this population based on a 3-stage, female-based population projection matrix to determine which of these vital rates had the greatest influence on overall population growth rate. We determined each vital rate's sensitivity (i.e. the effect of a small change in each vital rate on population growth), elasticity (i.e. the effect of a proportional change in each vital rate on population growth), and ability to explain variation in population growth rate. Juvenile recruitment, female return for both age classes, and fledging success determine population growth because they have high sensitivity, elasticity, and explained large amounts of variation in population growth rate. Contrary to expectations, the number of nesting attempts, clutch size, and hatch rate have little impact on population growth rate. To stem Tree Swallow decline, and potentially the declines we see across the aerial insectivores, fledging success or overwinter survival must increase.