I estimated hourly mass change at stopover sites for 14 species of migrant passerines from 15 sites across southern Canada by analyzing size-corrected mass of birds at first capture as a function of time of day of handling. Mean mass gains were 0.40% of lean body mass/h during spring and 0.53% during fall. Mass gain estimates varied significantly with season, site, and species, and were negatively related to condition of birds in the early morning. However, standard errors were large, such that few individual estimates were significantly different. Several sites with consistently low rates of mass gain had characteristics that probably reduced local food supply. Swainson's Thrushes (Catharus ustulatus) also had consistently low rates of mass gain. I estimated the time required to accumulate sufficient mass to fuel a 10-h migratory flight, and found that the majority of estimated mass gains were sufficient for birds to refuel during <1 week of stopover in southern Canada. At mean rates of mass gain from this study, migrants in southern Canada could potentially refuel completely during 2–3 days in both seasons, but true periods are likely somewhat longer. Analysis of mass change along migration routes (instead of across them, as in this study) is needed to detect whether there are differences among species in timing and location of maximum fuel deposition, as has been found in Europe.