Stopover behavior and migratory pathways of neotropical migrant birds in Central and South America have received little study. We examined stopover ecology of Swainson's Thrush (Catharus ustulatus) on the Osa Peninsula, Costa Rica, during spring migration, 2002–2005. Capture rates per net hour were high (;amx = 21.8 individuals/100 net hrs) suggesting large numbers pass through lowland coastal areas in spring. Mean passage date of males was ∼6 days earlier than females. Timing of passage by age class was variable; after-second year (ASY) birds preceded second-year (SY) birds in 2 years, SYs preceded ASYs in 1 year, and both had the same mean passage date in 1 year. We also observed annual variation in relative abundance of the two age groups. Only 1.7% of marked Swainson's Thrushes were recaptured on subsequent days suggesting most individuals left the immediate area soon after initial capture. Stopover lengths ranged from 1 to 9 days, although most were <4 days. Many individuals had some energy stores on arrival (fat score ≥ 1) but reserves varied between years and tended to be lower in SYs compared to ASYs. Regression of body mass against time of day indicated that individuals tended to gain mass throughout the morning (0.67% of lean body mass/hr on average). Our findings for stopover lengths, rates of mass gain, and recapture rates are within the range observed at North American stopover sites in spring. However, our capture rates were relatively higher, perhaps because the small land mass of southern Central America concentrates individuals at stopover sites in these regions.