Stopover sites used to accumulate the energy that fuels migration, especially those used prior to crossing ecological barriers, are regarded as critically important for the survival of Nearctic−Neotropical migratory birds. To assess whether South American stopover sites are used to store the energy required to cross the Caribbean Sea and the Gulf of Mexico to North America by a Neotropical migratory landbird, we studied Gray-cheeked Thrushes in northern Colombia through constant effort mist-netting during spring migration in 2010 and 2011. We combined stopover duration estimates and models of body mass change based on recaptures to estimate departure body mass and potential flight range from our study site. We recaptured 62 birds, the majority of which gained mass. Models indicated significant differences in rates of mass gain between years and age groups and with arrival date. Estimated total stopover durations varied between 15.4 (2010) and 12.5 days (2011). Predicted departure mass ranged between 41.3 and 44.9 g, and potential flight range was estimated at between 2727 and 4270 km. Gray-cheeked Thrushes therefore departed our study site with sufficient energy reserves to cross the Caribbean Sea and the Gulf of Mexico (2550 km). As the first demonstration that birds departing from South American stopover sites can reach North America without refuelling, this has important implications for stopover site protection. Strategic conservation measures in the Sierra Nevada de Santa Marta could protect habitats in which up to 40% of the energy required to complete spring migration is stored by a Neotropical migratory land bird.