Empirical investigations of source-sink dynamics are needed to relate metapopulation theory to spatially-structured, temporally varying population dynamics in the real world. The difficulty of acquiring the data needed to estimate vital rates has often constrained analyses of source-sink dynamics to static or simulated systems. We used 26 years of data on reproduction, movement, and survival to estimate annual source-sink dynamics across the entire range of the endangered Everglade snail kite (Rostrhamus sociabilis plumbeus) during the invasion of a novel prey species. Populations underwent striking changes in source-sink dynamics over time, varying with time since invasion. Both source and sink populations depended on immigration to offset high emigration rates. Source populations are often prioritized for conservation, but in a metapopulation composed largely of sinks, or sources with high emigration and immigration, an emphasis on maintaining connectivity or expanding available breeding habitat may be more important.