Identifying the existence of population sinks is critical for conservation and management. However, because of density-dependent dispersal, sinks can sometimes be masked by immigration events, especially during phases of population growth. We present a large-scale, empirical demonstration of within-population source-sink dynamics using the feral horses (Equus ferus caballus) of Sable Island National Park Reserve, Nova Scotia, Canada, as a model. We tracked the fates and movements of 98.7% of the female population (n = 190–237) across 3 demographic clusters (subunits) during a period of rapid population growth (2008–2010; 24.7% increase in density). All subunits experienced increases in population size each year (λ > 1.0). Our individual-based analysis showed that western Sable Island, where water availability was greatest, behaved as a source and would have grown with or without immigration in all years. However, the central (and fastest growing subunit) would have declined from 2008–2009 (λ = 0.951) without immigration. Further, the eastern subunit would have declined in 2 intervals (λ = 0.932, 0.999) without immigration. Our study demonstrates that the propensity of habitat to act as a sink can be masked during a period of population growth because of density-dependent immigration from adjacent habitats. These findings present a caution to managers charged with conserving wide-ranging species with long population cycles for which effects of immigration on local population growth rate can be difficult to isolate using standard methods of enumeration. © 2013 The Wildlife Society.