The ideal despotic distribution, whereby the lifetime reproductive success a territory's owner achieves is unaffected by population density, is a mainstay of behaviour-based population models. We show that the population dynamics of an island population of Seychelles warblers (Acrocephalus sechellensis) cannot be modelled with an ideal despotic distribution, and suggest the effects of both territory shrinkage and territorial disputes on reproductive success must be included to adequately model the population dynamics of this species. To do this we introduce two different approaches. The first is reductionist, using data on how population density affects individuals’ reproductive success to predict population growth rates. Because such a model is mechanistic, it can be used to predict population dynamics in novel environments, making it a desirable long-term solution. However, because territorial populations are typically tightly regulated, birth and death rate data at low population densities are often unavailable. Hence, our second approach statistically infers the relationship between population density and per territory reproductive success, and thus provides a stop-gap solution for the shorter term. Our analysis indicates that although the Cousin population of Seychelles warblers is highly resilient to environmental stochasticity, the degree of resilience is considerably underestimated by approaches that ignore intrinsic regulation through territory shrinkage and territorial disputes.