The red-throated diver (Gavia stellata) is listed on Annex I of the European Union Birds Directive and requires protection within Special Protection Areas (SPAs), among other conservation measures. The Outer Thames Estuary, east England, was known to support almost half of the Great Britain wintering population of red-throated divers but delineating an SPA boundary there was not straightforward as existing boundary-setting methods did not perform well. The number and distribution of red-throated divers in the Outer Thames Estuary was determined using visual aerial survey methods. Bird observations were smoothed using kernel density estimation (KDE) and were combined to create a mean modelled density surface. A threshold density was identified using maximum curvature and an SPA boundary was drawn around all cells on the density surface with a density greater than the threshold density. The SPA boundary contained 6301 red-throated divers in an area of 3937 km², which was 77.5% of the estimated Outer Thames Estuary red-throated diver population in an area of only 38.9% of the study area. This boundary was adopted in 2010 when the Outer Thames Estuary SPA was classified. This method is suited to a species that is aggregated at medium to large scales and for which appropriate environmental covariates are lacking. It is relatively simple, which is advantageous when communicating methods to non-scientists, and provides an objective method for drawing an MPA boundary that is robust to challenge by stakeholders who may wish the MPA to be larger or smaller.