The Montagu's harrier (Circus pygargus) is a ground-nesting raptor that breeds mainly in cereal crops in western Europe. We evaluate how the use of agricultural habitats may affect population sustainability in this species, based on simulation analyses, and explore how conservation measures could be optimized. Probability of extinction increased with decreasing harrier productivity, and this trend was accentuated when the carrying capacity (maximum number of breeding pairs) decreased. Harrier productivity in agricultural habitats is strongly affected by harvesting activities. An average of 60% of nestlings in agricultural areas of France and the Iberian Peninsula would perish in the absence of conservation measures. These losses would make populations unsustainable, if no immigration occurred. Simulation analyses showed that connectivity between populations through natal dispersal could allow persistence of threatened populations even in the absence of conservation measures. The probability of extinction of four hypothetical populations connected through natal dispersal would be lowest if one of those populations were fully protected (or fully productive), even if the other ones were unprotected. Montagu's harriers are semi-colonial, and populations could be considered as a compound of subpopulations (the colonies). Additionally, Montagu's harriers bred more frequently in areas where food abundance was high, and where the number of fledglings produced in the previous reproductive attempt was high. These factors could be used to develop sustainable and efficient conservation plans, identifying and protecting the most productive and stable colonies in agricultural areas, and further exploring experimentally factors that are likely to attract and maintain harriers in protected areas.