The Great Bustard Otis tarda is globally endangered. Many populations are in decline for unknown reasons .The only viable populations are now restricted to the farmlands of the Iberian Peninsula. An effective conservation plan is urgently required to prevent the extinction of thes species. Research on breeding biology is now in progress, but essential information on habitat requirements is lacking. Through a comparative study of a "healthy" population at Villaffifila (NW Spain) and an "endangered" population near Madrid, the principal questions to be addressed by this project are: 1. Which habitats are used and which are avoided? 2. Which diets are selected and why? 3. Does food depletion have a role in habitat selection?? 4. What are individual home range requirements? 5. Can the planting of favoured crops be used to increase the species range? 6. How are various parameters of population dynamics (e.g. juvenile mortality rates etc.) related to habitat quality at the two study areas? The proposed comparative approach has particular utility in predicting the consequences of habitat changes to Great Bustard populations. With this information it will then be possible to recommend precisely which agricultural practices should be encouraged to ensure the survival of this species.