The Military Macaw (Ara militaris) is an endangered species with a fragmented distribution and declining populations. Their diet has not been described in detail but the species is considered to be a specialized granivore, and individuals may make seasonal movements in search of food resources. This study examined the diet of Military Macaws in tropical dry and oak (Quercus spp.) forests of Cuicatlán, México. We followed macaws to identify diet composition, and quantified the nutritional content of all items which they consumed. Military Macaws used 10 plant species during the year having a standardized Levin´s niche breath index of B = 0.18. Their diet consisted principally of seeds but also included fruits, leaves, and latex. Foods rich in protein and lipids composed the diet of macaws, especially during the breeding season. Macaws preferred some plant species using them more than predicted by their abundances. Food resources must be considered when managing and preserving habitat to protect this endangered species.