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I conducted observations on the diet of the Floreana mockingbird (Mimus trifasciatus) during its breeding season in February and March 2011. The Floreana mockingbird is a critically endangered species restricted to Gardner and Champion Islets off the coast of Floreana Island, in the Galápagos Islands, Ecuador. During 11 days, 172 feeding bouts of adult and nestling mockingbirds were observed. The majority of feeding bouts of adults (31%; 19 feeding bouts) involved the consumption of nectar and pollen of Opuntia megasperma. Another important food item consisted of Lepidopteran caterpillars (27%; 17 feeding bouts). The majority of food items fed to nestlings consisted of Lepidopteran caterpillars (26%; 29 observations), followed by adult spiders (19%; 21 observations). The reintroduction of the species to its historical range on Floreana Island is currently being planned with an emphasis on the control or eradication of invasive cats and rats. To identify key areas for reintroduction, a study on the year-round diet of the species as well as availability and variability of food items is recommended. Nectar and pollen of Opuntia megasperma was an important dietary item for the species during its breeding season. This slow-growing plant species was widespread on the lowlands of Floreana Island but introduced grazers removed Opuntia from most of its range. In the context of the potential reintroduction of this species to Floreana Island, it is important to establish if this high-energy resource is key for breeding, and consideration should be given to a supplementary food program as it has been successfully implemented for bird species elsewhere.