The Horned Guan (Oreophasis derbianus) is endemic to humid montane forests of southern Mexico and Guatemala. This species is considered endangered because of their small populations, the loss and fragmentation of habitat, illegal trade, and overexploitation by subsistence hunters. We update information about the species' diet and foraging behavior by integrating the results generated during two and a half decades of research on the Horned Guan's ecology at the El Triunfo Biosphere Reserve, in Chiapas, Mexico, with additional published information compiled from other areas. Based on nearly 450 hrs of direct observations of free-ranging guans and 530 discrete feeding events, we found that during the breeding season Horned Guans feed primarily on fruits from six species of plants and leaves from one species. Horned Guans were not observed eating animal matter, corroborating its specialized frugivore-folivore habits. Our study increases the known plant taxa found in the Horned Guan's diet in El Triunfo from 40 to 63 ( Supplemental Material ()), and globally to 101 species ( Supplemental Material ()). For 48 taxa in El Triunfo, only fruits were consumed, while for eleven taxa consumption was restricted to leaves, and to flowers for one species; for four taxa both fruits and leaves were consumed. We found significant differences between males and females in the location of foraging on trees and diet composition. Young birds are fed fruits of Citharexylum mocinnii and leaves of Solanum appendiculatum by their mothers, both of which are rare in the diet of adult males. The conservation of the Horned Guan requires the long-term protection of suitable habitat that maintains the plant species important in their diet.
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