I recorded the presence of a male Brown-headed Cowbird (Molothrus ater), probably immature, with a beak deformation in the locality of Petaquillas, municipality of Chilpancingo, Guerrero. Studies are needed to know the causes of beak deformation in this and other species, and to clarify if it is an isolated case or a phenomenon that affects other individuals or species in the area. It is
... [Show full abstract] important to elucidate if there is any relationship between this malformation and the environmental pollution in the area due to the irrigation of crop fields with wastewater from the city of Chilpancingo.