We report the first record of Bronzed Cowbird (Molothrus aeneus) brood parasitism on a sedentary tropical population of the Blue-gray Gnatcatcher (Polioptila caerulea), which probably represents the subspecies P. c. deppei. Our observations represent the 46th known true host species parasitized by this cowbird species, the 104th host and victim, and the 3rd known brood parasite for the family Polioptilidae. “Victims” are all species parasitized by cowbirds and “hosts” are those species that hatch, feed, and successfully raise cowbirds. Our assignment of these records to the true host list are based on feeding behavior observations at a nest in Veracruz, Mexico. We provide additional information on nest characteristics of an understudied gnatcatcher population.