Brood parasitism usually imposes several costs on individual hosts, one of which is the increased probability of brood reduction. We analyzed the effect of cowbird (Molothrus spp.) parasitism on the probability of brood reduction in the Brown-and-yellow Marshbird (Pseudoleistes virescens), a common host in the Argentinean Pampas. Using a nine-year database of nesting success of the Brown-and-yellow Marshbird, we compared the number of host fledglings produced in parasitized and unparasitized nests, and assessed the effect of year, parasitism (if the nest was parasitized or not), overall brood size (including cowbird and marshbird nestlings), and "hatching interval" (interval between the first- and last-hatched young) on the probability of brood reduction. Parasitized nests produced fewer Brown-and-yellow Marshbird fledglings. About 22% of nests suffered brood reduction and 68% of these nests were parasitized by cowbirds. A logistic regression model showed that hatching interval was the main factor associated with brood reduction in this species, being associated with both the number of cowbird eggs present in the nest and overall brood size. Our results reinforce the idea that cowbird egg-laying synchronization imposes additional costs on hosts, as it increases the probability of host brood reduction.