As part of the coevolutionary process between brood parasites and their hosts, the latter have developed different strategies to discriminate and reject parasitic eggs. This recognition–rejection process is the primary host defense against costly brood parasitism. The Red-crested Cardinal (Paroaria coronata) is an occasional host of the generalist Shiny Cowbird (Molothrus bonariensis) that successfully rejects all parasitic eggs. We studied the cues used by Red-crested Cardinals to recognize and reject foreign eggs by experimentally adding real parasite and host eggs painted as mimetic or nonmimetic of host eggs and analyzing whether eggshell coloration and/or shape were used as cues for egg rejection. Rejection rates, mostly through egg ejection, were high for all nonmimetic eggs (95% for unpainted cowbird eggs and 100% for painted nonmimetic cowbird and host eggs). On the contrary, they were low for mimetic host eggs (6% for unpainted host eggs and 20% for painted mimetic host eggs), but intermediate for painted mimetic cowbird eggs (55%). We also found that egg width significantly affected the probability of rejection, with wider parasitic eggs (i.e. more different from host eggs) more frequently rejected. We report for the first time that egg width is an important cue for recognition and ejection of cowbird eggs in an open-cup-nesting host. Our results show that coloration is a reliable cue used by Red-crested Cardinals to discriminate and reject parasitic eggs, but when coloration alone does not allow discrimination of foreign eggs, this host uses egg width as an additional cue.