Experiments showed that most Cedar Waxwings reject cowbird eggs placed in their nest during that part of the nesting cycle when cowbirds typically parasitize nests. It is reasonable to interpret the waxwing’s rejection behavior as an antiparasite adaptation that evolved in response to brood parasitism because evidence presented here shows that cowbird parasitism is a selective pressure on the Cedar Waxwing. Natural parasitism was detected at 7.5% of 334 nests. Parasitism is probably greater than 7.5%, as many cowbird eggs may be ejected before being seen. If accepted, a cowbird egg is likely to reduce the waxwing’s reproductive output. Cowbirds hatch sooner, are larger at hatching, and develop more quickly than waxwings and hence would normally outcompete the latter. Most cowbird eggs laid in waxwing nests are rejected and others may fail because of the waxwing’s frugivorous diet. Even if only 7.5% of the waxwing nests are parasitized, a considerable proportion of the cowbird’s reproductive effort is wasted. Despite this inefficiency selection may still have produced the most adaptive breeding strategy possible within the confines of the cowbird’s innate abilities. Several models are presented to explain the ways in which cowbirds choose host species. Each model accounts for the fact that the cowbird’s breeding strategy is not optimal.