We presented freeze-dried models of a nondescript sparrow (control), a female brown-headed cowbird (Molothrus ater, Icterinae) and an avian nest predator to nesting eastern kingbirds (Tyrannus tyrannus, Tyranninae) during the egg-laying and nestling stages to test for nest defense against the threat of cowbird parasitism. Both kingbird parents responded during 81% of the 183 trials. Kingbirds responded aggressively toward the cowbird model, but only slightly more so than to the control, and less than to the predator model. The level of aggression directed toward the predator increased between the laying and nestling stages. The results suggest that kingbirds at Delta Marsh, Manitoba, did not recognize the cowbird as a unique or dangerous threat, possibly because of low selection pressure on this rejector species for aggressive nest defense against cowbirds.