Whereas most host species parasitized by Brown-headed Cowbirds (Molothrus ater) accept cowbird eggs in their nests, others reject the foreign eggs or desert parasitized clutches. Chipping Sparrows (Spizella passerina) are a nest deserting host of cowbirds, but many of their nests continue to be incubated with cowbird eggs suggesting that cowbird eggs per se do not necessarily trigger desertion. We demonstrate that encounters with an adult female brood parasitic cowbird rather than with a cowbird egg in the nest, elicit nest desertion. We observed that a total of 33 (67.3%) of 49 nests were naturally parasitized in Illinois. Parasitized nests fledged less than one third of the number of host young than unparasitized nests did, indicating that parasitism is extremely costly in this species and that an evolved response to parasitism would likely spread through the population over time.