Grassland passerines are purported to tolerate parasitism by Brown-headed Cowbirds (Molothrus ater) because of adaptations by cowbirds that constrain egg discrimination and removal by their hosts, i.e., evolutionary equilibrium, rather than because of an absence of these defensive behaviors, i.e., evolutionary lag. We tested these hypotheses by experimentally parasitizing six grassland species with cowbird-like eggs and non-mimetic (blue) eggs in south-central Saskatchewan. Sprague's Pipits (Anthus spragueii), Vesper (Pooecetes gramineus), Savannah (Passerculus sandwichensis), and Baird's (Ammodramus bairdii) sparrows, and Chestnut-collared Longspurs (Calcarius ornatus) accepted all or nearly all cowbird eggs, but ejected or attempted to eject between 9 and 20% of blue eggs with 54% of rejected eggs not removed from the nests, i.e., failed ejection attempts. Western Meadowlarks (Sturnella neglecta) ejected an intermediate number of cowbird eggs (67%) and most blue eggs (92%), none of which was recovered. Ejection of non-mimetic eggs versus cowbird eggs by each species suggests that similarity in appearance of cowbird eggs and hosts' eggs impeded discrimination and may represent a form of cowbird egg mimicry. The low number of blue eggs ejected by five of the six species, their failure to remove these eggs from the nest sites, and the damage wrought on some hosts' eggs during ejection suggest the morphology of cowbird eggs also constrains ejection behavior. These results support the evolutionary equilibrium hypothesis as the better explanation for acceptance of cowbird parasitism observed in these grassland passerines.