Some species of North American passerines nearly always reject nonmimetic eggs placed in their nests and have apparently evolved this behavior in response to brood parasitism. Experiments presented here examined the specific egg parameters to which ’rejecter’ species respond, the relative tolerances rejecters show towards nonmimetic eggs and the degree to which rejection is limited to eggs of the brown-headed cowbird (Molothrus ater), the only parasitic bird widespread in North America. Relative to cowbird eggs, American robin (Turdus migratorius) eggs are larger, blue rather than white and immaculate rather than spotted. Experiments using 10 egg models at 137 nests showed that robins respond to each of these differences but do not usually reject an egg that deviates from their own by only one difference. Eggs that differ in any two of the three parameters are usually rejected. This built-in tolerance reduces the likelihood that robins will reject their own eggs if these are typical in size or coloration. Small egg size was the most important parameter eliciting rapid rejections (i.e. within 1 day), probably because differences in size can be detected by both visual and tactile perception. By contrast, small egg size was the least important parameter determining whether eggs were eventually rejected (i.e. within 5 days). In terms of their eventual response, robins may be more sensitive to egg coloration than to size because the latter parameter is less reliable in distinguishing between robin and cowbird eggs. Experiments were also carried out at 37 nests of the gray catbird (Dumetella carolinenesis), whose immaculate blue-green egg is only slightly larger than a cowbird egg. Catbirds are much more responsive to white ground color than to maculation perhaps because color is more reliable in distinguishing between catbird and cowbird eggs. Rejecter species exhibit degrees of tolerance towards foreign eggs that are proportional to the divergence between their eggs and those of the cowbird. Birds with eggs strongly divergent from cowbird eggs benefit from being relatively tolerant because they avoid rejecting their own eggs but still act against cowbird eggs. Species with cowbird-like eggs must be relatively intolerant to maximize the chances that cowbird eggs are rejected. Experiments show that rejection is not specific to cowbird eggs. Thus, birds have apparently responded evolutionarily to brood parasitism by developing recognition of their own eggs, rather than by developing recognition and rejection specific to parasitic eggs.