Egg-capping is the slipping of a hatched eggshell fragment over an intact shell. It is a rare phenomenon among avian species, probably because most eggs in the same clutch hatch synchronously, are similar in size, and parents typically remove debris—including shell fragments—from nests. Hatching asynchrony and egg-size differences are typically more pronounced in the context of interspecific brood parasitism, making conditions for egg-capping more likely. Indeed, egg-capping of host eggs occurred in 33% of clutches of Eastern Phoebes (Sayornis phoebe) parasitized by Brown-headed Cowbirds (Molothrus ater), always by the earlier hatched cowbird's eggshell, whereas it was not detected in nonparasitized clutches near Ithaca, New York. To determine experimentally if host parents rejected their own eggs capped by the parasite's shell, cowbird shell fragments were introduced into nonparasitized phoebe clutches either as slipped over intact phoebe eggs (caps) or simply placed into the nests (noncaps). The latency for cowbird egg-fragments to disappear was greater for caps (1–13 days) because all noncaps were removed in <1 day. Whether phoebe eggs hatched was also related to the outcome of the capping treatments: eggs that remained capped for >1 day had a greater probability of hatching failure (0.6) than noncapped eggs (0). Those data suggest that egg-capping by shell-fragments of earlier hatched cowbird eggs reduces phoebes' reproductive success and may represent a fitness cost to hosts in other parasitized species as well.