We testedw hether a common host of the brood parasitic Brown-headed Cowbird (Molothruast ater),the Red-winged Blackbird (Agelaius phoeniceus),accepts cowbird eggs in its nest because the blackbird does not recognize foreign eggs(evolutionary-lag hypothesis)or because the cost of rejecting eggs or deserting the nest and re-laying exceeds the benefits of egg rejection (evolutionary-equilibrium hypothesis). We reasoned that, if egg acceptance is due to the high cost of rejection, increasing the cost of acceptance should cause the blackbirds to attempt to reject cowbird eggs, or desert the nest and renest elsewhere. We increased the cost of acceptance by adding one, two or three cowbird eggs to Red-winged Blackbird nests, and by removing one host egg for every cowbird egg added. We found no rejection at any level of parasitism. Additionally, we found that Red-winged Blackbirds incubated parasitized clutches that contained none of their own eggs. We conclude that evolutionary lag is the reason for egg acceptance in Red-winged Blackbirds.