Variation in the appearance of eggs within a clutch has been hypothesized to decrease the likelihood of rejection of brood-parasite eggs, but tests of the hypothesis have produced mixed results among cuckoo (Cuculus canoras and Clamator glandarius) hosts. The hypothesis has not been tested in cowbird hosts because they typically show little intraspecific variation in responses to cowbird eggs. However, the Common Grackle (Quiscalus quiscula) is unusual among potential hosts of the Brown-headed Cowbird (Molothrus ater) because it shows both acceptance and rejection of cowbird eggs and it demonstrates high intraclutch variation in egg appearance, which makes it one of the few potential cowbird hosts suitable for a test of the intraclutch variation—host rejection hypothesis. We tested whether intraclutch variation in the appearance of Common Grackle eggs influenced the likelihood of egg rejection and found that Common Grackles with greater variation in intraclutch egg appearance were more likely to accept cowbird eggs when these were added prior to clutch completion. These results are the first to indicate that even when a host has eggs that are dramatically different from those of a parasite, variation in its own eggs may increase the threshold required to elicit egg rejection.
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Vol. 127 • No. 4