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Host rejection of parasitic eggs is the most important defence against avian brood parasitism Here, we examined the influence of egg size differences between host and parasite eggs on egg rejection behaviour in a population of Reed Warblers Acrocephalus scirpaceus parasitized by Common Cuckoos Cuculus canorus To this end, we experimentally parasitized host clutches using real Chinese Quail Column: chinensis and conspecific eggs Both egg types were painted immaculate blue The Chinese Quail eggs differed from host eggs in both size and colour, while the conspecific eggs differed only in colour There were no differences in the rate of rejection rate of the two types of experimental eggs However, Chinese Quail eggs were rejected primarily by nest desertion, whereas conspecific eggs were mostly ejected. Moreover, clutches with Chinese Quail eggs were deserted significantly sooner in comparison with the ejection of conspecific eggs Therefore, egg size differences apparently affect the mode and speed but not the rate of egg rejection in this host population.