<p>The relationship between brain size and cognitive ability remains debated across animal species. Avian brood parasites impose heavy costs on their hosts, which would lead to selection for hosts with a bigger brain to improve egg-rejection behavior, a process that typically requires cognitive skills. Daurian redstarts (<i>Phoenicurus auroreus</i>), a common host of common cuckoos (<i>Cuculus canorus</i>), defend against brood parasitism mainly by rejecting the parasitic egg from the nest, but with a large between-individual variation. In this study, we tested whether hosts’ head size (used as a proxy for brain size) is related to egg-rejection behavior in Daurian redstarts. Egg-rejection behavior includes three stages: egg recognition, decision-making, and the act of ejection. Here, we first examined the relationship between female head size and the probability of recognizing the parasitic egg. Then, we investigated whether female head size was related to the probability of ejecting the egg. However, we found no relationship between redstart females’ head size and either the recognition or ejection of the parasitic egg. Our results suggest that, in Daurian redstarts, head size is not associated with egg-rejection behavior. Although high costs from brood parasitism might drive the evolution of a larger brain in hosts, this does not appear to be the case for Daurian redstarts.</p>