Host learning selects for the coevolution of greater egg mimicry and narrower antiparasitic egg-rejection thresholds

Abstract Egg rejection is an effective and widespread antiparasitic defense to eliminate foreign eggs from the nests of hosts brood parasitic birds. Several lines observational critical experimental evidence support a role for learning by in recognition versus own eggs; specifically, individual that have had prior or current experience with parasitism are more likely reject eggs. Here we confirm experimentally altering subsequent egg-rejection decisions American robin Turdus migratorius, free-living host species obligate parasite, brown-headed cowbird Molothrus ater. We then model coevolutionary trajectory both extent mimicry host’s egg thresholds response increasing recognition. Critically, learning, see evolution narrower (more discriminating) greater parasites. Increasing clutch size (number eggs/nest) parasite load (parasitism rate) also narrowing effects on threshold. Together, these results suggest may play important coevolution egg-mimetic lineages parasites refined defenses hosts.