Risk of cuckoo parasitism does not predict egg rejection behaviour in two sympatric breeding hosts

Abstract Obligate brood parasitism imposes high reproductive costs on the host, leading to the development of anti-parasitism strategies across different spatial and temporal scales. Some studies have suggested that hosts adjust their egg rejection intensity by evaluating changes in cuckoo parasitism risk. However, host adaptive performance is highly variable with their level of anti-parasitic cognition. Leveraging a system of two sympatric hosts of the common cuckoo (Cuculus canorus), the grey bushchat (Saxicola ferreus) and Siberian stonechat (Saxicola maurus) that differ in their egg recognition abilities, we created two egg rejection levels (moderate and low) using different experimental eggs to test whether these hosts adjusted their rejection intensity around the cuckoo’s migratory arrival. Our results showed that neither host cognitive ability nor parasitic egg type had a significant effect on the egg rejection rates before and after the cuckoo’s arrival. Therefore, the hypothesis of adjustment in egg rejection to within-season shifts in parasitism risk was not supported. Future studies should investigate various host life history traits, such as anti-parasitism adaptation patterns, breeding experiences associated with age structure and mating systems, to further explore the composition and maintenance dynamics of egg rejection within a host population.