Complete egg clutches as memory template for host egg recognition

Abstract

Egg recognition and rejection are among the most prevalent forms of host defense against brood parasitism. However, there is a lack of comprehensive data on the ability of hosts to update and modify their egg rejection strategy throughout the breeding cycle. Here we examined two cavity-nesting bird species, the green-backed tit Parus monticolus and coal tit Periparus ater, to investigate their egg rejection strategies at different breeding stages. The results revealed that neither of these tit species rejects foreign eggs during the egg-laying stage; however, both species exhibited egg rejection after starting incubation. During this stage, both the green-backed tit and the coal tit exhibited high rejection rates against foreign eggs, and the green-backed tit maintained a high egg rejection rate during the nestling period as well. There was no significant difference in foreign egg survival time between the two species. Additionally, there was no difference in egg survival time between the egg-laying treatment group (foreign egg addition during the laying period, rejection during the incubation period) and the incubation treatment group for either tit species. Our study revealed that both green-backed tits and coal tits use the entire clutch as a memory template for comparing and recognizing foreign eggs, and that both species share a consistent egg rejection strategy. This is the first reported case that coal tits do not reject foreign eggs during the egg-laying period.