Complete egg clutches as memory template for host egg recognition

The study site is located in Liupanshan National Nature Reserve (106°09′–106°30′ E, 35°15′–35°41′ N), Ningxia Hui Autonomous Region, in the northern part of the Loess Plateau, in which the highest elevation reaches 3,000 m. The study was carried out during the breeding season (April–August) in 2024. Coal tits and green-backed tits are small secondary cavity-nesting birds in the family Paridae within the order Passeriformes, and often nest in the crevices in rock and walls, eaves, and artificial nest boxes. Although the green-backed tits breeding in this area have been established to possess strong egg recognition abilities (Liu et al. 2024a), it remains uncertain whether coal tits have similar abilities.In this regard, egg recognition and rejection is among the most prevalent forms of host defense. Egg recognition and egg rejection differ among host species, as well as among different geographical populations of the same host. Here we examined two cavity-nesting bird species, the coal tit (Periparus ater) and the green-backed tit (Parus monticolus) to investigate their egg recognition strategies at different breeding stages. The results revealed that neither of these tit species reject foreign eggs during the egg-laying stage; however, both species exhibited this behavior after commencing incubation. During this stage, the rate of foreign egg rejection was lower in coal tits than in green-backed tits, and the survival time of foreign eggs in the nests of green-backed tits was shorter than that in the nests of coal tits. These observations suggest that green-backed tits demonstrate a quicker recognition and rejection of foreign eggs than coal tits. This is the first reported case of cuckoo hosts that does not reject foreign eggs during the egg-laying period. Our study revealed that both coal tits and green-backed tits use the entire clutch as a memory template for comparing and recognizing foreign eggs.