Interactions between egg colors promote egg recognition in a cavity-nesting bird

The Liupanshan National Forest Park (35°15’ - 35°41’ N, 109°09’ - 116°30’ E) is located in Jingyuan County, Guyuan City, Ningxia Hui Autonomous Region, northwestern China, at an altitude of 2,060 - 2,931 m. It has a total area of 6.78 ha with forest coverage of 72.8% and is the largest natural secondary forest in Ningxia (Liu et al., 2023a). The green-backed tit is a small secondary cavity-nesting bird, which belongs to the family Paridae and order Passeriformes. They often build nests in rocks, walls, eaves, and artificial nest boxes, and have strong egg recognition ability (Liu et al., 2024; Yang et al. 2022).Data collection and field experimentsDuring the breeding season (April to August) from 2023 to 2024, we hung artificial nest boxes (n = 200) in the Liupanshan National Forest Park. We regularly inspected the artificial nest boxes to determine the breeding status of birds. For nests in the incubation stage, this study designed the following experiments. In experiment 1, one red (long wavelength) model egg (n = 24), one blue (short wavelength) model egg (n = 29), and one purple (short wavelength) model egg (n = 25) were added separately to nests, and the rejection rates of the three groups were compared. In experiment 2, one red model egg together one blue model egg (n = 24), as well as one purple model egg together with one blue model egg (n = 15), were added to the nest simultaneously. The rejection rates of the same-colored eggs were compared between experiment 1 and experiment 2 (the simultaneous addition cohort). Model eggs of the same color were added to the experimental nest only once and the experimental period was 6 days. If the model egg disappeared, had pecking marks, was buried, or was abandoned during the experiment, it was considered as egg rejection; if the model eggs still existed and were normally incubated after the experiment, it was considered as egg acceptance; if the nest was preyed upon or damaged during the experiment, it was not included in the experimental results (Liang et al., 2016; Liu et al., 2023a).In this study, egg recognition strategies were examined by adding differently colored model eggs to the nests of green-backed tits (Parus monticolus), a secondary cavity-nesting bird. The results showed that green-backed tits rejected blue and purple (short wavelength) model eggs at similar higher rate compared to red model eggs (long wavelength), with significant differences in rejection rates between long wavelength and short wavelength model eggs. However, the rejection rate of red model eggs was significantly increased from 55.8% to 83.3% via the facilitation of blue model eggs. Thus, our results were consistent with the multiple threshold decision rule. This study provided experimental evidence that egg color has an important effect on egg recognition in cavity-nesting birds, and also demonstrated that interactions between egg colors promote host egg recognition.