Data for: Shy-boldness cannot predict egg rejection in the Japanese tit

During the breeding season (April-June) of year 2022, when Japanese tits were found occupying nest boxes, breeding nests were regularly monitored to determine the dates of the first eggs. Female Japanese tits usually started incubation the day after laying their last egg (Zhang et al. 2020), and therefore, we defined the date when the females laid their last eggs as day 0 of the incubation period. After determining that the Japanese tits had entered the incubation period, we began an escape behavior test and a foreign egg recognition experiment among the female Japanese tits. In order to measure escape behavior, researchers slowly approached the nest box from about 10 m and gazed at the hole to record whether the females flew away, though the orientation of the approaching was adjusted according to location of focal nest boxes. If the female did not fly out from nest boxes when the experimenter went under the tree hung up nest box, we climbed the tree. We defined females that flew away from the nest box as shy individuals and those that did not as bold individuals during this process. If the females were not in the nest at the time of the first experiment, this experiment was repeated on the second day until escape behavior of the incubating females was recorded. After waiting for the departure of the females, we placed one foreign egg in the nest cup after clutch completion (i.e., on the first day when the host began to incubate) (Figure 1a), while recording the experiment date and the clutch size in the experimental nest. In this study, most egg experiments were conducted on the day when the clutch of tits were completed or the next day, and only a small number of nests could not be determined for their incubation time. However, none of the experimental nests hatched after 6 days of the egg recognition experiment (see below), suggesting that all of these nests should have been in the early to mid-incubation period (i.e., 10-15 days for Japanese tits) at the time of egg experiments. The foreign eggs used in the experiment were the commercial non-fertilized eggs of white-rumped munia (Lonchura striata), colored with a blue non-toxic and odorless marker, which were slightly smaller in size than the experimental tit species (see Liu et al. 2019 for the exact egg sizes of Japanese tits and white-rumped munias). We set the length of the egg rejection experiment to 6 days, and considered females of the experimental nest not to reject foreign eggs if the incoming eggs remained in the experimental nest and the nest was not abandoned within 6 days. Females of the experimental nest were considered to reject foreign eggs if within 6 days, the incoming eggs disappeared or remained in the nest box but the females did not incubate them (i.e., buried under the nest material or not in the nest cup, as shown in Figure 1b) (i.e., Yang et al. 2015; Liang et al. 2016; Liu et al. 2019). Some studies have suggested that nest abandonment is also one of the ways host birds reject foreign eggs (Hanley et al. 2016), but nest abandonment was not found in our experiments. If the experimental nest succumbed to predation within 6 days, experimental results from that nest were not counted.