We found a moderate rate of cuckoo parasitism in our study population of Daurian redstart, with 15.6% of nests parasitized, and the rate was remarkably lower than in common redstarts (more than 20% in any population). Effective parasitism rate in Daurian redstarts (14.8%) was higher than that in common redstarts (11.6% at most), because a large number of cuckoo eggs was laid outside the nest cup in common redstart nests [18, 19, 23]. Still we found a number of adaptations to cuckoo parasitism in Daurian redstarts in terms of rejection of cuckoo eggs from second rather than first clutches. A change in laying date may be an adaptation to parasitism, and a reduction in parasitism rate and a change in nest site towards villages among second compared to first clutches. We will briefly discuss these and novel adaptations to cuckoo parasitism.
Parasitism and Eviction Success
In our study, only one cuckoo egg was found outside the nest cup, and one egg was found during the nest-building stage. In the other word, nearly all cases of cuckoo parasitism were effective and successful. In common redstarts, although higher natural parasitism rates were found (generally more than 30%), less than half of all cuckoo eggs were found inside nest cups, while the remainder were either on the nest rim or on the ground outside nest boxes [18, 19, 23]. This was not caused by ejection by hosts, but rather by mislaying parasites [19, 23]. Different designs of entrances of nest boxes might contribute to this difference [19, 23]. Moreover, this difference in nest hole size might be an efficient defence against cuckoo parasitism for cavity-nesting hosts.
All cuckoo chicks in Daurian redstart nests succeeded in evicting all host eggs or nestlings. Studies of common redstarts showed that cuckoo chicks do not always succeed in evicting all host offspring [18, 19, 23]. The nest structure of the common redstart is a likely explanation, because cuckoo chicks in steeper nest cups showed lower eviction success . Another possible reason is the location of the nest cup relative to the box walls . Sometimes nest cups are built just next to the back wall of the nest box, and there is no space for evicted eggs and chicks, so they fall back into the nest cup again after being evicted by cuckoo chicks . Cohabitation with host nestlings was likely to cause cuckoo chicks to have higher nest mortality [18, 19; but see 23].
In Daurian redstarts, some nest cups were built next to the rear wall or in the corner of the rear or the side wall. According to our observations of eviction behavior of cuckoo chicks, they sometimes failed since host offspring fell back into the nest cup. However, several minutes later, cuckoo chicks showed eviction behavior again, evicting host eggs or chicks to another side. In the end, cuckoo chicks succeeded in evicting all host offspring and they lived solitarily in host nests. That may be the reason why cuckoo chicks in Daurian redstart nests showed high fledging rate with 31 of 36 cuckoo chicks fledging.
Response to Brood Parasitism
Comparisons of nest desertion rates for different categories of nests did not reveal any statistically significant difference between control and (naturally or artificially) parasitized nests. Thus, we suggest that nest desertion is not a specific response to cuckoo parasitism in Daurian redstarts. A similar result was also found in the European common redstart [23, but see 19]. Desertion behavior as a defensive strategy against parasitism has been studied frequently [29, 30]. However, nest desertion often occurs in small-sized hosts such as chipping sparrows Spizella passerina, meadow pipits (Anthus pratensis), and blue-grey gnatcatchers (Polioptila caerulea) that rarely eject parasite eggs [31-33]. They are physically unable to puncture or grasp and finally eject parasitic eggs . However, some medium- and large-sized hosts such as grey-backed thrushes (Turdus hortulorum), blackbirds (Turdus merula), and common grackles (Quiscalus quiscula) do not desert parasitized nests instead of ejecting parasitic eggs [35-37]. Moreover, nest desertion can be triggered by other factors such as parental mortality, human disturbance, or egg loss [38-40]. In fact, human disturbance might be the most obvious explanation for nest desertion in our study, since we hardly found deserted nests with daily checks.
Egg recognition and rejection are the most common and efficient host defences against brood parasitism . Daurian redstarts eject parasitic eggs, but has specific egg morphs, with hosts laying white clutches showing significantly higher egg ejection rates than those laying blue clutches. Daurian redstarts laying white clutches ejected 33.3% naturally parasitized real cuckoo eggs and all artificially parasitized real cuckoo eggs, while hosts laying blue clutches ejected no real cuckoo eggs (n = 26). However, Daurian redstarts laying blue clutches could eject mimetic cuckoo eggs, which implies that they also have the ability to recognize and eject cuckoo eggs, although their ejection rates were significantly lower than their white counterparts.
We propose the following hypothesis: If Daurian redstarts resist cuckoo parasitism by egg ejection, then egg ejection rates in the second breeding attempt with high risk of parasitism will be higher than in the first breeding attempt, when there is no risk of cuckoo parasitism. From egg experiments, we found that ejection rates of mimetic cuckoo eggs in the second breeding attempt were significantly higher than in the first breeding attempt both in blue and white clutches, which in line with the hypothesis.
Different cuckoo host races are known to differ in timing of breeding, breeding habitat and potentially other phenotypic traits that contribute to isolation among races . Here we have shown that parasites of Daurian redstarts differ in timing of breeding, as expected for isolation by timing of breeding. This finding also suggests that such isolation by timing of breeding may affect the variance in timing of breeding, by common redstart showing greater variance than Daurian redstart. Hence both mean and variance in timing of breeding may contribute to such effects of timing as shown in this study.
Daurian redstarts laying white clutches showed significantly higher egg ejection rates in all categories. A possible explanation is that the classic color of parasitic cuckoo eggs is pale blue, closely matching those of Daurian redstart eggs of the blue morph . This suggests a long co-evolutionary history with strong selection from the host by rejecting non-mimetic eggs . Co-evolutionary interactions between cuckoos and hosts support egg polymorphism [43, 44]. Egg polymorphism in hosts is a specific adaptation to and a defence against cuckoo parasitism, since one female cuckoo is confined to lay one egg morph , which can only match its corresponding morph in host nests. Therefore, it favours hosts laying other egg morphs to discriminate and reject matching parasitic eggs [15, 46], which will reduce the rate of successful parasitism in cuckoos. In turn, to increase parasitism success, cuckoos are predicted to increase the frequency of the corresponding egg morph . Most (12 of 13) known species of Phoenicurus redstarts lay blue egg morphs, and nearly all (5 of 6) parasitized species of Phoenicurus redstarts display white egg morphs [47, 48]. This suggests that a blue redstart egg is the ancestral state, and, likewise, the blue egg morph is an ancient trait in cuckoos . Here, we inferred that white egg morphs are relatively recent in Daurian redstarts, and that it has evolved to cope with cuckoo parasitism. Significantly higher egg rejection rates in white clutches in our study strengthens this idea. Moreover, the large difference in ejection rates of real blue and white cuckoo eggs may be the reason why cuckoos lay eggs mostly in blue clutches [49, but see 50].
In the second breeding attempt in 2018, we found a purple cuckoo egg in a blue clutch, which was laid by a different cuckoo subspecies (see Results). We hypothesized that this may be due to mislaying by the female cuckoo. Alternatively, she may not have been able to wait to find a suitable nest, since after three days when the purple cuckoo egg was found, and the host eggs hatched.
Proximity to human habitation
The rate of parasitism in nests far from villages was significantly higher than that in non-parasitized nests, implying that there is a lower parasitism risk in nests close to villages. That may be the reason why remarkably more redstarts chose to breed near villages in the second than the first breeding attempt with high parasitic risk.
Proximity to human habitation has been proven to be an effective strategy for resisting cuckoo parasitism . In a study of Oriental reed warbler (Acrocephalus orientalis), Møller et al.  found that cuckoo parasitism rates increased with distance from the nearest human habitation. That is because most parasitic cuckoos tend to avoid close proximity to humans [51, 52], so there is lower risk of cuckoo parasitism in villages. A similar pattern occurs in magpies (Pica pica), which tend to choose nest sites with low risk of cuckoo parasitism .
We only found parasitized nests in the second breeding attempts, while there were no parasitized nests in the first attempts. This difference between first and second breeding attempts is due to the common cuckoo not having arrived yet when the Daurian redstart has already initiated its first breeding attempt. Thus, we suggest that hosts advancing their timing of breeding can also effectively prevent cuckoo parasitism .