Common cuckoo eggs are more resistant to puncture by the host

The study site is at Sifangtuozi Farm (46°00′-46°22' N, 123°46'-123°57' E), which is located in Jilin Province, Northeast China. Every year during the breeding season (June-August), large numbers of Oriental reed warblers migrate to the reed beds surrounding agricultural ridges, diversion canals, ditches, and ponds in the region to breed (Yan et al. 2024). The Oriental reed warbler is one of the main hosts of common cuckoos, with a parasitism rate of 34.3–65.5% (Wang et al. 2020, 2021, 2022; Yang et al. 2014, 2016, 2017). From June to August 2022, we conducted relevant field experiments in the study area.In the study area, farmers often cut part of the reeds on the ridges for cultivation. Heavy rain and wind can cause many reeds to fall over, which can lead to the abandonment of nests by Oriental reed warblers that had previously nested in these areas; however, the warblers would re-select other areas for nesting and breeding. Therefore, we collected these abandoned nests and eggs (including eggs of parasitized common cuckoos already in the nest) from Oriental reed warblers during the breeding season (Yan et al. 2024). We reconditioned and reinforced the collected nests and measured the collected Oriental reed warbler eggs (egg length: 21.87 ± 0.92 mm, egg width: 15.87 ± 0.64 mm, egg weight: 2.88 ± 0.27 g, n = 16) and common cuckoo eggs (egg length: 22.33 ± 0.99 mm, egg width: 17.01 ± 0.52 mm, egg weight: 3.64 ± 0.37 g, n = 12). In addition, we tested budgerigar (Melopsittacus undulatus) eggs in our experiments, that were white unfertilized eggs purchased from the market for breeding, and these egg parameters were measured (egg length: 20.08 ± 0.90 mm, egg width: 15.31 ± 0.61 mm, egg weight: 2.03 ± 0.16 g, n = 25).To explore whether the eggshells of common cuckoos (Cuculus canorus) are resistant to puncture-ejection by their common hosts, Oriental reed warblers (Acrocephalus orientalis), we designed experiments to investigate if and how breeding Oriental reed warblers peck at foreign eggs that includes common cuckoo, Oriental reed warbler and budgerigar (Melopsittacus undulatus) eggs. The results showed that, given the same frequency of egg pecking, the probability of eggshell breakage was 87.5% for eggs of the Oriental reed warbler and 0% for eggs of the common cuckoo, with a significant difference (P =0.001, Fisher`s exact test). Our study shows clearly that common cuckoos’ eggshells are less susceptible to puncture-ejection than those of Oriental reed warblers and budgerigars.