DOI: 10.6084/m9.figshare.29555939.v1
title: <b>Nest-site selection by a cuckoo host in response to parasitism risk</b>

description: Sifangtuozi Farm (SFTZ) (46°00′-46°22′ N, 123°46′-123°57′ E) is located in Jilin Province, northeastern China. It lies in the Nen River Basin, a tributary of the Songhua River, with terrain gradually descending from northwest to southeast, and rich in water resources flowing through. SFTZ has an abundance of water, with thriving lush reeds and cattails. Extensive stands of cattails and reeds grow in areas surrounding the farm, including agricultural ridges, irrigation channels, streams, and ponds (Trnka et al. 2023). In our study area, both ORWs and CCs are summer migrant (Liu et al. 2025). ORWs were identified as the main hosts of CCs and exhibited strong aggressive behavior towards cuckoos (Li et al. 2015, Trnka et al. 2023). In China, there has long been a high degree of coevolutionary adaptation between ORWs and CCs (Wang et al. 2022), with cuckoo parasitism rates ranging from 34.3% to 65.5% (Liang et al. 2014; Yang et al. 2016, 2017). Data collection Field data collection on ORWs was performed during the breeding season (late May to August) in both 2022 and 2023. After the onset of the ORW breeding activities, systematic searches were conducted in the study area. For each newly discovered nest, GPS recording, photography, and numbering were performed. Miniature video recorders (Ou Chuang A8; Xiamen Shangyu Huajin Electronic Technology Co., Ltd., Xiamen, China) were used when necessary for real-time video recording. Regular visits were made to each nest at least once per day, and the final reproductive outcome of each nest was recorded. If at least one ORW chick successfully fledged from the nest, it was recorded as a successful breeding nest; any other outcomes were recorded as breeding failures. Furthermore, factors contributing to the failure of all unsuccessful nests were documented, such as human activities, predation, parasitism, weather conditions (Yang et al. 2011). In addition, if nests were successfully parasitized by the CC during the breeding season, the number of parasitism events was recorded (Figure 1). However, if the parasitic eggs in the nest did not hatch successfully, the actual outcome of the nest (breeding success or failure) was recorded, along with any other factors contributing to its failure (Antonov et al. 2007c). In addition, we measured and recorded statistical parameters of the ORW nest-sites (Li et al., 2016): (1) distance to perch (m): distance of the nest from the nearest perch point such as trees, utility poles, wires, and netting where cuckoos can stop and observe; (2) distance to house (m): distance to the nearest house; (3) distance to neighbor (m): distance between the nearest active nests of the same warbler species. The nest distances of ORWs to houses, perches, and neighbors that were ≥200 m were uniformly recorded as 200 m. Distances &lt;200 m were recorded according to the actual measured values. The instrument used to measure the distance was SNDWAY (SW-1000A; SNDWAY Electronic (Guangdong) Co. Ltd., Dongguan, China).When avian brood parasites and their hosts interact, the “frontline” defense of the host is crucial to stave off the detrimental effects of parasitism. By comparing the characteristics of the nest-sites of Oriental reed warblers (Acrocephalus orientalis, ORWs) that are parasitized by common cuckoos (Cuculus canorus, CCs) versus those that are not, this study aims to elucidate the nest-site selection strategy employed by the ORW host to combat cuckoo parasitism. Results showed that the distance of parasitized nests [166.7±56.6 m, mean ± standard deviation (SD)] to houses was significantly larger than that of unparasitized nests (159.7±57.4 m) (P=0.045). Similarly, the distance of parasitized nests (20.7±14.9 m) was significantly closer to cuckoo perches than that of unparasitized nests (90.6±73.5 m) (P&lt;0.001). However, no significant difference was observed between parasitized nests (101.2±69.8 m) and unparasitized nests (97.2±69.3 m) in terms of distance from the nearest neighbor (P=0.214). Our findings support the proximity of human hypothesis, suggesting that Oriental reed warblers reduce the risk of parasitism by nesting and breeding near inhabited houses. Additionally, our results also align with the perch proximity hypothesis, indicating that nests located closer to cuckoo perches are more susceptible to parasitism. However, our findings do not support the nearest neighbor hypothesis. This study indicated that parasitism risk affects nest-site selection by cuckoo hosts.