Medusa in the nest: Human hair causes the death of breeding barn swallows (Hirundo rustica)

One study site was located in Danzhou City, Hainan Province (19°11′–19°52′ N, 108°56′–109°46′ E), in the northwestern region of Hainan Island in south China. The site has a tropical monsoon climate, with a mean annual temperature of 23.1–25.3°C and annual precipitation of 1,387–2,791 mm (Yan and Liang 2024). Fieldwork was conducted and nest-site parameters were recorded during the breeding season from March to June 2024, when active barn swallow nests were located in the vicinity of Danzhou urban center.The other study site was located in Zhalong National Nature Reserve, Qiqihar City, Heilongjiang Province (46°48′–47°31′ N, 123°51′–124°37′ E), situated on the Songnen Plain in northeastern China. The site has a temperate continental monsoon climate, with a mean annual temperature of 0.7–4.2°C and annual precipitation of 400–550 mm (Yan and Liang 2024). Fieldwork was conducted and nest-site parameters were recorded during the breeding season from June to August 2024, when active nests were located in Zhalong, Qiqihar City.The barn swallow is a common summer migrant in China, distributed across all provinces. Migration southward begins in autumn, with only small numbers overwintering in southern Yunnan, Hainan Island, Taiwan, and the Xisha Islands (Zheng 2023). The breeding season in Hainan spans from March to June (Liang et al. 2013), and that in Heilongjiang spans from May to August (Li et al. 2019). Barn swallows typically raise two broods per year, with the first generally from March to June and the second from June to August (Liang et al. 2013; Tian et al. 2024).Data collectionAt both study sites, we located buildings with active barn swallow nests. In Danzhou, swallows typically nest under the eaves of residential buildings, with one to two active nests per household (free-standing, unattached house). In Zhalong, nests are more commonly found inside storage buildings and livestock enclosures (e.g., cattle, horse, sheep, and chicken pens), with individual structures often containing ten or more active nests. Once an active nest was confirmed, we monitored it at regular intervals of approximately 2–3 days throughout the breeding period. At each inspection, we directly examined the interior of the nest and photographed its contents to accurately identify the materials present.Previous research on entanglement mortality associated with anthropogenic nest materials is geographically and taxonomically biased, with few studies conducted in Asia and most focusing on seabirds. In this study, we aimed to assess the prevalence of anthropogenic materials in barn swallows (Hirundo rustica) nests and their association with entanglement injury or mortality. We examined the nest material composition of 193 active barn swallow nests from two geographically distinct populations in China (Hainan in the south and Heilongjiang in the north), finding that 38.3% of nests contained anthropogenic materials, principally hair (22.3%) and plastic (21.8%), with both occurring at significantly higher rates in the south Hainan population. We report, for the first time, seven cases (3.6%) of entanglement mortality in breeding adults and nestlings caused by hair and plastic (fishing line) incorporated into nests. Although the number of individuals killed by entanglement in hair and plastic was not large, our findings indicate that the ubiquitous presence of hair and plastic in the environment may pose a direct threat to bird survival. A comprehensive, multidimensional understanding of the interactions between birds and anthropogenic materials will contribute to a broader assessment of the threats that environmental pollution poses to avian populations.