<b>Barn swallows (</b><b><i>Hirundo rustica</i></b><b>) prefer nesting in inhabited buildings: A field test of the human proximity hypothesis</b>

Study Site 1 was located in Danzhou City, Hainan Province (19°11′–19°52′ N, 108°56′–109°46′ E), in northwestern Hainan Island, China. The site has a tropical humid monsoon climate, with a mean annual temperature of 22–23°C and annual precipitation of 900–2,200 mm (Yan and Liang 2024). Data were collected during the barn swallow breeding seasons of 2014 and 2023–2024. Study Site 2 was located in Leishan County, Qiandongnan Miao and Dong Autonomous Prefecture, southwestern Guizhou Province (26°02′–26°34′N, 107°55′–108°22′E), China. The site has a mid-subtropical humid monsoon climate, with a mean annual temperature of 14–15°C and annual precipitation of 1,310 mm (Zhang et al. 2023). Data were collected during the barn swallow breeding season of 2023. Study Site 3 was located in Qiqihar City, Heilongjiang Province (46°13′–48°56′N, 122°24′–126°41′E), 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). Data were collected during the barn swallow breeding season of 2024 (Figure 1). Study species The barn swallow belongs to the family Hirundinidae, order Passeriformes (Zheng 2023). As a synanthropic species, barn swallows historically may have preferred to nest in natural cavities such as cave entrances or rock crevices, constructing open cup-shaped nests (Speich et al. 1986). Over time, they have increasingly shifted toward nesting in anthropogenic structures near human habitation, including houses, barns, and bridges (Teglhøj 2018; Kim et al. 2023). At the Hainan and Guizhou study sites, barn swallows typically nest inside or outside of occupied residential buildings (Liang et al. 2013). At the Heilongjiang site, they more commonly nest in livestock enclosures (e.g., cattle, horse, sheep, and chicken pens) and storage buildings, consistent with findings reported by Ambrosini et al. (2002) and Han et al. (2026). Field data collectionDuring the 2014 survey in Hainan Province, we selected 10 county-level municipalities across the eastern, western, southern, northern, and central regions of Hainan Island, taking into account the island’s topographic diversity, and sampled coastal villages, lowland foothill villages, and montane forest villages as survey units. The study areas comprised predominantly older, established village settlements, some of which had been partially vacated as residents relocated to newly built housing, leaving former dwellings unoccupied. In some cases, entire villages had relocated to new sites nearby, leaving large numbers of abandoned buildings; in other cases, settlement density remained high. Therefore, our survey sites included a representative distribution of both inhabited and uninhabited buildings. At each village, we conducted systematic counts of inhabited and uninhabited buildings and recorded the number of barn swallow nests in each building type (recorded as zero where none were found), also noting whether each nest was located inside or outside of the building. In total, we surveyed 69 villages, encompassing 4,613 buildings, and documented 1,288 barn swallow nests. Between 2023 and 2024, we conducted detailed nest-site surveys in Danzhou (Hainan), Leishan (Guizhou), and Qiqihar (Heilongjiang), focusing on active nests with confirmed breeding activity. Upon locating an active nest, we recorded a series of nest-site parameters, including nest height (the vertical distance from the nest floor to the ground), building age (categorized as “new” if ≤ 5 years old and “old” if > 5 years old, as reported by the building’s occupants), occupancy status, and nest position (indoor versus outdoor). Livestock enclosures visited regularly by people for animal husbandry were classified as “inhabited.” In total, we recorded detailed nest-site data for 942 nests: 373 in Hainan, 360 in Guizhou, and 209 in Heilongjiang. Nest-site selection is critical to avian reproductive success. Although human activity is known to influence nest-site choice in birds, whether humans facilitate or impede avian reproduction remains a matter of debate. The human proximity hypothesis proposes that birds breeding near human habitation experience reduced rates of nest predation and brood parasitism. To test this hypothesis, we investigated nest-site selection in barn swallows (Hirundo rustica) across three provinces in China: Hainan, Guizhou, and Heilongjiang. Occupancy rates of the breeding swallow nests were 34.6% in inhabited buildings but fell sharply to 0.9% in uninhabited buildings. Among all active nests recorded, 98.1% were located in inhabited buildings, and swallows showed a strong preference for older buildings (91.9%). Outdoor nests (75.1%) significantly outnumbered indoor nests (24.9%), and outdoor nests were placed at significantly greater heights than indoor nests. Our findings support the human proximity hypothesis, indicating that barn swallows maintain close associations with humans and use human presence as an effective barrier against nest predators and brood parasites.