Nest site selection is critical for reproductive success in birds. Several bird species including Barn Swallow (Hirundo rustica gutturalis) typically nest near human settlements, presumably because this reduces their risk of predation. Here, we investigated the nesting habits of Barn Swallows in South Korea. We predicted that abandoned houses would not be chosen even if these houses provided similar nesting conditions compared to occupied houses. We conducted surveys across South Korea and recorded human occupancy, house structure, the number of active and old nests, and the deterioration condition of houses. Among the recorded active nests, 95% were located in inhabited houses (n = 38), whereas only 5% were observed in abandoned houses (n = 2). Interestingly, human presence was a significant factor in the occurrence of active nests, whereas house structure had no appreciable effect. Among the nest site characteristics, the number of old nests was significantly higher in inhabited houses than in abandoned houses. Further, ∼97% of inhabited houses were not damaged, whereas 50% of the abandoned houses were damaged, potentially affecting the nesting site selection of Barn Swallows. Our results are consistent with our hypothesis that Barn Swallows prefer to nest in human-inhabited houses. As house structures suitable for nesting are reduced due to rapid urbanization, and as swallow nests are actively eliminated by human residents, the reproductive success of Barn Swallows in South Korea can be significantly reduced.

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