Where are Barn Swallow nesting records most abundant in anthropogenic landscapes? Testing the Goldilocks hypothesis along urban–rural environmental gradients

Urban and rural environments are often treated as contrasting habitat types, but they can also be interpreted as the two ends of a continuous land-use gradient shaped by human activities. Along such gradients, organisms may experience trade-offs among different ecological resources. Here we introduce the Goldilocks hypothesis, which predicts that breeding activity is maximized under intermediate environmental conditions between urban and rural environments. To test this hypothesis, we investigated the distribution of Barn Swallow (Hirundo rustica) nests along urban–rural gradients in three regions of Kyushu, Japan (Nagasaki, Fukuoka, and Kagoshima). Environmental gradients were quantified using principal component analysis (PCA) based on land-cover composition within 500 m grid cells. Nest abundance showed a consistent unimodal pattern across all regions, peaking at intermediate environmental conditions. Potential nesting sites increased toward the urban end of the gradient, whereas potential foraging habitats increased toward the rural end, indicating a spatial trade-off between key breeding resources. These findings support the Goldilocks hypothesis and highlight the ecological importance of intermediate landscapes where elements of contrasting human-modified environments overlap, providing insights for landscape management and biodiversity conservation under ongoing urbanization.