The use and preference towards different artificial shelters by European tree frog (Hyla arborea) in Škocjanski zatok Nature Reserve

Monitoring amphibians in non-breeding habitats is often constrained by low detectability and potential disturbance to sensitive ecosystems. We tested the effectiveness of artificial shelters - PVC pipes - as a non-invasive method for monitoring the European tree frog (Hyla arborea) outside the breeding season in Škocjanski zatok Nature Reserve (Koper, Slovenia). Over three years (2022–2024), we installed 40- and 50-mm diameter shelters at 24 sites and conducted regular inspections. A total of 338 encounters were recorded, with frogs of all sizes using the shelters. Overall shelter occupancy declined across years, likely reflecting a combination of factors including reduced recruitment, vegetation changes, and demographic fluctuations. Preference analysis revealed a statistically significant, consistent trend: narrower (40-mm) shelters were used 1.6 to 1.7 times more often than wider (50-mm) ones, despite no significant difference in the body size of frogs occupying each type. With this approach, we greatly enhanced the detectability of Hyla arborea in the Škocjanski zatok Nature Reserve compared to classical monitoring methods such as acoustic surveys or netting. Importantly, a non-invasive approach using artificial shelters minimized the risk of disturbance to breeding and migratory birds, which is essential in areas with high avifaunal conservation value. Our results support the broader use of artificial shelters for monitoring arboreal amphibians in ecologically sensitive areas and suggest future research should explore finer-scale shelter design to improve detection and ecological interpretation across life stages.