Distance sampling with vehicle-mounted cameras for the estimation of farmland bird abundance over a wide area.

Farmland bird populations are declining due to farmland abandonment and agricultural intensification. Effective conservation strategies require appropriate monitoring approaches, including efficient and scalable survey methods. In this study, we developed a large-scale monitoring method for herons and egrets (Ardeidae) using vehicle-mounted video cameras and distance sampling models that incorporate location uncertainty. The survey was conducted in and around the evacuation zone of the Fukushima Daiichi nuclear power plant accident. A total of 7,031 km of roadside video footage was recorded, covering 24.41 km{superscript 2} of farmland. Ardeidae were observed only outside the evacuation zone and were entirely absent within it. Predicted abundance differed greatly between areas inside (0.0279 {+/-} 0.0307/km{superscript 2}) and outside (4.57 {+/-} 5.36/km{superscript 2}) the evacuation zone. Incorporating location uncertainty into the distance sampling model had little effect on the estimates (4.57 {+/-} 5.36 vs. 4.51 {+/-} 5.29/km{superscript 2} with and without integrating location uncertainty, respectively). This suggests that our video-based roadside survey method is robust to location uncertainty in structured landscapes such as Japanese rice paddies. The accuracy may be attributed to the study system, where levees and roads divide paddy fields, limiting potential error in observer-target distances Our method can be applied to other open habitats, such as natural grasslands and wetlands, especially in areas lacking artificial markers, by incorporating measurement uncertainty into the model. This combination of roadside surveys with vehicle-mounted cameras and distance sampling provides a practical and transferable approach for monitoring large-bodied species in open landscapes, enhancing both the efficiency and spatial coverage of biodiversity assessments. HighlightsBroad-scale farmland abandonment following the Fukushima Daiichi nuclear power plant accident led to sharp declines in large farmland bird abundance. Efficient large-scale roadside surveys were conducted using vehicle-mounted video cameras. Ardeidae abundance was estimated using distance sampling models that incorporated location uncertainty. The survey method is robust and applicable to open habitats such as farmlands, grasslands, and wetlands.