Infrastructure for Humans, Nesting for Birds: Snow Poles as Unintended Avian Habitats in Hokkaido, Japan

Structures made by humans are shaped by regional culture, history, climate, and social needs. Although such structures are not designed for wildlife, birds often use them as nesting sites, suggesting that human infrastructure can unintentionally influence avian behaviour and population dynamics. This study examined a striking example of this in Arrow poles widely installed in Hokkaido, Japan. Developed as a winter road-safety feature in the 1980s, Arrow poles consist of a hollow horizontal aluminium pipe that is often left open-ended, creating cavities for bird nesting. To our knowledge, this is the first systematic assessment of these structures as avian nesting sites. Focusing on Eurasian Tree Sparrows (Passer montanus) and Russet Sparrows (Passer rutilans), we used Google Street View (February 2024) and field surveys (June 2023) to estimate Arrow pole numbers, nest occupancy rates, and total nest numbers. Of the approximately 281,560 poles estimated across Hokkaido, 5.0% may be occupied by Eurasian Tree Sparrows and 7.67% by Russet Sparrows, corresponding to 2.52% and 8.80% of the total nest numbers for each species, respectively. These estimates were derived from Monte Carlo simulations to incorporate uncertainty. Further analysis using long-term bird monitoring data (1997-2002 and 2016-2020) and statistical comparisons (Mann-Whitney U test and linear regression) suggested that Russet Sparrow populations have increased more in Hokkaido than in mainland Japan, particularly along roads with Arrow poles. These findings highlight how infrastructure developed for human use can unintentionally support wildlife and highlight the potential for integrating biodiversity considerations into infrastructure planning.