2023 Volume 19 Pages A21-A30
I used records from the second (1997 to 2002) and third (2016 to 2020) Breeding Bird Surveys in Japan to characterize the population decline of Eurasian Tree Sparrow Passer montanus observed between the two surveys, including what type of environment exhibited the decline. The study sites with significant sparrow declines had characteristically large agricultural areas and high temperatures. The decline of the species could not be explained by changes in land use. There are three reasons postulated to be responsible for the large population declines of the species at these study sites: 1. because the initial population of the species was high in the suitable habitat with large agricultural areas, the number of birds represented by the declines was high even if the rate of decline was the same as other type of environment; 2. due to the decline of insects, the food availability of the species has decreased without land use change; 3. the nesting sites of the species have decreased without land use change because rebuilt buildings have no gaps for nesting. Taking into account the habitat of the species, I estimated the extent of the population decline across Japan. The population of the species was estimated to have declined by 62.1% in the 18 years between the second and third surveys. This trend corresponds to the decline of the Eurasian Tree Sparrow population by half every 26 years or so.
