Population Declines of the Oriental Magpie Pica serica (Corvidae) in the Saga Plain, Japan: Relationships With Land Use Patterns, Nesting Sites, and Breeding Success

Urban areas are hotspots of biological invasions where land use and other factors are associated with the success of non-native species. Non-native species alter their habits during the process of adapting to new environments, making them important subjects for studying organismal environmental adaptation. The Oriental Magpie Pica serica was introduced and established a substantial population in the Saga Plain, Japan, from the Korean Peninsula, probably by the early 17th century. As an adaptation to urbanization, this species has gradually begun to prefer to nest on utility poles rather than trees since around the late 1970s, but has declined its population since the 1990s. In this study, we hypothesized that the recent magpie decline is related to the changes in land use patterns and that magpie density affects its breeding success. We investigated the nesting density of magpies in the Saga Plain and compared the density with previous surveys. In addition, we surveyed the breeding success in areas with low and high magpie densities. As a result, the magpie densities rapidly decreased from 2008 to 2019, whereas nesting rates on artificial structures increased significantly. In our analysis, building and farmland areas themselves had a positive effect on magpie density. Over the recent decade, the building area has increased, whereas the farmland has decreased in our census sites. Then the negative impacts of the farmland reduction were considered larger than the positive impacts of the increase in building areas. These suggest that urban developments critically affect the magpie decline. Breeding success rates were significantly higher in areas with high magpie density than in low-density areas. In low-density areas, success rates from egg hatch to fledging are significantly lower than in high-density areas, suggesting that low-density trends of magpies in most areas facilitate breeding failure and further negative impacts on the populations.