Monitoring the changes of land bird populations in Japan from 2009 to 2013

Abstract

Birds breeding in forests have been monitored by point count methods at 21 sites throughout Japan between 2003 and 2009. We analyzed the population trends of 25 species that were recorded at more than 10 survey sites. Population indices and long-term trends of each species were calculated using TRIM (TRends and Indices for Monitoring data). Bush Warblers Cettia diphone and Siberian Blue Robins Luscinia cyane showed significant declines, while Narcissus Flycatchers Ficedula narcissina showed a significant increase. Twenty-two other species of bird showed no significant changes in populations. Bush Warblers and Siberian Blue Robins use the forest understory for foraging and breeding. Data from other studies have shown that the forest understory has decreased at many sites in Japan due to the impact of browsing by Sika Deer Cervus nippon (Saeki 2013). When the bird monitoring data was evaluated in conjunction with the degree of browsing impacts by deer, the abundance of the two bird species tended to be lower in the sites with heavy deer browsing. These results suggested that populations of Bush Warblers and Siberian Blue Robins decreased in part due to the impact of increased deer browsing. Those bird species were also decreasing in the sites with light deer browsing, suggesting that further studies are needed to understand the full range of mechanisms involved in their decrease.