研究内容要約
Research
Summary |
近年、多摩川流域の山地帯では、東京都の花粉対策事業などにより、大規模な森林伐採が行われるようになった。伐採は、開放的な環境を創出し、様々な生物に影響が生じると考えられる。本調査研究では、鳥類のヨタカとサシバについて、2015年と2016年に現地調査を実施し、その生息状況と伐採地の関連を明らかにすることを目的とした。
ヨタカについては、伐採地34箇所で生息調査を行ったほか、伐採地と同様に開放的な環境である採石場等、河川敷でも調査を行った。また、伐採地がない森林でも調査を行った。調査の結果、ヨタカが最も高い頻度で生息するのは採石場等であったが、伐採地でも調査地23箇所で生息が確認され、ヨタカの重要な生息場所であることが分かった。さらに伐採地での生息環境を分析した結果、伐採面積が4.3ha以上になるとヨタカが生息する傾向にあることが分かった。
サシバについては、伐採地24箇所、草地5箇所で調査した結果、伐採地4箇所、草地3箇所で生息を確認した。1990年代に都内で行われた調査結果と比較すると、サシバの生息地が回復した可能性がある。サシバの営巣地を2箇所で発見し、雛への給餌物を2つの巣で調査した結果、伐採地で得られるトカゲ類やバッタ類などが含まれることが分かった。しかし、それらの全体の給餌物に占める割合は2巣で違いが見られた。それぞれの営巣地の周辺環境の違いに応じ、伐採地を採餌場所として使用する頻度に違いがあることが考えられた。
Recently, in the mountainous watersheds of the Tama River, large scale deforestation is being conducted by the Tokyo Metropolitan Government for reasons such as pollen-based allergy prevention. As a result, much open space is created and this is seen to have great impact on various living things. In this study, we conducted field studies of the grey nightjar (Caprimulgus indicus) and grey-faced buzzard (Butastur indicus) in 2015 and 2016 to find out the relationship between their inhabitation and deforested areas.
For the grey nightjar, inhabitation studies were conducted at 34 deforested areas as well as at quarries in the river beds, that is open space similar to the deforested area. We also did a study in forests without deforested areas. While we found that grey nightjar inhabited quarries at the highest frequency, we also confirmed their inhabitation at the 23 deforested areas that we studied, suggesting that they are also important habitats for grey nightjar. We further found after analyzing their inhabitation environment in the deforested areas that they tend to inhabit the areas when the deforested area exceeded 4.3ha.
For the grey-faced buzzard, we studied 24 deforested areas and 5 areas of open grasslands, and confirmed their inhabitation at 4 and 3 areas, respectively. When compared to the study results conducted in Tokyo metropolitan areas in the 1990s, this may imply that their habitats are being restored. We found their two nesting grounds, and found lizards and grass hoppers in two nests to feed the baby birds. These prey items are the ones commonly found in deforested areas. The ratio of these prey items to all feed found in the nests, however, differed among the two nests, which suggests that depending on the surrounding environment, there are differences in the frequency of using the deforested areas as their feeding spots.
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