Present Condition of the Avifau of Mukojima, Ogasawara Archipelago, Following the Eradication of Feral Goats

27 Mar Present Condition of the Avifau of Mukojima, Ogasawara Archipelago, Following the Eradication of Feral Goats

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Abstract

聟島は小笠原諸島の北部に位置する無人島である．この島の植生は第二次大戦後に野生化したヤギ（以下ノヤギ）によって植生の大部分が破壊された．同時に鳥類相も戦後にかけて変化が見られ，戦前生息していた小笠原の固有亜種ハシナガウグイスCettia diphone diphoneと，固有種メグロの亜種ムコジマメグロApalopteron familiare familiareが確認されなくなった．ノヤギが近年すべて駆除されたことから，島の植生は回復しつつある．今回，聟島の陸鳥の鳥類相の現状を明らかにするため，2007年3月から6月の期間，島内の2ヵ所でルートセンサスを行い，さらにルート区間の植生を記録した．結果，外来種メジロが多くの森林で，在来種イソヒヨドリがわずかに開放地で観察された．調査期間中観察され，繁殖が確認されたのはこの2種のみであった．その他の小笠原在来陸鳥のノスリButeo buteo，ヒヨドリHypsipetes amaurotis，トラツグミZoothera dauma，ウグイスCettia diphoneは一時的に少数が観察された．今後これらの鳥類の飛来，定着が聟島の植生が回復するにつれて増加するかもしれない．外来種メジロは小笠原在来鳥類との間で餌資源の競合が生じる一方で，種子散布者，花粉媒介者として島の植生回復に貢献すると考えられる．

English Abstract

Mukojima is an uninhabited island in the northern part of the Ogasawara archipelago. The island’s, vegetation was drastically disturbed by feral goats Capra aegagrus hircus after 1945. The change in the avifau resulting from this habitat disturbance has previously been described. Japanese Bush Warbler Cettia diphone and Bonin Honeyeater Apalopteron familiare became locally extinct. In recent years, the goats have been extermi ted and the vegetation is now recovering. To clarify the current stituaion of the avifau on Mukojima, we counted bird numbers along two routes from March to June, 2007. We also recorded the vegetation along the routes. Many Japanese White-eye Zosterops japonicus (an introduced species) were observed in the forest, and several Blue Rock Thrush Monticola solitarius were observed in open areas. These were the only two species observed throughout our study period; both were confirmed to be breeding. Other tive land birds, Common Buzzard Buteo buteo, Brown-eared Bulbul Hypsipetes amaurotis, White’s Trush Zoothera dauma, Japanese Bush Warbler Cettia diphone were less abundant, with only a few individuals of each species observed for short periods. In the future, such vistors to Mukojima may increase as the vegetation recovers. Japanese White-eye will contribute to the recovery of the vegetation as they are both seed dispersers and polli tors, although they may compete with indigenous species for food.