Environmental Characteristics of Bridges Selected by Nesting Formosan Whistling Thrush (Myiophoneus insularis)

Abstract

台灣紫嘯鶫(Myiophoneus insularis)會利用橋面下的橋墩縫隙築巢，我們研究橋樑環境特質對本種巢位選擇的影響。民國86年5至10月間調查台北縣郊區的三峽、石門、萬里、深坑、石嘉定、平溪、坪林等鄉鎮山區的橋樑，記錄各橋樑的環境特質以及被紫嘯鶫利用的狀況。所調查的68座橋樑中，有紫嘯鶫築巢的有31座，橋齡少於五年者未有被使用之紀錄。以卡方分析(Chi-square test)檢驗各項環境因子的資料，顯示在巢位高度、橋長、平均坡度方面，其選擇有顯著差異(P<0.05)，紫嘯鶫傾向於在橋墩縫隙距離地面6公尺以下、橋長20公尺以下、四周平均坡度大於70度的橋樑築巢。以橋長、巢位高度、平均坡度三個因子進行複邏輯迴歸分析，所得的預測模式正確率為78%(n=41) ，但三因子中僅平均坡度具顯著預測力。人對橋樑周圍環境的利用會造成坡度及植被的改變，可能因此影響本種對橋樑的選擇。此外，人們在溪邊的遊憩活動也可能影響其對橋樑的選擇，但無顯著差異。建議未來建橋時，可根據本種對橋樑特質的偏好施工，以利於本種生存。

The Formosan Whistling Thrush (Myiophoneus insularis) is known to use the interstice between the bridge and abutment as a nesting site in the breeding season. We studied the characteristics of 68 bridges in the mountainous area around Taipei County from May to October in 1997. Thirty-one were used by the species. Bridges less than 5 years old (n=4) were not used for breeding. There were significant differences in nest-site height, bridge length, and average slope between used and not-used bridges (X2test, P<0.05). The Formosan Whistling Thrush tended to nest on bridges with abutments < 6 meters, bridge length < 20 meters, and average slope of surrounding terrain> 70°. Analyzing these three factors with multiple logistic regression resulted in a model with a correct prediction rate of 78% (n=41); but only the average slope was a significant factor. Land use type and vegetation type resulting from human settlement were strongly correlated with slope around the bridge, possibly explaining the role of slope in bridge selection. Human use of the stream habitat for recreation may also affect the choice of the bridge, however, there were no significant differences. The characteristics of the bridges selected by this species are recommended for use in future bridge construction.