台灣中部黑枕藍鶲之鳥巢掠食風險與巢位特徵

Nest-Site Characters Related to Predation Risk for the Black-Naped Monarch (Hypothymis azurea) in West-Central Taiwan

鳥巢天敵是威脅鳥類能否繁殖成功的重要因素之一。本研究我們假設鳥巢覆蓋度、巢樹大小、鳥巢可接近性、巢枝支撐度、巢樹類型、水源及人類活動頻度會影響黑枕藍鶲（Hypothymis azurea）的鳥巢天敵掠食風險。2007-2008年繁殖季，我們於台灣中部雲林縣東側以竹林及次生闊葉林爲優勢植被的丘陵地，尋找黑枕藍鶲鳥巢，隨後監測發現之鳥巢以追蹤各巢最終繁殖命運。繁殖成功（以有1隻或以上幼鳥成功離巢定義之，n=22）及被天敵掠食鳥巢（n=27）間的特徵差異以邏輯迴歸分析檢測。以訊息理論研究法（AICc）進行模式比較的結果顯示巢可接近性及巢樹大小，分別由巢距冠層邊緣最近距離及巢樹高代表，最爲研究資料所支持（累積ωi=0.785）。另由Odd ratios來看，巢距冠層邊緣距離每增加1m，繁殖成功率上升約87%，而巢樹高度每增加1m，則提升約18%。研究結果顯示黑枕藍鶲透過將巢築在不容易接近及（或）搜索的位置來降低天敵掠食的風險。

Nest predation is one of the most sever threats to the breeding success of passerine birds. We examined nest-site characters (variables), such as nest covers, nest inaccessibility, nest supported plants (species, sizes and stem strength), and distances to water sources and human activities, for the black-napped monarch (Hypothymis azurea) in a hill area covered predominantly with cultivated bamboos and secondary broad-leaf trees in the west-central Taiwan. We tracked nesting success, quantified their outcomes, and determined the causes of the nesting failures for two breeding seasons in 2007-2008. We compared nest-site variables between succeed nests (fledged at least one offspring, n=22) and depredated nests (n=27) with binary logistic regression. The results of model comparisons with Akaike's information criterion (AICc) showed that nest inaccessibility and plant sizes expressed, respectively, as distances of the nests from foliage edges and plant heights were found to be the most important variables determining the nesting success (accumulated ωi=0.785). Based on the odds ratios, an increase in 1m each of the distance and the height increased the breeding success of 87% and 18%, respectively. Apparently, the black-naped monarch makes nests at inaccessible sites to avoid predation.