黑翅鳶(Elanus caeruleus)為棲息於平原開闊環境的日猛禽。由於台灣過去缺乏棲息於平原環境的留棲性日猛禽，在2001年出現首筆繁殖記錄後，黑翅鳶很快佔領此棲位，族群快速增加並在2010年遍佈全台平原地區，可能與類似棲位的物種發生競爭。紅隼(Falco tinnunculus)為來台度冬的平原性猛禽中數量最多的物種，與黑翅鳶有相似的棲位，但過去並無研究探討黑翅鳶出現對紅隼的影響。本研究分三部分，首先整理2003至2017年台灣8座機場的鳥相監測資料、ebird資料庫2000至2019年的目擊記錄，以及2014至2020年台灣新年數鳥嘉年華的資料，瞭解2物種歷年的數量變化。第二部分則從食性及覓食活動時段來探討棲位重疊的程度，並使用Pianka's index進行分析。第三部分以標本測試黑翅鳶與紅隼發現入侵者時的反應。結果發現，無論機場或是ebird資料，度冬紅隼的數量均呈現逐年減少，黑翅鳶則呈現逐年增加的趨勢。新年數鳥則因監測年份有限而無法反應長期的族群趨勢。齧齒類為兩種猛禽最常見的獵物，分別佔黑翅鳶與紅隼獵物生物量的94.3%和61.6%。Pianka's index以頻度計算的結果為0.74 ，以生物量計算的結果為0.97，顯示兩物種在食性上有很高的重疊度。Pianka's index以覓食活動時段計算的結果為0.89，顯示兩物種的覓食時段有很高的重疊度。行為互動實驗發現黑翅鳶對出現於領域內的入侵者皆會攻擊，但在度冬前期(10至12月)對紅隼有最高的攻擊頻度(紅隼2.9次 vs. 黑翅鳶1.1次)，且會隨著紅隼逐漸北返(1至4月)而下降(紅隼1.5次 vs. 黑翅鳶0.8次)；紅隼僅記錄會對紅隼進行驅趕。本研究顯示，黑翅鳶對入侵者的攻擊行為提高平原環境的競爭壓力，在棲位高度重疊的情況下很可能促使紅隼轉而尋找其他地區作為度冬地，最終導致來台度冬的紅隼數量減少。

The black-winged kite (Elanus caeruleus, abbreviated as BWK) is a small-sized open land raptor. Due to the lack of resident diurnal raptors in the plains environment of Taiwan, after the first breeding record in 2001, the BWK quickly took over this niche, with population size increasing rapidly and spreading throughout the lowland regions of Taiwan, possibly competing with species that have a similar niche. The Eurasian kestrel (Falco tinnunculus, abbreviated as EK) is the most abundant of the open land raptor that wintering in Taiwan and has a similar niche to the BWK, but no studies have investigated the impact of the occurrence of BWK on the EK in the past. This study is divided into three parts. Firstly, I collected data from three different sources to evaluate the long-term population trends of these species, including 1) avian fauna monitoring from eight airports in Taiwan; 2) sighting records of BWK and EK from eBird basic database; and 3) the Taiwan New Year Bird Count. The second part examined the degree of niche overlap in terms of diet composition and foraging time and used Pianka's index for analysis. Secondly, I analyzed the degree of niche overlapping using diet composition and foraging time by Pianka's index. Thirdly, I used specimens to examine the response of BWK and EK when facing conspecific and interspecific intruders. Both the airport (2003-2017) and eBird (2000-2019) data showed that the number of wintering EK has declined while the number of BWK has increased year by year. The NYBC (2014-2020) data cannot reflect long-term population trends of EKs due to the relatively limited time scale of monitoring. Rodents accounted for the highest contribution to prey biomass, with 94.3% in BWK and 61.6% in EK, respectively. The diet overlap calculated with Pianka's index was 0.74 by frequency and 0.97 by biomass. The foraging time overlap calculated with Pianka's index was 0.89, indicating that the two raptors have a high degree of overlap in foraging time. BWK attack both EK and BWK specimens during the early wintering period (October to December), with the average frequency of attacks on EKs higher than on BWKs (2.9 vs. 1.1 times); but the frequency decreases as EKs return to breeding ground (January to April) (1.5 vs. 0.8 times). EK was only recorded to attack an EK specimen. This study shows that the attack of the BWKs on invaders increases the competitive pressure, which may reduce the chances of EKs spending the winter in Taiwan, and eventually lead to a reduction in the number of wintering individuals.

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