棲位變異假說（niche variation hypothesis）解釋為何棲位變異是產生與維持生物多樣性的機制之一；此假說預測擁有較寬棲位的族群，與此棲位相關的形值變異量應該較大，而擁有較窄棲位的族群，與此棲位相關的形值變異量應該較小。雀形目鳥類生物多樣性高，適合用來研究棲位變異。鳥類的形態（體型大小、嘴喙形值等）在許多研究中皆已被證實與其食性有高度相關性。本研究與國立東華大學許育誠研究室合作，於2009-2017年在臺灣東部沿不同海拔（28公尺至2668公尺）的10個樣點以霧網捕捉三種常見雀形目留鳥（粉紅鸚嘴Sinosuthora webbiana、山紅頭Cyanoderma ruficeps、綠繡眼Zosterops japonicus） 做為研究物種。研究方式為測量各鳥種之個體形值（嘴喙長、寬、深，以及全頭長、跗蹠長與最大翼長）並採集其胸部絨羽分析穩定碳氮同位素值（δ13C、δ15N），以分別量化族群形值變異量與食性棲位寬度。本研究的假說為：（1）各物種個體形值與同位素食性棲位間具相關性，亦即確認本研究選用之形值是與同位素食性棲位利用有關的形值；（2）各物種擁有較寬食性棲位的族群，形值變異量會較大，反之擁有較窄食性棲位的族群，形值變異量會較小，亦即檢測棲位變異假說是否在這三個物種上得到支持。結果顯示，山紅頭與粉紅鸚嘴個體的δ13C值受形值影響，顯示本研究所選用的嘴喙與體型形值可能與這兩個物種食物碳源利用有關；而綠繡眼個體的δ13C值以及三物種的δ15N值，與形值並沒有顯著或一致性的關係。山紅頭與粉紅鸚嘴族群的食性棲位寬度與嘴喙形值變異量有顯著正相關，而其他形值則沒有相關性，另外山紅頭在各體δ13C食性棲位分析與棲位變異假說驗證的形值並沒有一致性，因此並不完全的支持棲位變異假說的預測；而綠繡眼族群的食性棲位寬度與各形值變異量皆無相關性，不符合棲位變異假說的預測。在本研究中棲位變異假說得到了部分的支持，這三個鳥種的形值演化可能受食性棲位影響的程度不同。未來研究可以嘗試以多維棲位寬度，並包括形值以外的其他性狀變異（例如覓食行為變異），對棲位變異假說進行更完整的驗證，如此應能對棲位變異與生物多樣性的維持有更深入的瞭解。

The niche variation hypothesis (NVH) is proposed as one of the mechanisms that could explain the generation and maintenance of biodiversity. The NVH predicts that populations with wider ecological niches should exhibit more variation in the morphologies that are related to niche use. Passerines are an ideal group of bird species to study niche variation, given the high level of biodiversity they exhibit. Previous studies have shown that many aspects of bird morphology (e.g. body size, bill morphology) are correlated with their diets. In collaboration with Dr. Yu-Cheng Hsu at National Dong-Hwa University, we captured passerines across ten sites in eastern Taiwan along an elevational gradient (28m - 2668 m) from year 2009 to 2017, which allowed me to test the NVH on three common resident species (Sinosuthora webbiana, Cyanoderma ruficeps, Zosterops japonicus). We measured the length, width and depth of bill, as well as head length, tarsus length and wing length of each captured bird to quantify individual morphology and population-level morphological variation. We collected feather samples from their chest region to analyze their stable carbon and nitrogen isotopic compositions (δ13C, δ15N), which can be used to quantify individual trophic positions and population-level trophic niche width. I hypothesize that, for each of the three species: (1) Individual morphological values should correlate with their trophic positions, which helps confirm that the morphologies used in this study are related to isotope trophic niche; (2) Population niche width should correlate positively with morphological variation, which is a direct test of the NVH. The results showed that individuals’ δ13C values for C. ruficeps and S. webbiana can be explained by their morphological values, suggesting that the morphologies used in this study could be related to the use of dietary carbon sources. However, individuals’ δ13C values for Z. japonicus, as well as individuals’ δ15N values for all three species, were not related to the morphologies. There was a positive correlation between trophic niche width and bill-size variation in C. ruficeps and between trophic niche width and bill-shape variation in S. webbiana. However, the other morphologies examined for these two species were not correlated with their trophic niche width. In addition, bill size variance correlated with niche width but, individuals’ δ13C values cannot explained by bill size and these two analysis are not consistent, so the NVH was partially supported in C. ruficeps. Furthermore, there was no correlation between trophic niche width and morphological variation in Z. japonicas. In summary, the NVH was partially supported in this study. The role of trophic niche variation in shaping morphological evolution might be different in different species. In the future, multi-dimensional niche, as well as phenotypic values beyond morphology (e.g. behavior) can be incorporated to test the NVH, which should provide more insights into the role of niche variation in shaping biodiversity.

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