鷺科 (Ardeidae) 鳥類普遍分佈於台灣西南部沿岸區域。 其中，鷺亞科 (Ardeinae) 類群，例如此區域常見的小白鷺 (Egretta garzetta (Linnaeus, 1766); Little Egrets)、中白鷺 (Egretta intermedia (Wagler, 1829); Intermediate Egrets)、大白鷺 (Ardea alba Linnaeus, 1758; Great Egrets)、蒼鷺 (Ardea cinerea Linnaeus, 1758; Grey Herons)、黃頭鷺 (Bubulcus ibis (Linnaeus, 1758); Cattle Egrets)、及夜鷺 (Nycticorax nycticorax (Linnaeus, 1758); Black-crowned Night-Herons) 等，常會形成覓食混群 (mixed-species flocks)，且能利用不同型態棲地覓食，甚至包括人為塑造或干預的環境，如魚塭等。因此，除自然棲地的消長外，人造環境的型態與其變化亦可能造成這些鷺鳥在數量多寡、分佈範圍、及覓食行為互動模式上的變動。 例如冬季時，魚塭陸續收成且候鳥聚集，此時魚塭的狀態變異較大，可能使鷺鳥出現不同的利用方式。
本研究主旨為探討鷺鳥在台灣西南部沿岸區域的棲地利用模式及混群覓食行為。 樣地位於台南市鹽水溪至七股溪間的沿岸區域，南北長約14 km，東西寬約10 km。 全區共劃設成3398格200 × 200 m樣方，並在每格中區分魚塭、廢棄水池、深水域、淺灘、紅樹林、一般林地、開闊陸域、與人工棲地等8種棲地類型。 本研究於2010-2011年間冬季 (11月至2月)，以分層隨機取樣 (stratified random sampling) 挑選樣方，並在鳥類活動最活躍的日出後3小時內與日落前3小時內，共調查35.0% 樣地面積中的棲地比例與個體數量以了解上述6種鷺鳥的棲地偏好與分佈。 針對鷺鷥皆會利用水陸域棲地的特性，我測試假說 (1) 覓食鷺鷥的數量會受棲地多樣性影響，且預期在棲地多樣性越高的樣方中有較多覓食的鷺鷥個體。 鷺鳥在台灣沿海常利用紅樹林繁殖或棲息，且覓食個體數量會與群聚處距離成反比，因此我測試假說 (2) 覓食鷺鷥的數量會受紅樹林的出現影響，並預期有紅樹林出現的樣方中有較多的覓食鷺鷥個體。
結果顯示鷺鷥的棲地利用具有偏好。 8種棲地中，魚塭為覓食鷺鳥最偏好的類型，而鷺鷥個體密度在深水魚塭高於放乾魚塭，但與棲地面積無關。 鷺鳥覓食或非覓食時皆偏好廢棄水池，尤其為大白鷺及蒼鷺的非覓食個體最偏好之棲地類型，而整體非覓食個體最偏好的棲地則為面積僅占1.1% 的紅樹林。 紅樹林亦為此區域內主要的鷺鷥營巢棲地，且紅樹林的出現會增加在其附近覓食鷺鷥的數量。 棲地多樣性並不影響覓食鷺鷥的數量而不支持所提假說。 以選擇指數而言，面積僅佔 5.0% 的廢棄水池與佔 42.3% 的魚塭對覓食鷺鷥而言具有相同的重要性。
為了解混群的形成、驅趕行為與進食率 (fish intake rate，魚/分鐘) 之間的關聯、及群體大小與周遭物種對鷺鷥進食率的促進，本研究接續在2011年6月至2012年7月於鹽水溪沿岸三處廢棄水池，此最常有小白鷺與中白鷺穩定混群覓食的棲地類型中，記錄日出時段個體進出、降落位置與不同群狀態下的啄食率 (strike rate，啄食次數/分鐘)、成功率、及進食率，並測試假說： (1) 鷺鳥降落在覓食混群時，會依據群中的物種選擇降落位置，並預期鷺鳥降落時會較靠近同種； (2) 第一隻覓食個體在新個體加入時是否表現驅趕行為會受第一隻覓食個體單獨覓食時的行為表現影響，預期出現驅趕行為的個體其覓食表現較好； 及 (3) 鷺鳥的覓食表現會受最近個體的物種影響，預期中白鷺與小白鷺皆會在靠近小白鷺時有較好的覓食表現。
結果顯示，個體降落時雖有靠近同種的趨勢，但缺乏顯著統計上支持兩物種的降落是非隨機的，因此不支持所提假說。 鷺鳥群是否形成與驅趕行為的出現有關，但第一隻覓食個體的覓食表現與其驅趕行為的出現與否沒有關聯。 當個體有驅趕行為時，後續個體的降落頻度在群成功形成或失敗間並無差別，因此降落頻度並不是群形成的主因。 兩物種相較而言，小白鷺具有較高的啄食率而中白鷺具有較高的成功率。 群的大小只與中白鷺的啄食率及進食率呈正相關，且差異僅出現在單獨覓食個體與較大群體 (11-30隻及 > 30隻兩組) 個體之間。 當靠近另一小白鷺時，小白鷺能經提高成功率而中白鷺則經提高啄食率而增加進食率。 本研究結果雖無法直接證實造成啄食率、成功率、及進食率差異的原因與機制，但由上述結果可知，鷺鳥若在覓食群與小白鷺出現的空間位置覓食可具有較高的進食率。
本研究提供了實際數據顯示鷺科鳥類在台灣西南部沿海區域為機會性棲地利用模式且當中具有棲地偏好。 其中，環境變動較小且無人為經營活動的廢棄水池則為鷺鷥活動的重要棲地，在此環境下，證據顯示群中小白鷺的存在能增進其他鷺鷥個體成功覓食，是影響鷺鳥進食率的主要因子。 因此小白鷺的出現可能在水鳥覓食活動上扮演重要的角色，至少可作為鷺鷥群覓食狀況的一個直接指標。 鄰近地區的溼地規劃上可連結本研究棲地選擇與覓食行為的結果，針對廢棄池子加以保留可增加當地物種多樣性，且若能吸引覓食群，尤其是小白鷺的加入時，則代表鷺鳥在此棲地中能具有較高的進食率，將有利於不同鷺鷥物種族群在當地的存續。

Herons (Ardeidae) are commonly distributed in the southwestern coastal Taiwan. Within Ardeidae, the common members of the subfamily Ardeinae in this area include Little Egrets Egretta garzetta (Linnaeus, 1766), Intermediate Egrets Egretta intermedia (Wagler, 1829), Great Egrets Ardea alba Linnaeus, 1758, Grey Herons Ardea cinerea Linnaeus, 1758, Cattle Egrets Bubulcus ibis (Linnaeus, 1758), and Black-crowned Night-Herons Nycticorax nycticorax (Linnaeus, 1758), which often form mixed-species foraging flocks and are able to use different habitats, even environments modified by humans. Therefore, the abundance, distribution, and interactions of foraging herons are affected by the succession of natural habitats, as well as by changes in the types of human-modified habitats. For instance, certain fishponds in the study area were harvested and migrants congregated in the winter, and thus, the variations in the fishpond status may change the use pattern of the herons.
This study investigated the habitat use patterns and foraging behaviors of mixed-species heron flocks in the southwestern coastal Taiwan. The study area was located between Yanshuei River and Cigu River in Tainan City, and covered approximately 13,592 ha. I set 3398 grids sized 200 × 200 m and distinguished the habitats into 8 categories: fishponds, unused ponds, deep-water areas, floodplains, mangroves, woodlots, open areas, and man-modified areas. I sampled by stratified random sampling and surveyed 35.0% of the total area within 3 h after sunrise and within 3 h before sunset between November 2010 and February 2011 to investigate the habitat preferences and distributions of the 6 mentioned herons. According to the character of using both aquatic and terrestrial habitats of herons, I tested the hypothesis that habitat diversity affects the abundance of foraging herons, and predicted that higher foraging abundance occurs in habitats with greater diversity. Herons are able to use mangrove as breeding and roosting sites in coastal Taiwan, and the abundance of foraging herons are negative correlated with the distance from colonies. Therefore, I tested the hypothesis that the appearance of mangroves affects the abundance of foraging herons, and predicted that it is higher in areas populated with mangroves than those without.
Herons showed preferences in their habitat uses. Fishponds were the most preferred habitat type of herons, and deep fishponds had a greater abundance of birds compared with drained fishponds, but the bird density showed no correlation to fishpond areas. Unused ponds were the only habitat type preferred by both foraging and non-foraging herons, especially by the non-foraging Great Egrets and Grey Herons. Mangroves were the most preferred habitat by resting individuals and were also the major habitat of heronries, and mangroves influence the abundance of foraging herons in nearby areas. I found no relationships between the abundance of foraging herons and habitat diversity, thus did not support the hypothesis. According to the selection index results, the unused ponds, with only 5.0% of the total area, were equally important as fishponds, which comprised 42.3% of the total area.
This study further examined the mixed-species foraging flocks of herons, the relationships between expelling behaviors and fish intake rates (fish/minute), and the effects of group sizes and neighbors on fish intake rates. Between June 2011 and July 2012, I investigated the formation of foraging flocks and the foraging performances of herons, including the strike rate (attacks/minute), the success rate, and the intake rate, by focal sampling Little Egrets and Intermediate Egrets in 3 unused ponds along Yanshuei River, Tainan. I tested the hypothesis that the present species in a flock may affect the landing position of newly coming herons, and predicted herons may tend to land closer to conspecifics. I also tested the hypothesis that the foraging performances of existent herons in a flock may affect the presence and degree of expelling behaviors when other herons join, and predicted individuals showing expelling behaviors to have higher food intake rates compared to those showing no expelling behaviors. For flocking Little Egrets and Intermediated Egrets, I tested the hypothesis that the neighboring species affects the foraging performance of herons, and predicted both species to have improved foraging performances when foraging closer to Little Egrets.
I found no evidence for non-random landing and flock joining in both species, although they tended to slightly land closer to conspecifics. The expelling behaviors of the first heron affected flock formation, although I found no relationships between the foraging performances of existent herons and the occurrence of expelling behaviors. Landing frequency did not affect flock formation when the first heron demonstrated expelling behaviors. For solitary herons, Little Egrets had higher strike rates, whereas Intermediate Egrets had higher success rates. Group size effects occurred mainly in Intermediate Egrets, which showed higher strike and intake rates in group sizes of 11-30 and over compared to solitary birds. When either species foraged closer to another Little Egret, the success rates increased in Little Egrets, whereas the strike rates increased in Intermediate Egrets, ultimately resulting in increased intake rates in both species. Although this study did not directly prove the reasons and mechanisms that triggered the differences in the strike rate, success rate, and intake rate, the results indicated that herons could have a higher intake rate in the flocking location of Little Egrets.
This study provided empirical evidence that sheds light on the opportunistic habitat use and habitat preference of herons in the southwestern Taiwan. Unused ponds were subject to minor environmental changes and human activities were important habitats for herons. In unused ponds, Little Egrets may play an important role in the foraging activities of water birds, or at least they can be considered a direct indicator of the foraging conditions of heron flocks. Wetland planning in nearby areas can apply the results of habitat selection and foraging behavior provided in this study, which indicated that maintaining unused ponds could increase local biodiversity. A habitat can attract specifically the foraging flocks of Little Egrets, indicating that herons can forage with a higher intake rate in such a habitat, which helps the existence of heron populations.

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