台灣西部沿海是許多水鳥（waterbirds）遷移之中繼站，原為冬候鳥的大白鷺（Great Egrets, Ardea alba Linnaeus, 1758）和中白鷺（Intermediate Egrets, Egretta intermedia（Wagler, 1829）），每年9月至隔年5月會來台度冬，夏季則北返進行繁殖，惟近年在西南沿海發現有繁殖的紀錄，其顯示台灣沿海棲地和濕地環境的重要性漸增。目前台灣並無詳細完整之大白鷺和中白鷺食性資料，因此，本研究在2010年5月至7月，於鷺鷥的繁殖期間，針對台南鹽水溪口濕地和七股鹽田濕地紅樹林中之大白鷺、中白鷺、小白鷺（Little Egrets, Egretta garzetta（Linnaeus, 1766））、夜鷺（Black-crowned Night-Herons, Nycticorax nycticorax（Linnaeus, 1758））及黃頭鷺（Cattle Egrets, Bubulcus ibis（Linnaeus, 1758））進行研究，以期能建立台南地區共域鷺鷥之基礎資料，以提供棲地保育經營者管理上之依據。本研究是以田野採集雛鳥食團（boluses），並將食團內的樣本鑑定分類後紀錄其數量和重量，並以生態指數和統計方法分析月份間和鳥種間的差異。
本研究結果顯示，66隻樣鳥的食團中，共有2179個個體樣本，計有11綱20目45科共80個項目，總重583.95 g。各月份數量最多的都是昆蟲（46.29% - 93.08%），而重量最多的是魚（69.57% - 81.60%）；相對重要性指數（Index of relative importance）數值最高的項目在5月為海鰱科、6月為麗魚科之吳郭魚、7月為虱目魚科；其豐富度指數（Margalef’ s richness index）最高者為6月，歧異度指數（Shannon-Wiener index）和均勻度指數（Pielou’ s evenness index）最低亦出現於6月。大白鷺、小白鷺和夜鷺取食數量最多者為魚類（43.75% - 74.51%），而中白鷺和黃頭鷺則為昆蟲（72.63%，95.39%），黃頭鷺之食物中重量最重者為昆蟲，佔80.53%，其他鷺鷥則是以魚類為主，分別佔85.70%至97.14%；大白鷺、中白鷺和小白鷺之相對重要性指數最高項目皆為虱目魚科，夜鷺是麗魚科之吳郭魚，黃頭鷺則為家蠅科之幼蟲；其豐富度指數最高數值與歧異度指數及均勻度指數最低數值皆出現在黃頭鷺。生態棲位寬度（niche breadth）最大是為大白鷺；最小為夜鷺。生態棲位重疊度（niche overlap）最高為小白鷺和夜鷺（1.36）；而黃頭鷺和其他4種鷺鷥的重疊度皆為最低。夜鷺平均取食物種數與平均取食個體數顯著低於大白鷺，黃頭鷺則顯著高於其他鷺鷥；平均昆蟲數量比例黃頭鷺顯著高於其他鷺鷥，但平均魚類數量比例則最低，其餘4種鷺鷥間無顯著差異。而6、7月平均取食重量則顯著高於5月。黃頭鷺的平均昆蟲重量比例顯著高於其他鷺鷥，但平均魚類重量比例顯著最低，其餘鷺鷥間沒有差異。根據本研究結果可得知，大白鷺、中白鷺、小白鷺和夜鷺的食物皆以魚類貢獻度最高，黃頭鷺食性明顯和其他4種鷺鷥不同，主要的食物資源是昆蟲，而取食魚、蝦等水域性食物的比例幾乎沒有，因此，黃頭鷺雖為鷺科（Ardeidae），但不應被歸類為涉禽（waders）。至於鷺鷥食性和環境因子、覓食行為、競爭關係之交互作用，則有待進一步研究。

SUMMARY

In recent years, Great Egrets (Ardea alba) and Intermediate Egrets (Egretta intermedia) were thought to be winter visitors but found breeding during summer in Taiwan. To protect the feeding habitats of them and other water birds, diet studies during breeding season are necessary. We collected boluses of chicks from Great Egrets, Intermediate Egrets, Little Egrets (Egretta garzetta), Black-crowned Night-Herons (Nycticorax nycticorax), and Cattle Egrets (Bubulcus ibis) from May to July of 2010 in Tainan where located at southwestern Taiwan. We found 2179 individuals in 66 boluses, classified into 11 classes, 20 orders, and 45 families, totally 80 prey items weighing 583.95 g. The most important prey was Chanidae for Great Egrets, Intermediate Egrets, and Little Egrets, but Cichlidae for Black-crowned Night-Herons, Muscidae larvae for Cattle Egrets. Pielou’ s evenness index were lowest in June and Cattle Egrets. The percentage of insects of Cattle Egrets was significantly higher than the others, and the percentage of fish was significantly lower, both by quantity and biomass. Because aquatic food was nearly none, Cattle Egrets should not be waders although they were Ardeidae.

INTRODUCTION

Tainan is located on southwestern Taiwan where on the route of East Asian-Australasian Flyway. There are lots of fish ponds, salt-pans, and wetlands along the coast of Tainan with plenty of fish, shrimp, and benthos that are able to provide abundant food for water birds. Great Egrets and Intermediate Egrets were thought to be winter visitors before. However, recent observation indicates that they breed during summer in Taiwan (Yen, 1992). Diet studies of sympatric herons in Tainan are necessary, since it gets more and more important to protect the feeding habitats of herons and other water birds. Previous studies show that herons are mainly fed on fish, crustaceans, and insects (Kushland & Hancock, 2005). The food samples were collected from stomachs of adults and boluses of nestlings (Miranda & Collazo, 1997; Montesinos et al., 2008).

MATERIALS AND METHODS

To avoid killing birds, we decided to analysis the food samples in boluses that vomited from living chicks. The study site were three communal colonies in mangrove, two located at Yanshui Estuary Wetlands, and the other was at Cigu Salt-pans Wetland. Chicks usually vomited boluses when reseachers approached them. We collected samples in day time once a week from May to July of 2010. Boluses were collected from 66 chicks of Great Egrets, Intermediate Egrets, Little Egrets, Black-crowned Night-Herons, and Cattle Egrets. The Samples that preserved in 95% alcohol were classified and recorded by quantity and biomass. The data are analyzed with occurrence frequency, Margalef’ s richness index, Shannon-Wiener index, Pielou’ s evenness index, index of relative importance (IRI), niche breadth, and niche overlap. We also analyzed the differences between months and species by one-way ANOVA.

RESULTS AND DISCUSSION

From our results, there were 2179 individuals in 66 boluses, classified into 11 classes, 20 orders, and 45 families, totally 80 prey items weighing 583.95 g. The most abundant prey group was insects (46.3% - 93.1%), but fish were dominant by biomass in every month (69.6% - 81.6%). The results of IRI demonstrated that Elopidae, Cichlidae, and Chanidae were the most important prey item in May, June, and July, respectively. Fish were numerically the main prey of Great Egrets, Little Egrets, and Black-crowned Night-Herons, contributing 43.8% - 74.5% of prey items, while insects make up 72.6% and 95.4% of prey eaten by Intermediate Egrets and Cattle Egrets. Insects were also dominant in prey of Cattle Egrets by biomass (80.6%), but other herons were fed mainly on fish (85.7% - 97.1%). According to IRI, the most important prey was Chanidae for Great Egrets, Intermediate Egrets, and Little Egrets, Cichlidae for Black-crowned Night-Herons, and Muscidae larvae for Cattle Egrets. Margalef’ s richness index was highest in June and Cattle Egrets, while Shannon-Wiener index and Pielou’ s evenness index were lowest. Great Egrets had the widest niche breadth, and Black-crowned Night-Herons had the narrowest one. Little Egrets and Black-crowned Night-Herons showed that the highest degree of niche overlap (1.36). Both by quantity and biomass, the proportion of insects of Cattle Egrets was significantly higher than other herons, on the other hand, the proportion of fish was the lowest, no obvious difference between the other four herons.

CONCLUSION

All the results of this research demonstrated that Great Egrets, Intermediate Egrets, Little Egrets, and Black-crowned Night-Herons were fed mainly on fish, but the most important prey of Cattle Egrets was insects, aquatic food such as fish and shrimp were nearly zero. According to the results, I considered that Cattle Egrets should not be waders, although they are a member of Ardeidae. The results concurred with previous studies mostly, but some prey items including shrimp, frogs and snakes, are few or lack, and the major item of insects predated by Cattle Egrets was Diptera instead of Othoptera . It is that herons are opportunistic, they usually fed on food resources which were the most abundant or easiest to get on local habitates. In this research, samples did not collected during unbreeding season, and no data in another year could be compared, these should be further works to know if herons change their diet between seasons or years. In addition, it needs more researches about environment factors, feeding behavior, and competition of herons to help understand the interaction between diet and factors above.

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