碩士 / 國立成功大學 / 生命科學系碩博士班 / 101 / 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.
| Identifer | oai:union.ndltd.org:TW/101NCKU5105121 |
| Date | January 2013 |
| Creators | Chih-KaiHuang, 黃郅凱 |
| Contributors | Ya-Fu Lee, 李亞夫 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 110 |
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