Thesis Abstract and Dissertation Abstracts
Population Ecology of the Red-footed Booby on the Xisha Archipelago
Abstract Category: Science
Course / Degree: PhD
Institution / University: Lanzhou University, China
Published in: 2005
The Red-footed Booby, the smallest of the Sulidae, has a pan-tropical distribution and breeds on coral and volcanic islands where it nests in trees and breeds annually. The species is pelagic, feeding on flying fish and squid. The sub-species occurring on the Xisha Archipelago is Sula sula rubripes. Collection of information on the worldwide distribution and numbers of the Red-footed Booby started in the early 1900s and on breeding ecology in the 1960s. Recently, the availability of miniature equipment has enabled study of foraging behaviour to be carried out.
Compared to elsewhere in the world, little is known about the ecology of Red-footed Boobies on the Xisha Archipelago. This study of Red-footed Boobies in the Archipelago was carried out from 2003 to 2005. The thesis is in five parts covering breeding distribution and population size, morphology, breeding ecology, breeding and foraging behaviour, and threats.
Xisha Archipelago, consisting of 34 coral islands and reefs located in the northern South China Sea, has a total land area of 7.97 km2. The Archipelago is an important breeding area for sea birds. The main study site was on Dong Island, which has a total area of 1.55 km2.
Random sampling was used to estimate the breeding population size, which was determined to be about 35,500 pairs. This is approximately 10% of the world population. Compared to 50 years earlier, the breeding distribution of the Red-footed Booby within the Archipelago has decreased greatly and probably numbers have declined also. More than half the Pisonia trees favoured by boobies for nesting are unused. Thus, it seems that food may be the main factor limiting numbers.
The morphological study, which included taking many photographs, provided information on morphs, variation in bare part colours and the plumages of adult and immature birds. Detailed measurements of adults and juveniles enabled comparisons to be made with colonies elsewhere in the world. These measurements show that Red-footed Boobies occur in two size groups, with Dong Island birds being in the smaller one.
The study of breeding ecology involved investigating breeding habitats, nest density, chick growth pattern and rate, breeding success, breeding regime (e.g. clutch size, timing of breeding, breeding cycle duration), food supplied to chicks and feeding frequency. The clutch size is one and the egg-laying period is from September to early-April, with the peak period being November and December. There was little variation over the three years of the study. The average fledging period is 111 days, with a further 100-150+ days required to reach independence. Assuming a incubation period of 45 days, the length of the breeding cycle is from 9 to at least 11.5 months. Compared to other colonies, the chick growth rate is slow, juveniles are lighter when they fledge and take longer to fledge. This result is consistent with the food delivered to the chicks per day, which is less than elsewhere. This indicates that the breeding regime of the Red-footed Booby and chick growth rate are all controlled by food supply and also implies that the quantity of food in the South China Sea is probably poorer than elsewhere. However, the breeding success is similar to other islands showing that Dong Island boobies have adapted to this situation.
Breeding behaviour compared to other colonies is generally similar, but the timing and duration of breeding is often different to elsewhere. Many photographs were taken of courtship, including changes in bare part colour, juvenile feeding behaviour, and the behaviour of fledged birds. The pelagic foraging habitat is patchy and foraging success rate is low. As boobies do not forage at night, they use more than 80% of the daylight period foraging for themselves and their chick.
The main threat to the colony is a herd of approximately 100 introduced cows which preferentially eat Pisonia tress, especially new growth. The result in time will be the destruction of the forest that the boobies use for nesting. Frigatebirds are a minor threat due to small numbers. The plan to develop the Xisha Archipelago for tourism poses a serious future threat.
More information on food supply and reliability, and how the birds find food, should provide answers to important questions arising from this study, e.g. what determines the colony size, the length of the breeding cycle, breeding success and the timing of peak egg-laying. Future research should focus on gaining a detailed understanding of the foraging strategy of Red-footed Boobies in the northern South China Sea, which is probably the key to understanding how Dong Island boobies have successfully adapted to their environment. Another important area for future study is definition of the subspecies within the Pacific using biometric data and genetic analysis.
Thesis Keywords/Search Tags:
Red-footed Booby, Breeding Biology, Breeding Ecology, Status, Foraging Ecology
This Thesis Abstract may be cited as follows:
Cao, L. Population Ecology of the Red-footed Booby on the Xisha Archipelago. PhD Thesis. Lanzhou University, Gansu Province, China
Submission Details: Thesis Abstract submitted by Lei Cao from China on 20-Jan-2006 02:06.
Abstract has been viewed 2907 times (since 7 Mar 2010).
Lei Cao Contact Details: Email: caolei@ustc.edu.cn
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