The ecology of the Red-vented Bulbul in Fiji.
This research investigated the ecology of the Red-vented Bulbul, an avian introduction to Fiji, and its role as an agricultural pest. The work is also of comparative interest as a study, with intensive sampling, of a tropical passerine. In Chapter One, a background description of the natural history of Fiji is provided. The probable manner of the accidental introduction of the Bulbul into Fiji is discussed and its present status and distribution in the Pacific is described. Chapter two describes the methods used to quantify protein and lipid condition. The total lipid content was obtained by extraction in a soxhlet apparatus and a protein index was derived from the lean dry weight of the wing muscle block. The latter contained four flight muscles; the Pectoralis Major, the Subcoracoideus, the Supracoracoideus and the Corobranch- 1 i "'�ei:1 ialis, together with the Sternum and Coracoid bone. It is Sft6Wll that the c,/oeS ...,.,{ bone component of the wing muscle block is 1cJ:Rlikely t0 undergo weight +I.us changes and~the weight can be accurately predicted by a size measurement. It was necessary to correct the index of protein condition to allow for variation in the body size of collected individuals because there is a close correlation between body size and the lean dry weight of the wing muscle block. No correction was needed for the index of lipid condition since the lipid content and body size are unrelated. The indices of protein and lipid condition are not equivalent to metabolizable reserves although directly related to them. Chapter Three describes the breeding biology. There is a distinct breeding season which occurs during the rains. The Bulbul has a mean clutch size of 2. 5. There is a high rate of egg and nestling loss: 72% of eggs do not hatch and 53% of nestlings do not survive. Bulbuls have an extended period of parental care of fledglings and are unlikely to raise more than one brood in a season. Fledgling survival appears to be good and the annual recruitment rate is probably about 30%, although in one year it was considerably less. The moult of the Bulbul in Fiji is described in Chapter Four and the timing and duration of the moult in all native and naturalized populations throughout the range of the species are discussed. The Bulbul has a fixed moulting season everywhere and there is marked synchrony within all populations . 'Ihe individual moult probably takes 100-120 days. In India, t he moulting season generally occurs in the rainy season but in the Bulbul's naturalized range in the Pacific, the association with rainfall is not so clearly defined. In all localities, irrespective of climate, the moult commences soon after the summer solstice with the moulting season of t he populations in the southern hemisphere being displaced six months from that of the northern hemisphere birds. '!his provides circumstantial evidence that photoperiod i s a proximate factor initiating moult in this tropical species. Breeding and moulting are mutually incompatible in the Bulbul although late breeding females do commence their moult while feeding fledglings. In Chapter Five the dispersion and movements of Bulbuls are discussed. Like most pycnonotids, the Bulbul is a sedentary species or a short distance wanderer. It does not form large ,cohesive flocks and such population movements as do occur, are carried out by small groups or individuals ranging in search of food. In the agricultural areas of the Sigatoka Valley, Bulbul numbers built up during the vegetable growing season, but not in direct response to agricultural foods. The density of Bulbuls at tjle beginning of the breeding season exceeded 150 birds/sq. Km-2 and when peak numbers were recorded the density exceeded 350 birds/sq. Km- 2� The use of individual crops and non-agricultural habitats is described . The feeding ecology of the Bulbul, which was studied by stomach content analysis supplemented by standardised field observations , is the subject of Chapter Six. Although insect and plant material are taken at similar frequencies., plant material constitutes about 75% by volume of the stomach contents. In the wet season , when Bulbuls are breeding and subsequently moulting, insects assume greater importance in the diet. Three invasive weeds, which are available the year round, form the bulk of the plant food. It appears that the extent to which two of these plants are eaten is determined by the energy requirements of foraging rather than their availability per se . Lepidopterous larvae are the most important insect food. The diets of birds from separate conununal roosts differ, but only in food items with patchy distributions. Insects form a more important part in the diet of nestlings and fledglings than in that of adults, and in the dry season, more juveniles feed on insects than do adults who turn predominantly to plant material. Changes in body condition are described in Chapter Seven. Bulbuls undergo diurnal (4%) and seasonal (a. 14%) fluctuations in fresh weight. The protein condition of adults is best in the two months prior to the start of the breeding season. Females lose protein condition while laying eggs, but the protein condition of males starts dropping before the onset of breeding. The protein condition of both sexes remains constant and at a low level during the moult, but it improves soon after its completion. 1here is evidence that juveniles lose both protein and lipid condition during the moult. For most of the year, Bulbuls go to roost with nearly two grams of lipid, but during the early part of the dry season they lay down large lipid deposits whose adaptive value is not fully understood. Bulbuls from separate comnrunal roosts display differences in lipid but not protein condition, a feature which is probably related to differences in feeding ecology. Both diurnal and overnight changes in body condition take place and these are described and discussed. In Chapter Eight, the importance of the Bulbul as an agricultural pest is examined. Although the timing of the vegetable season coincides with the period in the annual cycle of the Bulbul when damage is likely to be greatest, the damage caused is unlikely to be of economic importance except to a few individual farmers. 1he damage to the crops most commonly attacked is discussed, its extent and management procedures which would reduce it are described. � 1he role of the Bulbul as a disseminator of serious weeds is discussed in Chapter Nine . It is probable that the Bulbul is an important biological agent involved in the dispersal of several serious weed species in Fiji, although it is not primarily responsible for the spread of Guava, for which it is commonly blamed. 1he limited distribution of certain weed species in Fiji, which form the bulk of the diet of the Bulbul , is probably responsible for its restricted distribution. In Chapter Nine, the effects of the Bulbul and other introduced birds on the indigenous avifauna are discussed. At present there are eleven introduced birds established in the Fiji Islands. All of these are essentially restricted to man-modified habitats, although the Bulbul is regularly seen in areas of immature secondary vegetation in the forests. Very little interspecific aggression involving the Bulbul and indigenous birds was observed and there appears to be little scope for direct competition for food resources between introduced and native species. Only the Polynesian Starling, Aplon.is tahuensis might be restricted to certain habitats because of the presence of the intr oduced birds. lhe final Chapter comprises a general discussion of five topics ; a comparison of the feeding ecology and habitat preference of the Bulbul in Fiji and India; the characteristics of t he f r ui t supply exploited by the Bulbul; the factors affecting the distribut ion of t he Bulbul in the Fiji Islands; the timing of the annual cycle and proximat e f actors initiating breeding.
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