Comparative breeding biology of some seabirds of Ascension Island

The work of which this study is an account was carried out while the author was Deputy Leader of the British Ornithologists' Union Centenary Expedition to Ascension Island , from November 1957 to April 1959.
The objects of the expedition were to investigate the general breeding biology of the resident tropical seabirds with a view to discovering how the timing of their breeding was controlled. In most temperate birds the controlling factors are changes in day-length, temperature, and availability of food; the particular interest of the eleven species at Ascension was that they were living in an environment with no seasonal change in day-length or climate, and apparently a uniform availability of food.
The author was responsible for studies on three of these species, the White Booby Sula dactylatra, the Brown Booby Sula leucogaster, and the Fairy Tern Gygis alba. A few observations were also made on the ten or so pairs of the Redfooted Booby Sula sula which were present. The bulk of this study is an account of the comparative breeding biology of the White and Brown Boobies. The Fairy Tern is not closely comparable to them, and only those aspects of its biology relevant to the general problem (breeding, food, and moult) are dealt with, in an Appendix.
The study is divided into nine sections, of which four deal with the breeding of the boobies, and three with other observations on the species' biology, vis, moult and food (both of which were found to have an important relation to the breeding biology), and behaviour, which had neither been fully described nor analysed before.
Section I is introductory, the aims, scope, and methods of study being described, together with the habitat. Ascension Island lies roughly in the middle of the South Atlantic (8°S, 14°25'W). It is a peak of the Mid-Atlantic Ridge, triangular in shape, with sides of about eight miles, and rises from coastal plains to 2,800 ft in the middle. Its volcanic origin is clearly seen in the numerous extinct craters, ash-fielda, and lava-flows, which are little weathered by the uniformly warm and sunny climate and the continuous south-east Trade-winds. Vegetation is confined to the slopes above about l,000 ft.
As a result of man's introduction of rats and cats, the seabirds are no longer found on the main island and, with the exception of the Wideawake Tern, are now confined to off-shore stacks and islets. The expedition's main work was therefore done on Boatswain-bird Island, a volcanic plug some 300 ft high and 400 yds, across, about 300 yds, off the south-east corner of Ascension. Only intermittent visits could be paid to this island, and the author spent about 130 days, spaced over 15 months, on it.
Section II deals with the colonies and breeding seasons. There were 1200-1300 pairs of White Boobies breeding on Boatswain-bird Island, and one or two pairs elsewhere. 600-700 pairs of Brown Boobies bred at Ascension, of which about two-thirds were on Boatswain-bird Island and the reminder on small stacks.
In both species there were clearly-marked peaks of laying, with intervening periods when the number of new clutches was very small. In the White Booty breeding appeared to occur annually (only one fun season was studied, but deductions were made about the preceding and following ones), in the Brown about every eight months (two full seasons were seen, and again deductions were made about others). In both species the time taken from laying of eggs to fledging of chicks was the same, six to seven months. Individuals of both species conformed to the breeding seasons of the population, and if out of phase for some reason, they had a longer or shorter "rest" period as necessary to bring them into phase again at the next season, these two discoveries at once suggested that external factors were modifying the birds' internal physiological cycles and controlling the time of breeding. What these factors might be is discussed later in the study, in the light of subsequent discoveries about the species' breeding biology.
The two species differed not only in periodicity of casual cycle but also in the time of year at which laying took place. The periodicity was such, however, that every two years the Brown Boobies would lay at almost the same time as the White. The significance of this, and its possible relation to annual variation in oceanic conditions with their origin in the melting of the Antarctic ice, is discussed, together with published information about the species' breeding seasons in other parts of the world.
Section III deals with clutch-size and incubation. Both species were found to lay two eggs, with very few exceptions, but only one chick was raised. Incubation is described, and the attentive spells at the nest analysed; the attentive spells of both species were found to be variable, those of the White Booby being about 48 hours and those of the Brown about 24 hours. This probably indicated a difference in the birds' feeding range (partly confirmed by a study of their food), important in the consideration of the two species' ecologlcal differences. Some desertions occurred during the study of attention spells, and the circumstances of these strongly indicated that the birds were experiencing difficulty in finding food, this view subsequently being supported by other events.
In Section IV the feeding, care, and growth of the chick are described. Records of growth rates of both normal, and abnormal chicks were obtained, and these provided further evidence of the operation of a food shortage. Losses in weight and reductions of growth rate occurred in chicks of varying ages but at roughly the sane date, August and September 1958.
The second chick of the clutch hatched about five days after the first and never lived more than two or three days. The curious circumstances of this are described the smaller chick was apparently expelled from the nest by the larger, and not starved to death as a result of the larger chick's more vigorous demands, as has been shown in some other species of birds. Experiments with twins were carried, out to investigate this situation further; the larger chick's ability to establish a supremacy was found to be so strong as to operate even when the difference in size between artificial twins was very small; and some parents were able to raise twin chicks at apparently the normal rate of growth for two weeks or more. Possible reasons for this striking behaviour amongst the chicks and its relevance to clutch size and breeding success are discussed.
In Section V, breeding success is described. Both species had a low breeding success, and big losses of eggs and chicks of the White Booby occurred in August and September 1958, supporting the other evidence concerning shortage of food. In one area of the White Booty colony studied only 4.5% of the eggs laid gave rise to flying young, in another area the figure being 9%. In the Brown Booby 5% of the eggs laid gave rise to flying young in one season, while in the following season the figure was 13%; the difference here was probably due to shortage of food in the first season causing late deaths among chicks.
Section VI deals mainly with moult in the White Booby. Less information was obtained about the Brown Booty but the procedure appeared to be the same as in the White; in view of the Brown Booby's shorter sexual cycle, however, more information than could be obtained would have been interesting. The sequence of primary moult was discovered when examining juvenile White Boobies which had returned to the island after a post-fledging dispersal. The change from juvenile to adult plumage took more than two years, the shedding and regrowth of the primaries, from the innermost outwards, occurring in three spaced concurrent cycles. This remarkable procedure was found to operate in adults as well. There was also a period of pauae in the primary moult of some adults, correlated with the time when most birds had chicks, but there were exceptions to this and some birds were moulting their primaries throughout their breeding. The primary moult of some of the other species of Ascension seablrds was examined in the light of these discoveries, and the results indicated a prolonged moult with a period of pause during breeding. This, together with the rotation of moult to the availability of food and the breeding seasons in tropical seabirds, is discussed.
Section VII is an analysis of the food of the boobies with some observations on their feeding behaviour. Both species regurgitated freely, the contents of all such food samples being noted and specimens preserved for later identification. Flying-fish were found to be prominent in the diet of both species, but both also took small fish on occasion, sometimes in large numbers, and, particularly in the case of the Brown Booby, to the exclusion of flying-fish. White Boobies generally concentrated more on large flying-fish (about 20 cm. long) than Brown Boobies, and the latter took a higher proportion of small fish of more species than the White Booby. When they did take large fish, the Brown Boobies took a slightly greater variety than the White. Only Brown Boobies were seen regularly to fish close inshore. Thus there was an overlap in the two species' food, but they appeared to differ in feeding methods, in the variety of species taken, and in size preference.
Section VIII is a comparative account and interpretation of behaviour patterns observed in the course of other observations on establishment of territories, conduct of mated birds, pairing, territorial behaviour, and permanence of pair-bonds. These patterns and the situations in which they occurred have not been described before and are important for the understanding of the derivation of certain behaviour patterns in other members of tho family Sulidae. Some of the behavioural differences between White and Brown Boobies were apparently related to differences in their nesting habits.
Section IX contains the general conclusions from the above findings and a discussion of their relation to the relation of seabird rubbers and the control of breeding times of birds in a tropical marine environment. Particularly stressed are the importance of studies on the moult cycle and availability of food in rotation to the breeding seasons.
It is argued that the two boobies may have evolved the observed differences in breeding biology as a means of exploiting, in different waye, cyclical variations — perhape small — in the availability of food, these being a result of variations in oceanic conditions.