COMPARATIVE BIOLOGY OF THE WHITE BOOBY AND THE BROWN BOOBY SULA SPP. ATASCENSION

Abstract

SUMMARYThe breeding cycles and related biology of the White Boorby Sula dactylatra and the Brown Booby S. leucogaster at Ascension were investigated between November 1957 and April 1959.In both species there were marked peaks of laying, with intervening periods of very few new clutches. In the White Booby, although only one full season was studied, an annual cycle was deduced; in the Brown Booby (two full seasons) the cycle appeared to be eight‐monthly. The breeding season (laying of eggs to fledging of chicks) was however roughly the same length in both species, namely six to seven months. Individuals of both species for the most part conformed to the breeding seasons of the population as a whole, i.e. if out of phase for some reason, had a longer or shorter “rest” period as necessary to bring them into phase again for the next season. These discoveries indicate that external factors are controlling the times of breeding. The proximate factors are not known; at Ascension there is no appreciable seasonal variation in day‐length or climate, and possible cycles in the surrounding sea could not be investigated.The two species' periodicity was such that, if maintained, every two years they would lay at almost the same time of year, July–August. The possible relation of these observations to a single underlying factor, perhaps annual variation in oceanic conditions with their origin in the seasonal melting of the Antarctic ice and consequent effect on the food cycles, is discussed.Details of incubation, desertion, feeding, care and growth of the chick are given. The regular death of the second chick as a result of asynchronous hatching, and experiments with twins are described. A low breeding‐success, and deaths of chicks from starvation, indicated that both species suffered a shortage of food over an ill‐defined period around August‐September 1958. Regurgitations of food examined throughout the study, and a few observations on feeding, indicated a big overlap in the two species' diets, mainly flying‐fish, but some differences did appear.Details of moult, mainly in the White Booby, are given. Shedding and re‐growth of the juvenile primaries began at about seven months of age, proceeding from the innermost outwards in three spaced concurrent cycles, this remarkable pattern continuing throughout adult life. A period of pause in the primary moult was correlated with the time when most birds had chicks, but some birds were known to be moulting their primaries while breeding. A prolonged moult with a pause during breeding was indicated by examination of some other Ascension species; the relevance of this to the availability of food and breeding seasons in tropical sea‐birds is discussed.