THE BIOLOGY OF ABBOTT'S BOOBY SULA ABBOTTI

SUMMARY 766 pairs of Abbott's Boobies Sula abbotti were located on Christmas Island (Indian Ocean), the species' sole breeding ground, and a further 1,058 pairs conservatively estimated to have been missed. To these may be added an arbitrarily chosen figure of two‐thirds the number of juveniles which were on the island at the time of the estimate, to represent sub‐adults which were probably away at sea, giving a total of 4,200‐5,100 individuals. An independent assessment, based on the number of birds flying in to the island, gave a conservative figure of 3,500‐4,500 individuals. This represents a very large increase over previous figures which were merely guesses. The distribution of abbotti is mapped in detail. Its correlation with topographical features is described. Nests are usually placed over 70 feet high in dense jungle, above the 500 foot contour. Abbotti is the only sulid that nests relatively solitarily, but even so there may occasionally be three nests in a tree. The most densely populated area held 90 pairs in 876,000 m 2 . The morphology of all ages is described and weights and measurements given. Adult males averaged 1,503 g, females 1,600 g. The species can be sexed on bill colour (pink in female, blue‐grey in male) but the juvenile is virtually indistinguishable from the male. Adults apparently suspend moult during breeding and resume it as their offspring approaches independence. The breeding cycle occupies more than 15 months and egg‐laying seems restricted to April‐July. Breeding must therefore be biennial (or less frequent). The large egg constitutes more than 7% of the female's weight, more than twice as much as in S. bassana (also uniparous) and considerably more than any other sulid. Incubation takes 56 days and incubation stints average about 52 hours. The growth of abbotti is remarkably slow. A comparison is made with that of S. leucogaster on the same island at the same time. The chick is at first fed by complete regurgitation, a habit not shown by any other sulid. Feeds average one per day until the chick is about two months old, and are iess frequent thereafter. Males and females take approximately equal shares in feeding the young. The fledging period is probably 24 weeks or more and is followed by three to four months of post‐fledging dependence. Several features of abbotti's breeding biology suggest that it is a distant, oceanic forager. Abbotti is markedly unaggressive, both in rival and pair situations, and the effects of this are traced in the composition of its behaviour repertoire. It is suggested that marked inhibition of close‐range aggression has evolved to reduce the likelihood of falling from the tree tops, a particularly potent danger since a grounded abbotti is doomed. Some other aspects of behaviour (e.g. longdistance pair interactions, restrained chick begging) support this interpretation. Abbotti's chief ritualised behaviour pattern is the “ecstatic” meeting ceremony Mutual Wing Waving, which is probably important in maintaining the pair bond, in a species needing such prolonged co‐operation in breeding. Wing Waving is also used as a territorial (agonistic) display. Head Jerking, the only other conspicuous ritualised display, is chiefly aggressive but frequently occurs in the pair context. The main elements of abbotti's behaviour are summarised (Table 8) within the sulid framework. The Discussion considers the probable adaptiveness of abbotti's plumage and morphology, the evidence for classing abbotti as a distant forager, its behavioural peculiarities compared with the family norm, and the phylogenetic implications. The future prospects for Sula abbotti are a matter for concern, since Christmas Island is destined for further phosphate mining, involving substantial destruction of cover. It has been suggested that “tree islands” in the key areas would help conserve a significant proportion of the world's abbotti population. It is hoped that a survey of abbotti's numbers and distribution around 1972 will help evaluate the effect which destruction of habitat has already had and provide a basis for estimating future prospects.