THE TROPIC BIRDS (GENUSPHAETHON) OF ASCENSION ISLAND

SUMMARY This paper is an account of investigations into the breeding cycles and breeding biology of two species of tropic bird, Phaethon ae. aethereus and Phaethon lepturus ascensionis, on Boatswain Bird Island, Ascension Island, in the southern Atlantic Ocean. Investigations covered the period November 1957–March 1959, and involved 700 marked nests and over 1,400 marked birds. Both species were found to lay in every month of the year, although the number of eggs laid per month varied according to cyclical patterns peculiar to the two species. Individually, birds were found to breed at intervals of five to ten months (P. lepturus) or nine to twelve months (P. aethereus) according to the success or failure of their previous cycle; in either species those which were previously successful laid again towards the higher end of the range, those which were previously unsuccessful laid earlier, according to the length of time taken over incubating or rearing before loss. The interval between successive layings was determined in successful breeding by the length of the sexual cycle, i.e. the time taken by courtship, incubation, the rearing of the chick and the complete moult of body feathers, and was some 20% longer in P. aethereus than in the smaller P. lepturus. Unsuccessful breeders occasionally re-laid soon after the loss of egg or chick, but were more likely to lose their nest-site to usurpers, fail to lay again, and undergo a post-nuptial moult similar to that of the successful breeders. The overall pattern of breeding, i.e. the shape of the curve showing the number of eggs laid per month, was in either species determined by (a) the ratio of successful to unsuccessful breeders and (b) the length of the successful sexual cycle. Red-billed Tropic Birds (P. aethereus) followed a pattern showing marked peaks and low points of activity, with peaks spaced eleven months apart. Yellow-billed Tropic Birds showed a pattern in which successive peaks were spaced only nine months apart and laying was spread more evenly throughout the year. These differences are related to differences in the size of the two species, the larger P. aethereus taking longer to breed and being more successful in interspecific competition for nest-sites. Details of breeding behaviour, food, egg measurements, rates of growth of chicks, nest mortality and fledging success are given, with especial reference to P. lepturns which on Boatswain Bird Island outnumbered P. aethereus approximately in the ratio 2:1. A partial sexual dimorphism of colour in P. lepturus is also described.