The Breeding Ecology of Abbott Booby Sula abbotti

Abstract

A survey of 2 636 sites showed that Abbott's Booby nests mainly in Planchonella and Eugenia, the two main emergent forest trees, and Celtis and Trisferopsis. Most nests face north-west, which allows birds to land into the wind during the period when visits to the nest are most frequent. Comparison of present distribution with that in 1967 shows that preference for certain areas has been strongly maintained. Boobies do not colonize hitherto unoccupied areas when displaced. Substantial amounts of preferred habitat have been destroyed and there may be further fragmentation of the remainder.

Fidelity to site and mate is, from circumstantial evidence, high. Pairs that complete a successful breeding in August or September usually return the following April. The breeding cycle (c. 504 days) comprises some 66 days pre-laying activities, 55 days incubation, 151 days fledging period and 230 days post-fledging feeding. Seventy per cent of eggs are laid between mid-May and mid-July. Some pairs laid in the same year that their juvenile successfully departed and some laid later in the year in which their free-flying juvenile died, or in the year after their previous laying. These cases constitute important departures from normal biennal breeding.

In any year about 20% of site-owning pairs are not engaged in breeding. 1.27 eggs per pair per two years were laid and 0.196 young per pair per year, or one successful offspring in 5.1 years, were reared. Breeding success (independent young reared from eggs laid) is highly variable from cycle to cycle, on average between 26.5 and 33.3% but in some cycles less than 10%. Success appears to increase with experience. About 23.8 years are required for a breeding pair to replace themselves. Most of 169 deaths among young Abbott's were due to starvation, mostly in March. Cyclones may kill 20mo of free-flying juveniles. Starving, free-flying young may drop below 50% of adult weight. Weight at fledging is 85% mean adult weight.

Young Abbott's were observed at the nest-site on 98.1% of observer visits in January/February and 83.5% of visits during the six weeks prior to final departure. The young are fed on averages about twice a week in the monsoon period, though often much less. Most young take their first flight in December or January and are at risk from clumsy landings, and from attack by frigatebirds (Fregata spp) especially in partly-cleared areas. They depart abruptly, mainly in July to September.

On the whole of Christmas Island, around 900 pairs laid eggs in 1981 and 800 in 1982. The total number of pairs engaged in breeding in 1982 (tending eggs or young of any age) was estimated to be 1 136 pairs. There is a substantial number of non-breeders on the island.