BiologyBiology

Biology

Frigatebirds are a group of seabirds that are adapted morphologically for a highly specialized type of aerial feeding which affects their entire breeding ecology and behaviour. All have extremely prolonged breeding cycles. Frigatebirds in general, do not remain faithful to the same mate or nest site from year to year. Each new breeding effort involves setting up a new territory/nest site, and finding a new mate (Nelson 1975). There is no information on the nest site or mate fidelity of Christmas Island Frigatebirds, but they are probably similar to other frigatebirds.

It requires at least 15 months for a pair of Christmas Island Frigatebirds to raise one offspring to independence, although it is not known whether both partners are required for the entire period. Both Greater Frigatebirds and Lesser Frigatebirds share parental duties and only raise one offspring every two years. However, Magnificent Frigatebird (F. magnificens) males cease parental duties when the young are three to four months of age, leaving the females to raise the young to independence. As a result, males breed every year and the females every second year, and there is a skewed sex ratio (Diamond 1972). In Christmas Island Frigatebirds, females tend to feed the older juveniles more than males do (Gibson-Hill 1947, Marchant & Higgins 1990), but males have been recorded feeding free-flying young that were at least eight months old (Nelson 1975). Thus, it is not known whether male birds can attempt to breed annually. The sex ratio of Christmas Island Frigatebirds is not known.

At the beginning of each breeding season, adult males select sites from which they display to females. Males begin displaying in late December and begin forming pairs in late February (Marchant & Higgins 1990). The nest is built on the display site after a male has attracted a female (Nelson 1975). As many as 30 nests have been recorded in one tree (Nelson 1975; T. Stokes, pers. comm.).

Most young hatch between mid April and late June and are extremely slow-growing (Nelson 1975), but appear to grow more rapidly than other frigatebirds (Nelson 1972). Young frigatebirds first take to the air when the are c. 6 months old (Nelson 1975, Marchant & Higgins 1990), but remain dependent on the parents for at least a further 9 months (Nelson 1975). This extended period of dependence effectively means that the breeding cycle is biannual, since the time from laying to independence is approximately 15 months. Thus probably only about half of the adult population lay each year (Stokes 1988). Young birds attain adult plumage in the beginning of their fourth year (Gibson-Hill 1947) and the age of first breeding of frigatebirds, in general, is at least five to seven years, possibly older (Nelson 1975); it may even be as much as 9-10 years (Croxall & Rotherby 1991).

Probably 15 to 20% of eggs laid yield fledged young. However, Nelson (1975) found that Christmas Island Frigatebirds appeared to be more successful than other frigatebird species. Some groups on Christmas Island were able to successfully raise 60% of nestlings to at least half grown, but overall c. 30% at most reached fledging age; some groups had lower than 20% success (Nelson 1975). Probably no more than 20 to 30% of frigatebirds die during their first year of independence (Nelson 1975). It is likely that it takes a breeding pair at least 20 to 25 years to replace themselves. This would make recovery of population numbers from any decline extremely slow.