THE KLEPTOPARASITIC INTERACTIONS BETWEEN GREAT FRIGATEBIRDS AND MASKED BOOBIES ON HENDERSON-ISLAND, SOUTH-PACIFIC

Kleptoparasitic activities by Great Frigatebirds (Fregata minor) on Ma sked Boobies (Sula dactylatra) were quantified on Henderson Island in the South Pacific. The frequency of kleptoparasitic attacks increased towards dusk, the time when most Masked Boobies returned to the roost site. Masked Boobies returning to the roost low (< 30 m) over the wate r were significantly more likely to be chased than those returning at a higher altitude. Single boobies were also significantly more likely to be attacked than boobies returning as one of a group of birds. Howe ver, group size and height were positively correlated. There was no ef fect of group size on the likelihood of attack. Sixteen percent of cha ses were successful and the success rate was not influenced by either the height or the distance of the target from the roost. The duration of the chase was, however, influenced by the position of the target: c hases on distant or high targets lasted significantly longer than chas es on targets that were low or close to the roost. In addition success ful chases were significantly longer than unsuccessful ones. We discus s these results in relation to the chase tactics of frigatebirds, the avoidance tactics of boobies and the energetic costs and benefits of k leptoparasitism. Approximately 40% of daily energy expenditure of some individual Great Frigatebirds may be secured through kleptoparasitism . However, on average, frigatebirds may be meeting under five percent of their daily energy demands by this feeding method.