The Kleptoparasitic Interactions between Great Frigatebirds and Masked Boobies on Henderson Island, South Pacific

Kleptoparasitic activities by Great Frigatebirds (Fregata minor) on Masked 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 water 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. However, group size and height were positively correlated. There was no effect of group size on the likelihood of attack. Sixteen percent of chases 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: chases on distant or high targets lasted significantly longer than chases on targets that were low or close to the roost. In addition successful chases were significantly longer than unsuccessful ones. We discuss these results in relation to the chase tactics of frigatebirds, the avoidance tactics of boobies and the energetic costs and benefits of kleptoparasitism. 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.