Coping with variable and oligotrophic tropical waters: foraging behaviour and flexibility of the Abbott's booby Papasula abbotti

Seabirds of tropical oceanic waters have to cope with an extremely oligotrophic habitat, but little is known about their foraging behaviour and flexibility which enables them to catch sufficient prey for themselves and their chicks. In a 7 yr study encompassing contrasting oceanographic conditions, the foraging behaviour of chick-rearing Abbott's boobies Papasula abbotti, seabirds endemic to Christmas Island, Indian Ocean, was investigated using GPS- and diveloggers to examine (1) if the species exhibits foraging strategies that indicate specific adaptations to unproductive tropical oceanic waters, and (2) if (or how) the birds adjust their foraging behaviour to inter-annually varying marine conditions. Abbott's boobies displayed a number of distinct characteristics in their foraging behaviour: flight velocities were slower and diving activity lower than in other booby species. Foraging efficiency was enhanced by distinct temporal tuning of diving activity and trip timing, peaking in the morning and again in the afternoon. The birds exhibited some flexibility in foraging behaviour: when conditions deteriorated (i.e. when waters became warmer and less productive), their diet composition changed, they increased their trip durations, trip range and maximum dive depth, although other parameters such as diving activity and sinuosity did not change. Remarkably, the time spent on the water increased simultaneously with trip length. By those adaptations, Abbott's boobies were able to keep their body condition (as well as that of their chicks) stable even under poor marine conditions.