MEPS prepress abstract  -  DOI:

Seasonal shifts in foraging distribution due to individual flexibility in a tropical pelagic forager, the Ascension Frigatebird

Steffen Oppel*, Sam Weber, Nicola Weber, Derren Fox, Eliza Leat, Jolene Sim, Julia Sommerfeld, Mark Bolton, Annette C. Broderick, Brendan J. Godley


ABSTRACT: Predators exploiting tropical pelagic waters characterised by low fluctuations in seasonal temperature and salinity may require different foraging strategies than predators that can rely on persistently productive marine features. Consistent individual differences in foraging strategies have been found in temperate seabirds, but it is unclear whether such foraging specialisation would be beneficial in unpredictable tropical pelagic waters. We examined whether foraging trip characteristics of a tropical seabird were consistent between seasons and within individuals and explored whether seasonal changes could be explained by environmental variables. Ascension Frigatebird Fregata aquila trips lasted up to 18 days and covered a minimum travel distance up to 7047 km, but adult frigatebirds stayed within a radius of 1150 km of Ascension Island. We found that the 50% utilisation distribution of the population expanded south-westward in the cool season due to individuals performing more and longer trips into a southerly and westerly direction during the cool than during the hot season. Individual repeatability was low (R < 0.25) for all trip characteristics, and we were unable to explain seasonal changes in time spent at sea using oceanographic or atmospheric variables. Instead, frigatebird usage per area was almost exclusively determined by distance from the colony, and although individuals spent more time in distant portions of their foraging trips, the amount of time spent per unit area decreased exponentially with increasing distance from the colony. This study indicates that in a relatively featureless environment high individual consistency may not be a beneficial trait for pelagic predators.