Foraging ranges of incubating Sooty Terns Onychoprion fuscatus on Bird Island, Seychelles, during a transition from food plenty to scarcity, as revealed by GPS loggers

The foraging tracks of incubating Sooty Terns, nesting on Bird Island, Seychelles, were identified using GPS loggers attached to the central pair of rectrices. By chance, our 2014 study covered a transition from food abundance to food shortage. Incubation shifts during food abundance were mainly 1-2 d long but at the height of food shortage ranged 4-13 d, leading to temporary and permanent nest desertion by individuals left caring for the egg. The duration of foraging trips and the distance travelled also increased, from 151- 271 km for birds absent for 1-3 d, to 2 142-2 779 km by birds that were away from the colony for 4-10 d. This technique, coupled with spatial analyses that relate track paths and foraging locations to bathymetry, sea surface temperature and chlorophyll levels permitted more detailed investigations of the breeding birds’ use of their oceanic habitats. It also provides data to support and define areas to be recommended for Marine Protected Area status in Seychelles and elsewhere.