Adjustment of foraging trips and flight behaviour to own and partner mass and wind conditions by a far-ranging seabird.

Many animals are highly adapted to cover vast distances in search of ephemeral food resources. Pelagic seabirds have particularly wide-ranging foraging trips, made possible through efficient use of wind. During incubation, partners alternate long periods of fasting and so should adjust foraging and flight decisions according to the condition of the pair, as well as wind conditions experienced at sea. Here, we tracked incubating Juan Fernández petrels, Pterodroma externa , with GPS and immersion loggers, assigned at-sea behaviours using hidden Markov models, and weighed birds and their partners, to investigate the roles of wind and mass on flight and foraging behaviour, and the link between wind use and trip success. Birds conducted long anticlockwise looping trips, on average lasting 20.4 days and covering 10 741 km. They reached a region in the southeastern Pacific Ocean where prey search behaviour was concentrated, typically about 3400 km west of the colony. Outbound and return journeys appeared to broadly benefit from predictable southeasterly trade and westerly winds, respectively. Over finer scales, departure bearings were influenced by wind directions. Across trips, birds oriented predominantly with quartering tail winds which maximized ground speeds. Individuals experienced variable support from tail winds, and those that benefited more on outbound journeys (when winds were generally weaker) travelled faster, reached foraging areas more quickly and, over the entire trip, had higher mass gain per day at sea. Additionally, birds that were lighter on departure gained more mass and birds with heavier partners ranged further from the colony. Our results suggest that decisions involving where to go and how far, respectively, are based on prevailing wind patterns and an assessment of the condition of the pair. Consequently, while birds sought to benefit from wind assistance, those encountering greater tail wind support had more successful foraging trips, indicating that wind use may have direct fitness consequences. • Gadfly petrel foraging trips covered > 10 000 km in the southeast Pacific. • Birds maximized ground speeds by orienting with tail or cross-winds on an ocean scale. • Birds more aided by tail winds reached foraging areas quicker and gained more mass. • Lighter birds gained more mass and birds with heavier partners ranged further. • Wind and pair condition influenced foraging destinations and trip success. [ABSTRACT FROM AUTHOR]