Partitioning of diving effort in foraging trips of northern gannets

Abstract: Many species of seabirds are known to undertake foraging trips that vary in duration, lasting from a few hours up to several days. However, the important question of how individuals allocate their time during foraging trips of different durations has received relatively little attention until recently. Using activity loggers, we examined the for-aging behavior of chick-rearing northern gannets, Morus bassanus (L., 1758), during trips of different durations, and tested predictions concerning how foraging activity varies across trips. There was no evidence of a relationship between dive frequency during the first 3 h of a trip and trip duration, suggesting that the decision to continue on a longer trip was not affected by an adult’s initial rate of encounter with prey. Flight constituted approximately 50 % of total trip time, and the dive rate of birds per daylight hour was apparently unaffected by trip duration. Birds dived at similar rates on the outward and return sections of their foraging trips, which suggests that they may have been “topping up” on food on their return. Overall our results suggest that, unlike other pelagic seabirds, northern gannets at the Bass Rock do not adjust their individual foraging strategies among trips of different durations. Résumé: De nombreuses espèces d’oiseaux marins entreprennent des excursions de recherche de nourriture de durée variable, allant de quelques heures à plusieurs jours. Jusqu’à récemment, la question importante de l’allocation du temps par les oiseaux individuels durant les excursions de différentes durées n’a été que peu étudiée. L’utilisation d’enregistreurs d’activité nous a permis d’analyser le comportement de recherche de nourriture chez des fous d