Flexible foraging behaviour in a marine predator, the Masked booby (Sula dactylatra), according to foraging locations and environmental conditions
Résumé
Flexible foraging behaviour allows marine predators to exploit shifting prey diversity and adjust to spatio-temporal changes in the distribution of prey. In diving marine predators, the number of dives within a dive bout, distances between dives in a bout and distances between bouts have been related to prey patch size, density of prey patches and prey encounter rates, respectively. In addition, the predictability of prey encounters can be examined by studying individuals' foraging site fidelity. Based on this, we examined in chick-rearing Masked boobies (Sula dactylatra) from Phillip Island (South-west Pacific), Australia, using GPS and depth–acceleration data logger, how these parameters changed over time (two early breeding phases and one late phase over two years) and according to foraging locations and environmental conditions. Results revealed that birds foraged in two distinct areas: over a nearby shallow shelf (“local area”) and over distant deeper waters (“distant area”). Birds searched for prey in the distant area only during the two latest study periods, indicating less favourable foraging conditions within the local area during this time, although persistent higher chlorophyll-a concentrations in the local area were indicative of an upwelling. Our data suggest that birds experienced a trade-off between local and distant trips: although the strong foraging site fidelity within the local area suggests that prey encounters were predictable, smaller dive bouts indicated smaller prey patches, i.e. less prey nearby the colony during the two latest study periods. As a consequence, adults undertook distant trips to forage in larger prey patches, as inferred from larger dive bouts, but distances between these dive bouts were more variable and birds showed no foraging site fidelity, indicating less predictable prey encounters. Local trips presumably allow a high feeding frequency of the chick, while distant trips allow adults to replenish their own body reserves. Observed changes in foraging conditions, although the underlying causes were not determined, appear to be within the eco-physiological limitations of Masked boobies. However, further studies are required to determine the degree of this limitation, especially as changes in the spatio-temporal availability of prey can be expected to increase with global warming and changing oceanic processes.