How area restricted search of a pelagic seabird changes while performing a dual foraging strategy

Abstract

Movements and foraging strategies of marine predators should cope with the hierarchical spatial distribution of resources. Therefore, in order to predict the at-sea distribution of aerial predators, it is crucial to understand the factors governing trajectory decisions at different scales. Using first passage time (FPT) analysis on precision tracking information (GPS-loggers data) we were able to examine the foraging strategy of Cory's shearwaters Calonectris diomedea and to detect the adoption of area-restricted search (ARS), measuring the scale and duration of this behaviour. Data were collected from three different populations foraging in different oceanographic conditions. During long excursions birds only commuted between their colony and prey patches, while on their short movements birds increased the amount of looping movements. On short trips, birds addopted ARS behaviour at an average scale of 18 km and at a second nested scale of around 2 km. When engaging in long trips, first scale of ARS occurred on average at about 67 km of radii and than a second nested scale at a radii of 24 km. Overall, the different populations showed foraging patterns matching the habitats exploited: a) at smaller scales of ARS, sea-surface temperature, chlorophyll-a concentration and depth influenced the time of residence (i.e. FPT) of birds (with variations at a population level); b) at larger scales of ARS, FPT increased within regions of higher gradients of sea-surface temperature, chlorophyl-a concentration and depth. This study demonstrates that Cory's shearwaters adopt scale-dependent adjustments of movement in relation to the hierarchical distribution of the environment they exploit, matching the scale and duration of ARS with the hierarchical distribution of the environmental features.