Environmental determinants of foraging strategies in Cory’s shearwaters Calonectris diomedea

Abstract

Despite the advent of devices to track seabird movements, the extent to which produc- tive areas and oceanic winds influence foraging strategies is still not fully understood. We investi- gated the main environmental determinants of foraging strategies in Cory's shearwaters Calonectris diomedea by combining satellite-tracking information from 14 birds breeding on the Canary Islands with concurrent data on chlorophyll a (chl a) concentrations and oceanic winds. Additionally, we took blood samples at the end of each foraging trip and analysed carbon (C) and nitrogen (N) stable iso- topes to examine the use of trophic resources. All birds showed commuting trips, concentrating for- aging activity exclusively on the African continental shelf. Foraging locations showed a strong asso- ciation with chl a concentrations, suggesting birds select foraging areas according to prey availability. In contrast with other breeding colonies where Cory's shearwaters use a dual-foraging method, birds showed a unimodal strategy and did not show differences in C and N isotope sig- natures in plasma, confirming that close proximity to highly productive areas strongly influences foraging strategies. In addition, birds tracked during 2 consecutive trips foraged in the same area, suggesting that high resource availability promotes fidelity to feeding grounds also at coarse scales. Persistent northeast trade winds blew during the study period, and commuting trips followed a con- sistent clockwise movement with a southwest heading while the birds foraged along the continental shelf, suggesting that birds used tail winds to reduce their flying costs. Our results corroborate that oceanographic conditions in the vicinity of the breeding colony have a strong effect on foraging strategies of pelagic seabirds.