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 productive areas and oceanic winds influence foraging strategies is still not fully understood. We investigated 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 isotopes to examine the use of trophic resources. All birds showed commuting trips, concentrating foraging activity exclusively on the African continental shelf. Foraging locations showed a strong association 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 signatures 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 consistent 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.

Parent–offspring dietary segregation of Cory’s shearwaters breeding in contrasting environments

“…13 C signatures of SG adults were closer to the values found for adults from the Canary Islands, known to almost exclusively exploit the African coastal waters (Navarro et al 2007;Navarro and González-Solís 2009). The foraging trips also showed an intensive use of an oceanic-neritic gradient by SG shearwaters, and the occurrence of oceanic items on the chick diet was confirmed by conventional dietary analysis.…”

“…This idea is supported by the isotopic signatures of Cory's shearwaters from BE and Canaries, since adults from those colonies forage mainly along the coast and present much lower d 13 C variances (Navarro and González-Solís 2009). d 15 N isotopes are strongly affected by diet and usually represent the trophic position of the consumer (Kelly 2000;Bearhop et al 2004).…”

Methods for detecting and quantifying individual specialisation in movement and foraging strategies of marine predators

“…It is possible to study individual specialisation in space, and not only in trip characteristics, based on distances between the centroid of the locations at 2 different times for the same individuals tracked during the breeding season (Navarro & González-Solís 2009, Ceia et al 2014, or between centroids in different winters (Dias et al 2011, McFarlane Tranquilla et al 2014, Yamamoto et al 2014, Lea et al 2015. Distances can also be calculated between pairs of migratory routes (e.g.…”

“…evaluate the statistical significance of individual consistency), one approach is to use distances calculated for pairs of centroids or routes as the response variable in a (G)LMM with individual (same vs. different) as a random effect, and check for the significance of the random effect (Dias et al 2013). The second, and more widely used approach, is to compare the calculated within-individual distances with a null distribution of distances generated by reshuffling either locations or migratory tracks between individuals (Navarro & González-Solís 2009, Dias et al 2011, McFarlane Tranquilla et al 2014). This method based on distances does not take into account the spread of the locations around the centroids: hence, although useful to detect a shift in the general distribution, it would not detect a change only in range size.…”

“…Marine predators will often target regions characterised by local physical features or processes, including eddies, frontal systems, upwelling zones and shelf breaks, that increase primary production or serve to aggregate various types of prey (Kappes et al 2010, Louzao et al 2011, Pinet et al 2011, Baylis et al 2012, Arthur et al 2015, Wakefield et al 2015. Foraging behaviour and diet specialisations may therefore emerge as a result of the spatial and temporal availability and predictability of prey (Woo et al 2008, Navarro & González-Solís 2009, Patrick et al 2015, Sommerfeld et al 2015. Moreover, because marine predators frequently target such productive areas in the breeding and non-breeding seasons, and are central-place foragers during breeding, competition may be high and lead to the use of divergent foraging strategies (Estes et al 2003, Villegas-Amtmann et al 2008, Patrick et al 2014.…”