Shearwater Foraging Figure 7. Shearwater foraging in the Polar Frontal zone, in relation to oceanographic processes and copepod abundances. A. Longitudinal distributions in the Polar Frontal zone of water vertical velocity at 1095m depth (solid line, positive = upward) and water depth (dotted). B. Mean summer surface chlorophyll-a from SeaWiFS satellite estimates. C. Near-surface copepod density from continuous plankton recorder surveys (points = individual tow segments, black lines = mean6SE fit by generalised additive model). D. Shearwater relative foraging effort within 3u of latitude of the Polar Frontal zone (white = sooty shearwaters, black = South Australian short-tailed shearwaters, grey = Tasmanian short-tailed shearwaters). doi:10.1371/journal.pone.0010960.g007 Wind cost analyses One of the objectives for the current study was to evaluate whether the longitudinal distribution of sooty shearwater foraging effort in the Polar Front zone was related to patterns of oceanic winds. There is little in our results to suggest that the region directly south of New Zealand was sparsely utilized by the birds because of wind cost factors. However, wind patterns do appear to explain other broad-scale patterns in foraging, such as the lack of use of near-Antarctic waters east of about 150uE by South Australian short-tailed shearwaters. This use of wind on long foraging trips has previously been noted [e.g. 53] but not formally investigated. Both the sooty shearwaters and the Tasmanian short-tailed shearwaters made long foraging trips to the south-east of their colonies, into apparently energetically-costly areas. Shaffer et al. [13] noted the bias of the sooty shearwaters toward the south- eastern sector, and provided several possible explanations, including potentially higher prey abundances in that zone. Our wind cost analyses suggest two further factors. First, while such trips were costly overall, the outward flight legs were generally accompanied by favourable tail and hind-quarter winds. Long foraging trips are generally initiated when the adults have poor body condition [12,14] and so a foraging trip with minimal initial energy cost might be preferable to one that has a lower total cost (i.e. when the return leg also considered). Second, shearwaters breeding in south-eastern Australia do not appear to forage in this zone, possibly because of the high energy costs that they would incur in doing so. The south-eastern zone might therefore represent an area of reduced competition for the sooty shearwaters. There are a number of potential concerns with the wind cost analyses. Shearwaters generally fly close to the water, where wind speeds will be lower than the estimates provided by the NCEP2 data (which are for winds at 10m above the ocean surface, where shearwaters rarely fly). Flight close to the sea surface will reduce the induced drag and therefore the energy cost of flight [82], but perhaps more importantly, means that ocean waves become a potentially important consideration. Shearwaters are known to use wave troughs and wind gradients during flight [e.g. 83,84]. Dynamic flight techniques that exploit the interaction of winds and waves, such as gust soaring [50,51], have the potential to dramatically reduce the energy expenditure of seabirds in flight over the open ocean. The cost function used here does not account for such techniques. The decision not to incorporate these factors into the wind cost analyses was in part a reflection of the lack of fine-scale wind and wave data over the regions of interest (which would be required to model dynamic flight costs), and also the relatively coarse spatial and temporal resolution of the seabird locations (particularly those from the geolocation tags, which provide only one position fix per day and with relatively large spatial uncertainty). The wind cost analyses must therefore be interpreted in the context of relatively large-scale behaviours (of the order of days, or hundreds of kilometres). Further caution should be exercised in interpreting the insights into adult foraging behaviour obtained from tagged birds, since it is known that the handling of birds and presence of tags can reduce colony attendance and provisioning of chicks by the adult birds [43,85, but see also 86]. PLoS ONE | www.plosone.org 10 June 2010 | Volume 5 | Issue 6 | e10960