Shearwater Foraging Figure 3. Observed at-sea densities of sooty and short-tailed shearwaters in the Southern Ocean. A. Individual survey records (number of birds per 10-minute survey). B. Smoothed density surface fit with local scatterplot smoothing (see text). Locations of Southern Ocean fronts from north to south are shown in black: SAF-M, middle branch of the Subantarctic Front; PF-N, northern branch of the Polar Front; PF-S, southern branch of the Polar Front; SB-ACC, southern boundary of the Antarctic Circumpolar Current. doi:10.1371/journal.pone.0010960.g003 actual flight paths recorded from the tags. The simulated minimum-cost flight paths for South Australian short-tailed shearwaters foraging in regions south of the Polar Front zone involved a roughly southerly flight to near-Antarctic waters, followed by westerly flight using the polar easterly winds near the Antarctic coast as tailwinds (thick green line in Figure 5b), in good agreement with the observed flight paths of the tagged birds. These birds did not tend to forage to the east of their colonies south of the Polar Front, possibly because this would entail return flights against unfavourable winds. The minimum- cost return flight path from Antarctic waters was close to the most direct route (purple line in Figure 5b). Although none of the tags survived long enough to capture a return flight path, the at-sea densities (Figure 3) are consistent with reasonably direct return flights, as are previously published tracks from short- tailed shearwaters [53]. Physical oceanography of the foraging zones Strong, vertically-coherent, topographically-driven upwelling is present in several sectors of the Polar Front zone in the region of interest, notably around 140uE and 165uE, and east of 170uE (Figures 6 and 7a). These zones of strong upwelling extend over several hundred kilometres and are driven by the interaction of the eastward-flowing Antarctic Circumpolar Current with large-scale bathymetric features: the South-East Indian Ridge (140uE), and the Pacific-Antarctic Ridge (east of 165uE). These regions of upwelling correspond to the regions of high surface chlorophyll concentrations (Figure 7b), and with shearwater foraging effort in the Polar Front zone (Figure 7d). Productivity is generally higher south of the Polar Front, due to a range of factors including higher upwelling rates, iron input from melting sea ice, and shallower mixed layers [54,55]. The at-sea observations and the tracks of the South Australian short-tailed PLoS ONE | www.plosone.org 6 June 2010 | Volume 5 | Issue 6 | e10960