during February–April in the region 60uE–200uE, 40uS to the Antarctic continent. Data for short-tailed and sooty shearwaters were pooled, as these species are difficult to separate at sea. Shearwaters were sighted in 3266 of these surveys, with an estimated total of approximately 110000 individuals. The at-sea data were collected on Australian Antarctic resupply and scientific voyages, and so the survey effort is generally concentrated to the south and west of Tasmania, extending to the Antarctic continent. There was also survey effort in between Tasmania and Macquarie Island (154u529E, 54u379S), but only very limited survey effort in the region south of Macquarie Island. At-sea observations of shearwaters with the behavioural category of ‘feeding’ were extremely sparse: of the 3266 surveys in which shearwaters were sighted, only 38 contained a record of feeding shearwaters. We did not examine these records in detail. To assist in visual interpretation, the spatial distribution of observed densities was smoothed by local scatterplot (Lowess) smoothing applied to log10(x+1)-transformed densities [42]. The processing steps applied to the sooty shearwater geoloca- tion tag data in order to obtain daily position and diving activity estimates have been described [1,43]. The Tasmanian short-tailed shearwater geolocation data were processed similarly, although these data were not processed to determine dive locations. The South Australian satellite-tagged short-tailed shearwater positions were filtered using the method of McConnell et al. [44]. The archival tags deployed on the sooty shearwaters allow diving behaviour to be estimated. However, the tags using slow sampling rates are unlikely to correctly represent behaviours that occur on relatively short time scales. Thus, we examined dive depths only from fast-sampling archival tags. The times of local sunrise and sunset were calculated for each dive [45]. Sooty shearwater diving activity recorded with fast sampling tags was divided into two sets, such that dives south of 50uS (i.e. associated with foraging in Antarctic waters) were in one set, and the remainder in the other. The distributions of dive times and maximum dive depths were compared between the two sets. Diving information was not available for the short-tailed shearwaters, and so we used an estimate of habitat utilisation distribution to infer broad patterns in foraging effort. The habitat usage was computed using a kernel density estimator, with a cross- validation selection of smoothing parameter [46,47]. To gain a perspective on the overall effect of wind on potential foraging flights, grids covering the birds’ potential foraging grounds were constructed, with 2.5u longitude by 1.25u latitude bins. The wind cost of visiting each grid cell was calculated using method based on that of Felic´ısimo et al. [48]. Each simulated foraging flight was broken into 6-hour segments. For a given simulated flight segment, the associated wind was used to calculate the energetic cost of that segment. The speed of the bird over the ground (ground speed; Vg) can be written as a function of the speed of the bird through the air (airspeed; Va) and wind speed and direction [49]: qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ Vg~Vw cos bz Va2{Vw2 sin2 b where Vw is the wind speed and b is the direction of the wind with respect to the flight track. The theoretical power required for a bird in level flapping flight is given by [50,51]: P~qzr VazsVa3 where q, r, and s are constants that incorporate aerodynamical flight parameters such as wing span and body mass [50]. We Shearwater Foraging assumed that the birds fly at their maximum range speed (that is, the airspeed that yields maximum distance over the ground per unit energy expended). For a given wind speed and direction, Vmr, and the corresponding Vg and P, can be found by minimising P Vg [49]. An example of the resulting cost function is shown in Figure 2, and is minimal for tailwinds, moderate for cross-winds, and heaviest for head winds. For the flight parameters (q, r, and s above), we used a wing span of 1.03m and wing area of 0.0893m2 for sooty shearwaters and 0.93m and 0.076m2 for short-tailed shearwaters [52]. Sooty shearwaters embarking on a long foraging trip were assumed to have a body mass of 790g and carrying no food payload [12]. Adult birds gain body mass on long foraging trips, and on their return trips we assumed a body mass of 870g and a food payload of 193g [12]. For short-tailed shearwaters the corresponding values were 570g (outward body mass), 590g (return body mass), and 156g (return food payload) [14]. The cost of an individual simulated foraging trip was calculated as the sum of the costs of its 6-hour segments. We used near- surface wind data from the NCEP/DOE Reanalysis 2 data set (http://www.esrl.noaa.gov/psd/). These are model-based esti- mates of the 10m wind speed and direction, at 6-hour intervals and on a spatial grid of approximately 2u resolution. Outward flight legs were simulated as flights from the colony to the centre of the destination grid cell, and in the other direction for return flights. The intervening foraging time was not considered for cost calculations. 10000 foraging trips were simulated to each grid cell. The departure date of each simulated foraging trip was randomly sampled from the departure dates of the actual (recorded) foraging trips. In the absence of wind effects, the energy expended during flight at constant speed is proportional to the distance travelled. Including wind effects introduces variations about this relation- ship. Thus, to best illustrate the spatial variability in flight costs due to wind effects, the costs are presented as residuals (percentages with respect to a smooth fit of cost against distance, fitted using natural cubic splines in the R splines library). In order to make the residuals relevant to potential Southern Ocean foraging locations, only grid points south of the colony and within 3500km (sooty shearwaters and Tasmanian short-tailed shearwaters) or 5000km (South Australian short-tailed shearwaters) of the colony were included in the fit. Results Foraging flights and spatial distributions of shearwaters Shearwaters from all three tracking studies foraged in the vicinity of the Polar Front (Figure 1). Sooty shearwaters made use of two principal foraging areas: one to the south-west of New Zealand at approximately 140uE, and the other to the south-east at approximately 160uW (Figure 1a). Of the 31 long foraging trips made by sooty shearwaters, 21 were directed to the south-eastern foraging zone (by ten individual birds), and ten to the south-west (by four birds). Individual birds almost exclusively made long foraging trips in only one direction: only one bird made trips to both the south-western and south-eastern zones. The South Australian short-tailed shearwaters foraged from ,110uE–150uE, with particularly high usage of the region around 140uE (Figure 1b). Foraging flights were generally made with an approximately southward outward leg followed by east-to-west foraging. The South Australian short-tailed shearwaters also foraged south of the Polar Front, as far west as 55uE in waters at or south of the southern boundary of the Antarctic Circumpolar Current (,64uS). The foraging flights of the Tasmanian short- PLoS ONE | www.plosone.org 4 June 2010 | Volume 5 | Issue 6 | e10960