Shearwater Foraging Figure 2. An example of the wind cost function used for the cost simulations. Costs are shown for a range of wind speeds and angles. This example is for a sooty shearwater carrying no food payload. Angles are relative to the bird flight direction: an angle of 0u corresponds to a tailwind, 90u and 270u to cross-winds, and 180u a headwind. doi:10.1371/journal.pone.0010960.g002 tailed shearwaters were consistently made in a clockwise direction, heading to the Polar Front zone at ,165uE and then foraging east-to-west, with an approximately north-east return route to their colonies (Figure 1b). At-sea observations of shearwaters were generally distributed over offshore waters around and to the south of the Polar Front, from approximately 60uE to 150uE (Figure 3). The highest densities were observed in the vicinity of the Polar Front around approximately 140uE, and also close to the coast of Tasmania. The high densities of at-sea observations of shearwaters in the Polar Front zone at ,140uE were reasonably consistent across years. Six voyages intersected this zone (1993, 1994, 1998, 2000 (2), and 2006). On four of these voyages the observed densities of shearwaters were higher in this zone than the remainder of the voyage (Wilcoxon rank sum test, p,0.001 in each case). In the remaining two voyages the densities were not significantly different in this zone compared to the remainder of the voyage. Diving behaviour of sooty shearwaters Sooty shearwater dives recorded by fast sampling archival tags north of 50uS (n = 801 dives by seven individual birds) occurred from just before sunrise through to sunset, with broad peaks of dive activity centred on mid-morning and late afternoon (grey bars in Figure 4a). Dive activity south of 50uS (n = 933 dives by two individual birds) also occurred throughout the day, but showed distinct peaks just after sunrise and just before sunset (white bars in Figure 4a). The diel distributions of dive activity north and south 2 of 50uS were significantly different (p,0.001, x 12 = 245.1). The median maximum dive depth (fast tags; all dives combined, n = 1734) was 10.1m (interquartile range IQR 12.8m). The dives south of 50uS (median maximum depth 11.9m, IQR 14.5m) were deeper than those north of 50uS (8.5m, IQR 10.2 m; Wilcoxon p,0.001). The dive depths tended to be shallowest around sunrise and sunset, becoming deeper during mid-morning and mid- afternoon, and shallow again around midday (Figure 4b). Simulations of wind costs during long foraging flights The simulated wind costs for the three tracking studies showed similar broad patterns, with low-cost regions directly south of the colonies, and higher-cost regions extending in corridors roughly south-east and south-west from the colonies (Figure 5). The Polar Front region directly south of New Zealand, sparsely utilised by the sooty shearwaters, was both closer to the colonies and more energetically favourable (i.e. lower cost) than their observed foraging zones to the south-east and south-west of New Zealand. The observed South Australian short-tailed shearwater foraging activity corresponded to energetically-favourable areas (Figure 5b), whereas both the Tasmanian short-tailed shearwaters (Figure 5c) and the sooty shearwaters (Figure 5a) made outward flights to the south-east of their colonies, into apparently energetically-costly areas. Although variable, the prevailing wind over much of the Southern Ocean south of Australia and New Zealand was approximately north-westerly (i.e. blowing towards the south- east). South-easterly flight from the colonies was therefore accompanied by favourable (near-tailwinds) on the outward leg, but entailed return flights against headwinds. Because the wind cost function is asymmetric (that is, the penalties associated with head winds are proportionally higher than the benefits provided by tailwinds), such trips were more energetically costly overall. For short-tailed shearwaters the minimum-cost flight paths from the simulations generally showed good agreement with the PLoS ONE | www.plosone.org 5 June 2010 | Volume 5 | Issue 6 | e10960