Introduction 91

During the breeding season, pelagic seabirds have to return to their colonies at regular intervals and thus act as central-place foragers. One of the most extreme examples of this behaviour is manifest in albatrosses (family Diomedeidae), whose foraging trips may take them hundreds or thousands of km from their colonies, to remote patches of habitat, which are preferred due to high productivity or niche specialization (Nel et al. 2001, Hyrenbach et al. 2002, Phillips et al. 2005a, Pinaud & Weimerskirch 2005). Due to the great distances involved, the success of this strategy lies in maintaining

relatively low transport costs while ensuring that trips, particularly those to provision chicks, can be completed within given time constraints. During flight, the metabolic rates of albatrosses are exceptionally low (Bevan et al. 1995, Arnould et al. 1996). This is because they proceed almost exclusively by gliding, which is the least energetically demanding form of flight (Pennycuick 1982, Norberg 1986). Although the exact mechanisms that albatrosses use to glide are still under debate, they are thought to rely predominantly on exploiting wind velocity gradients close to the surface of the sea ('gust' or 'dynamic soaring', Tickell 2000, Pennycuick 2002).

Wind also plays a major role in dictating flight patterns at larger spatial scales. Satellite tracking has shown that some species tend to direct their flight paths relative to

synoptic-scale wind patterns, avoiding headwind flight (Weimerskirch et al. 2000b, Murray et al. 2003). However, many areas traversed by albatrosses are subject to strong, persistent prevailing winds. In such areas birds travelling to and from foraging

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patches may be more constrained in their choice of flight directions. This is of considerable consequence, because albatrosses rarely resort to flapping flight (Pennycuick 1982) so their groundspeeds, and hence transport costs, are likely to be affected by their orientation with respect to the wind at several spatial scales.

The ability to regulate flight speed may be particularly important for breeding birds (Hedenström & Alerstam 1995). Albatrosses, like all seabirds, experience changes in the severity of the central-place constraint as the breeding season advances (Shaffer et al. 2003). During incubation, which is carried out alternately by both parents, foraging trips may be long, their duration limited only by the fasting capability of the bird left on the nest (Weimerskirch 1995). After hatching, the chick is brooded or guarded for a few days or weeks (depending on the species) alternately by one parent while the other forages. Thereafter, it is left unattended and provisioned by both parents. During these latter stages, the length of foraging trips is usually much shorter because adults must satisfy both their own and their chicks’ energetic requirements (Weimerskirch et al. 1997b). Although several studies have shown that albatrosses contract their foraging ranges in response to these changing time constraints (Shaffer et al. 2003, Phillips et al. 2004b), the hypothesis that they also regulate groundspeeds in order to reduce time costs during more constrained stages, by changing flight direction with respect to wind direction, has not been tested. Furthermore, within individual foraging trips it is likely that groundspeeds measured at the scale of hours will vary with behaviour. For example, birds en route to and from foraging areas are likely to travel faster than those engaged in prey search and capture (Houston 2006).

The airspeed, and therefore groundspeed, of gliding birds is also constrained by their morphology. Theoretical airspeed is proportional to the square root of a bird’s wing loading (defined as the weight per unit wing area (Pennycuick 1989)). Wing loadings are greater in the larger species of albatross (Shaffer et al. 2001, Phillips et al. 2004b), in theory resulting in greater airspeeds. Furthermore, because albatrosses are sexually size dimorphic, theoretical airspeeds of females are lower than those of males. This prediction has led to the hypothesis that, females have concomitantly lower stall speeds due to adaption to flight in light winds, which explains the pattern of spatial sexual segregation observed in some species (Shaffer et al. 2001, Phillips et al. 2004b).

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However, empirical relationships between groundspeeds of males and females and wind speed and direction have not hitherto been tested.

In this chapter I examine several factors that may influence flight speeds of foraging albatrosses: Firstly, I use a combination of satellite telemetry and immersion logger data to quantify groundspeeds of wandering albatrosses Diomedea exulans, black- browed albatrosses Thalassarche melanophrys, grey-headed albatrosses T.

chrysostoma and light-mantled sooty albatrosses Phoebatria palpebrata at the scale of hours. I chose these species because they breed sympatrically and reflect the range of sizes and life history traits expressed by albatrosses. By careful selection of tracking locations, I minimize errors in groundspeed estimates, allowing me to model the response to relative wind speed. I compare observed interspecific and sexual differences in groundspeeds with those predicted by aerodynamic theory, testing the hypotheses that larger species fly faster than smaller ones. For each species I then model groundspeed in response to relative wind speed in more detail, considering the effects of breeding- and trip-stage, and testing the hypothesis that males fly faster than females. As albatrosses are more active during the day and on moonlit nights

(Weimerskirch & Guionnet 2002, Phalan et al. 2007), I also consider whether diel or lunar phase could influence my estimates of groundspeed. I then test the hypothesis that male birds frequent windier habitats than females. Finally, I compare observed flight directions with respect to wind during the outward and inward stages of foraging trips, when birds are more constrained in their choice of flight directions, and between species during the comparatively unconstrained middle stages of trips.