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Trans-equatorial migration of Short-tailed Shearwaters revealed by
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geolocators
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Mark J. Carey1*, Richard A. Phillips2, Janet R.D. Silk2 & Scott A. Shaffer3
1Department
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of Environmental Management and Ecology, La Trobe University,
Albury – Wodonga Campus, 3689, Australia
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*Corresponding author: Mark J. Carey
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Address: Department of Environmental Management and Ecology, La Trobe
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University, Albury – Wodonga Campus, 3689, Australia
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(T) +61 2 6024 9868
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(F) +61 2 6024 9888
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Email: markcarey82@hotmail.com
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Running head: Migrating Short-tailed Shearwaters
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British Antarctic Survey, Natural Environment Research Council, High Cross,
Madingley Road, Cambridge CB3 0ET, UK
3 Department
of Biological Sciences, San Jose State University, One Washington
Square, San Jose, CA, 95192-0100, USA
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Abstract.
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Until recent decades, details of the migratory movements of seabirds remained
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largely unknown because of the difficulties in following individuals at sea.
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Subsequent advances in bio-logging technology have greatly increased our
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knowledge of seabird migration and distribution, particularly of highly pelagic
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species. Short-tailed Shearwaters Ardenna tenuirostris (≈ 500 g) have been studied
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extensively during their breeding season, but our understanding of their movements
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outside this period remains poor. Here, we present the first tracks of the trans-
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equatorial migration of Short-tailed Shearwaters from a colony on Great Dog Island,
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Tasmania, Australia. Data were obtained from global location sensors (GLS loggers
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or geolocators), which enable the estimation of bird location twice per day based on
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ambient light levels. Following breeding, tracked shearwaters flew south of the
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Antarctic Polar Front to a previously unknown stopover site where they remained for
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several weeks before travelling rapidly northward through the western Pacific to
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coastal waters off Japan. Short-tailed Shearwaters spent the bulk of the Northern
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Hemisphere summer either in this region or further north in the Bering Sea, before
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returning south through the central Pacific to the breeding grounds. For the first
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time, our results document in detail the complete migration of this long-lived seabird,
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reveal individual variation in timing and distribution, and describe the environmental
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characteristics of their key nonbreeding habitats.
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Introduction
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Until recently, details of the long-distance migratory movements and nonbreeding
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habitats of pelagic seabirds remained largely unknown, despite their importance as
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top marine predators and as potential indicators of changes in climate and other
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environmental conditions (Brooke 2004; Raymond et al. 2010; Chambers et al. 2011;
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Montevecchi et al. 2012). Available information came traditionally from banding
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studies and at-sea observations, but these techniques have a number of drawbacks;
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they are rarely able to identify all the important oceanic stopover sites, migration
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corridors, and the range of individual variation in timing, behaviour and distribution
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(Serventy 1967). Advances in tracking technologies have revolutionised our
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knowledge of a wide-range of top marine predators, many of which are threatened
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by overharvesting of marine resources, pollution or incidental mortality in commercial
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fisheries (Petersen et al. 2008; Phillips et al. 2008; Montevecchi et al. 2012; Block et
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al. 2011). Although devices were initially heavy and deployments restricted to larger
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species (Phillips et al. 2005; Phillips et al. 2006), they have since become sufficiently
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miniaturised for use on seabirds less than 1 kg (Shaffer et al. 2006; González-Solís
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et al. 2007; Catry et al. 2009; Guilford et al. 2009, 2011; Egevang et al. 2010; Harris
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et al. 2010; Rayner et al. 2011a,b; Pinet et al. 2011). This is an important advance,
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as smaller species often occur in much higher abundance and are major consumers
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(Furness 1994; Brooke 2004). From a conservation perspective, there is also an
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increasing need to identify where birds spend their non-breeding season to preserve
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key resources and habitats (Birdlife International 2004; Phillips et al. 2008).
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The Short-tailed Shearwater (Ardenna tenuirostris) is a small procellariiform (≈
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500 g) with an estimated world population of 23 million breeding birds (Skira 1991),
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and is fifth in rank amongst seabirds in terms of global prey consumption (Brooke
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2004). The majority of the global population of Short-tailed Shearwaters breed on
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islands in the Furneaux Group, south-eastern Australia, where they nest in dense
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colonies. Previous observations of migrating Short-tailed Shearwaters have
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suggested that they conduct a trans-equatorial flight across the Pacific Ocean
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(Shuntov 1974; Marchant and Higgins 1990; Ito 2002). Early results from banding
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studies, which began in 1913, and reviews of museum collections, implied a broad
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‘figure-of-eight’ movement around the Pacific Ocean (Serventy 1956; 1961). Vessel-
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based surveys suggested that birds migrated in broad fronts in the western and
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central Pacific (Maruyama et al. 1986; Ito 2002). That these birds foraged regularly
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in Antarctic waters during the breeding season was not confirmed until 1980 (Kerry
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et al. 1983), with further information provided by a few short satellite-transmitter
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deployments in the 1990s and 2000s (Nicholls et al. 1998; Klomp and Schultz 2000;
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Raymond et al. 2010), and stable isotope studies (Weimerskirch and Cherel 1998;
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Cherel et al. 2005).
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Non-breeding Short-tailed Shearwaters are believed to leave the colonies
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early in March (Serventy 1967), followed by breeding adults in April and May, and
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then fledglings (Marchant and Higgins 1990). Birds were thought to take
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approximately 6 weeks to migrate to the eastern part of the Bering and Chukchi
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Seas and to remain there for the majority of the non-breeding period (Marchant and
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Higgins 1990). Huge numbers of Short-tailed Shearwaters were killed in high seas
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drift-net fisheries in the North Pacific until this fishing method was banned (DeGange
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and Day 1991; Uhlmann et al. 2005), and they still face threats from other fisheries,
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climatic change, and potentially pollutants, including plastic ingestion (Vlietstra and
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Parga 2002; Anderson et al. 2011). The timing of different phases during migration,
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and the exact route was unclear. It has been suggested that the return journey
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commences at the beginning of September through the western sector of the Pacific,
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although there is evidence that shearwaters pass through the Gulf of Alaska to
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waters off California before heading south to Australia (Serventy 1967; Skira 1991).
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Whichever route is used, birds are first seen around their nesting islands from early
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September, and return to their burrows by late September – early October (Marshall
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and Serventy 1956; Robertson 1957; Serventy 1967; Skira 1991).
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Until now, there was little available information on the exact timing, speed,
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flight paths and nonbreeding distribution of Short-tailed Shearwaters. Here, we use
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global location sensors (GLS loggers) to provide the first comprehensive data on
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their migratory journeys, and the location and environmental characteristics of key
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nonbreeding habitats. Information on distribution is critical for identifying marine
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threats, including from fisheries, and potential future impacts of climate change,
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overharvesting of resources or other environmental variation. In addition, we provide
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much greater detail on individual variability in movement patterns, which has
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implications for buffering threats from a changing environment.
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Materials and Methods
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GLS deployment and analysis
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In December 2007 and January 2008, 27 GLS loggers (Mk 13: British Antarctic
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Survey, Cambridge, UK) were deployed on breeding Short-tailed Shearwaters on
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Great Dog Island, Furneaux Group, Tasmania, Australia (40°15’S, 148°15’E). All
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adults were from burrows used in a previous study (Meathrel and Carey 2007) and
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were sexed by cloacal examination or by using a discriminant function (Carey
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2011a). Thirteen pairs and an individual female were caught in their burrows during
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early incubation and equipped with a logger using attachment methods described in
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Carey et al. (2009). Birds were recaptured and loggers retrieved the following year.
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Breeding in this species begins in late November when eggs are laid over a 16-day
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period (Meathrel et al. 1993), and ends when chicks fledge in April to May in the
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following year (Serventy 1967).
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The loggers measure light levels every 60-s, and store the maximum value in
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each 10-min recording interval. Light data were processed with ‘TransEdit’ and
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‘BirdTracker’ software (British Antarctic Survey, Cambridge). Sunrise and sunset
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times were identified based on light curve thresholds, with longitude calculated from
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time of local midday relative to Greenwich Mean Time, and latitude calculated from
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day length, providing two locations per day (Phillips et al. 2004). Locations derived
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from light curves with obvious interruptions around sunset and sunrise were
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identified and later excluded if appropriate, following Phillips et al. (2004). During
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processing, location estimates were filtered for unrealistic travel rates using a speed
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filter (McConnell et al. 1992) and a threshold of 50 km h-1 (Spear and Ainley 1997).
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Latitude estimates were unavailable around the equinoxes (Phillips et al. 2004).
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Although some of the birds in this study ventured into the Arctic Circle, they did not
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do so during periods of continuous daylight. Based on concurrent deployment of
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similar loggers and satellite tags on Black-browed Albatrosses (Thalassarche
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melanophris), locations were considered to have a mean accuracy ± SD of around
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186 ±114 km (Phillips et al. 2004).
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In total, 93% of locations were retained after filtering. Track lines for each bird
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were created from the remaining locations based on curvilinear interpolation (hermite
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spline; Tremblay et al. 2006) at 10-minute intervals. Total distance travelled and
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maximum distance (range) from the colony were estimated for interpolated tracks.
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Utilisation Distribution (UD) kernels were calculated from all subsampled locations
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(i.e. every 12 hours) to characterize the core nonbreeding distribution and patterns of
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habitat use following procedures outlined in Shaffer et al. (2009). In brief, 50, 75 and
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90% UD kernels were estimated using the Iknos Kernel program (Y. Tremblay,
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unpublished) developed in MatLab, with a grid size of 80 km, requiring a minimum of
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two individual birds within a grid cell for inclusion, and adjusting for bird effort by
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dividing each cell by the number of birds that contributed those locations. Area
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calculations for UD kernels excluded major land masses.
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Habitat analyses
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Characterisation of the habitats used by nonbreeding Short-tailed Shearwater was
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based on analysis of remotely sensed environmental data obtained from
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http://coastwatch.pfel.noaa.gov/ (see website for metadata on satellite sensors and
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parameters). These included primary productivity (chl a) estimated using methods
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described in Behrenfeld & Falkowski (1997) with resolution of 0.1°, Sea Surface
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Temperature (SST) which was a multiple-satellite blended product with a resolution
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of 0.1° (see Powell et al. 2008 for details on specific SST datasets), and 3-day
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average surface wind vectors (0.25° resolution) measured from the Seawinds sensor
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on the QuickSCAT spacecraft (Frielich 2000). Bathymetry was extracted from
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ETOPO2 (Smith and Sandwell 1997). Data for each environmental parameter were
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extracted from the global time series within a 1° longitude by 2° latitude grid (the
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approximate error of the geolocation method) centred on the date of each location
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following methods outlined in Shaffer et al. (2009). The mean ± SD of the data in
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each grid cell was used in subsequent analyses.
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Results
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Twenty Short-tailed Shearwaters (10 males and 10 females) of the 27 birds fitted
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with loggers (i.e., 74%) were recaptured at their breeding colony between November
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and December 2008. All birds returned in as good body condition as non-equipped
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adults (Carey et al. 2009; Carey 2011b). Of the 20 tagged birds, 14 were from both
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pair members, and the remainder from one partner. Twelve of 20 recovered loggers
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provided complete records from throughout the deployment period (breeding and
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migration), and a further four loggers provided partial tracks (n=16, 8 males and 8
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females).
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Preliminary examination suggested that there were no obvious differences in
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timing of movements or distribution between males and females, and data were
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pooled in subsequent analysis. At the end of the breeding season, tagged birds
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travelled south to a stopover region of deep, cold water south of the Antarctic Polar
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Front and north-west of the Ross Sea between 60°-70° S and 150°-180° E, in which
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they remained for an average of 26.1 ± 16.4 days (Table 1). Generally, failed
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breeders arrived at the stopover site earlier and stayed longer than successful
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breeders (Table 1). This previously unknown oceanic stopover for Short-tailed
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Shearwaters was located in cold, highly productive waters characterised by high chl
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a concentrations (Table 2). Between 15 April and 9 May 2008, all 16 birds departed
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on a northerly migration through the South Pacific, passing the Equator within a few
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days of each other (mean 26 April ± 6 days). Not surprisingly, birds experienced
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high ocean temperatures during this period, low chl a concentrations and low wind
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speeds (Table 2). After crossing the Equator, birds followed a north-westerly route
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towards the east coast of Japan until they reached 30° N, where seven birds
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diverged on a north-easterly bearing towards the south-central Bering Sea around
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the Aleutian Islands, and the remaining nine birds continued north-west to the east
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coast of Japan (Figure 1). All birds ceased their northward passage at
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approximately 40°-50° N, and shifted to a pattern of predominantly east-west
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movements (Figure 1). Movements across the Pacific Ocean Basin were rapid, with
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birds travelling a mean of 10,920 (± 732) km in just 13.0 (± 1.5) days, at a travel
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speed of 840 (± 121) km.day-1 (Table 1). All birds subsequently remained north of
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40° N and between 120° E and 150° W for the duration of the boreal summer (Figure
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2).
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Mean time spent in the North Pacific was 148 ± 9 days. Individuals used one
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of two major regions, either (i) around northern Hokkaido, Japan, or (ii) west of the
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Pribilof Islands in the Bering Sea (Figure 2). These areas were characterised by
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cold, highly productive waters (Table 2). All birds began their return migration to the
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breeding colony in mid-September to early October (Figure 1; Table 1). Birds
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travelled in a south-westerly direction through the central Pacific, west of the
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Hawaiian Islands, passing the Equator on 7 October within a few days of each other
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(± 7.7 days), as on the northbound journey. After passing the Equator, birds
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continued travelling in a south-westerly direction towards the Australian east coast.
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Birds then travelled south along the east coast of Australia, passing the Furneaux
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Group, Bass Strait, in mid to late October (13 October ± 6.5 days). Duration of the
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southward migration was 18 (± 2.6) days, at a travel speed of c. 692 km.day-1. The
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average distance travelled during migration and the non breeding period was
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approximately 59, 600 (± 15,700) km.
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Discussion
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Several authors have attempted to unravel the migratory behaviour of Short-tailed
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Shearwaters based on band recoveries and at-sea surveys (Serventy 1956; 1961;
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1967; Maruyama et al. 1986; Skira 1991), but only by tracking is it possible to
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determine individual variation in movements in any detail. Upon completion of
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breeding, tracked Short-tailed Shearwaters travelled south where they presumably
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engaged in intense foraging in cold, highly productive Antarctic waters. This would
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help birds gain sufficient body condition to undertake the subsequent migration
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through the relatively unproductive waters of the tropics. The rapid transit north
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suggests that they exploit the prevailing global wind systems as do other trans-
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equatorial shearwaters (Shaffer et al. 2006; Felicísimo et al. 2008; Hedd et al. 2012),
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and Arctic Terns Sterna paradisaea (Egevang et al. 2010). The efficiency of
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shearwater flight paths is illustrated by the speed of northward migration from the
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Southern Ocean to the North Pacific, a distance of approximately 11,000 km in only
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13 days (840 km.day-1), similar to Sooty Shearwaters (Ardenna griseus) that
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averaged 910 km.day-1 while crossing Equatorial waters on their northward migration
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(Shaffer et al. 2006).
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Tracked birds exhibited clear synchrony in timing of migration; all left the
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Southern Ocean stopover site, crossed the equator, and arrived at their non-
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breeding region within a few days of each other, and a similar pattern was evident
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during the return journey to the colony. This is in agreement with at-sea
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observations suggesting that flock sizes of migrating Short-tailed Shearwaters are
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large (100s – 100 000s) (Marchant & Higgins 1990), and also corroborates results
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from studies of several other migrant seabirds, which similarly indicate high levels of
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synchrony in timing of passage through oceanic flyways (Shaffer et al. 2006,
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González-Solís et al. 2009, Egevang et al. 2010, Hedd et al. 2012). There was no
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indication that birds from the same colony travelled together in the same flocks, and
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no evidence to support persistent associations during migration between individuals,
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including members of a pair.
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The tracking data reveal that Short-tailed Shearwaters spend their non-
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breeding period in two highly productive areas in the western and central North
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Pacific, (i) the Oyashio and Sōya Currents off Japan, and the Sea of Okhotsk and
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Liman Current in the Sea of Japan, and (ii) waters around the central Aleutian
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Islands and the south Bering Sea. The timing of their arrival in this region coincides
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with that of two other trans-equatorial shearwater migrants, Sooty (4 May ± 13 days;
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Shaffer et al. 2006) and Flesh-footed A. carneipes (11 May ± 8 days; Rayner et al.
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2011b) Shearwaters, during the period when oceanic productivity in the North Pacific
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exceeds that found in the South Pacific (Shaffer et al. 2006). In the boreal summer,
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the waters of the North Pacific are highly productive as a result of physical forcing,
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converging water masses, or coastal upwelling, all of which promote primary and
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secondary production that attract fish, squid, and krill, and support millions of
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seabirds (Gould et al. 2000; Hunt et al. 2002; Ito 2002; Kasai et al. 2010).
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The tracked shearwaters selected one of two major areas in which to spend
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the non-breeding period. However, some individuals that migrated initially to waters
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off Japan later moved to the Bering Sea before the migration south. In contrast,
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birds that travelled first to the Bering Sea did not later move to Japanese waters. A
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change in distribution before return migration has also been observed in Flesh-
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footed Shearwaters, which Rayner et al. (2011b) suggested was in response to
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retreating sea ice in the Sea of Okhotsk and an associated bloom in primary
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production, leading to enhanced foraging opportunities. The same may apply to
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Short-tailed Shearwaters that follow the retreat of sea ice in the Sea of Okhotsk and
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the north Bering Sea. Individuals that moved into the southern Chukchi Sea in
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August and September might be related to the availability of prey. Movement
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between nonbreeding areas was not observed for Sooty Shearwaters (Shaffer et al.
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2006), and whether Short-tailed Shearwaters do so because they can, or because
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they are forced to, is unclear. The implications of variability within and between
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individuals in terms of vulnerability to environmental change are becoming clearer
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(Dias et al. 2011; Catry et al. 2011a,b). Those species who exhibit behavioural
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plasticity will be more resilient to global climate change than those species with
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inflexible migration strategies (Dias et al. 2010). Whether some Short-tailed
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Shearwater individuals exhibit interannual nonbreeding site fidelity, as demonstrated
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recently for several, but by no means all seabirds (Phillips et al. 2005; Croxall et al.
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2005; Hatch et al. 2010; Dias et al. 2010), requires further investigation.
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The routes used for both the northbound and southbound migrations of the
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Short-tailed Shearwaters are strikingly different from those of Sooty Shearwaters
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from New Zealand, even though both species spend their non-breeding period in the
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Bering Sea or off Japan (Shaffer et al. 2006). This does not reflect the location of
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colonies from which birds were tracked. Although Flesh-footed Shearwaters from
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Australasia also migrate as far as Japan (Serventy et al. 1971), a recent GLS
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tracking study of three birds from New Zealand indicates this species has a
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northward migration route that is similar to that of Short-tailed Shearwaters (Rayner
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et al. 2011b). These comparisons illustrate the risk in assuming that similar species
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transiting to broadly overlapping nonbreeding regions use the same migration
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corridors. Thus, tracking data from multiple species and sites will be necessary
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before recent international initiatives, including by the Convention on Biological
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Diversity and the Convention on Migratory Species, are likely to achieve their goal of
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establishing a comprehensive network of ecologically or biologically significant
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marine areas (EBSAs), given that the intent is to include the important migration
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corridors (BirdLife International 2009).
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This study has revealed details of the migration strategies and nonbreeding
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habitat use of Short-tailed Shearwaters. This includes the identification of an
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important and previously unknown post-breeding stopover site in Antarctic waters,
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which is likely to be a key area in which birds build body reserves before undertaking
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the long northerly migration. Thus, long-term changes in duration and extent of
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winter sea ice, and declines in abundance of Antarctic krill in this region (Atkinson et
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al. 2004), should be viewed with considerable concern. Our results also indicated
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considerable flexibility in movement patterns of individuals, with some birds switching
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areas in the North Pacific during the nonbreeding period. This may be a key element
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of these birds’ migration strategy that helps to buffer temporal and spatial variability
308
in prey abundance. At the individual level, Short-tailed Shearwaters show apparently
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greater flexibility in their within season nonbreeding distribution than Sooty
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Shearwaters (Shaffer et al. 2006). However, Sooty Shearwaters exhibit a wider
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range of nonbreeding sites (western, central, and eastern North Pacific) at the
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population level. This key difference in individual variability in movement patterns
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has implications for buffering threats from a changing environment, although whether
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Short-tailed or Sooty Shearwaters will show greater resilience to environmental
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change remains equivocal. Individuals tracked over multiple years would provide
316
greater clarity.
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Increases in global temperatures and in the severity and frequency of El Niño
events may significantly influence major ocean ecosystems, including in the North
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Pacific (Baker et al. 2006). Declines in food availability or shifts in the distribution of
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key prey species could lead to population declines as a result of increased mortality
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during the nonbreeding period, or greater breeding deferral because adults are
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unable to attain sufficient body reserves before returning to the colony (Smithers et
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al. 2003; Surman and Nicholson 2099; Toge et al. 2011). Although there is no clear
324
evidence that populations of Short-tailed Shearwaters have declined, it seems likely
325
that this was the case in the 1980s and early 1990s, given the very high levels of
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adult mortality recorded in North Pacific squid drift net fisheries (Uhlmann et al.
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2005). In contrast, numbers of Sooty Shearwaters have definitely decreased, both at
328
major colonies in the absence of any land-based predators (Scott et al. 2008), and in
329
the California Current system (Veit et al. 1997), although the latter may to some
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extent reflect redistribution at sea (but see Lyver et al. 1999). Clearly, improved
331
monitoring of changes in population size and demography of Short-tailed
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Shearwaters should be a priority, as should further studies of annual variation in their
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nonbreeding distribution, given their obvious value as indicators of environmental
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change and its impacts on top predators in the North Pacific Ocean.
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336
Acknowledgements
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We thank all field staff involved in this project particularly the La Trobe University
338
Marine Ornithology Group, D. Black, F. De Natris, S. Kennedy, J. Mynott, V.
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McCartney, D. Smith, T. Karis, L. Savige and M. North. ANZ Trustees Foundation –
340
Holsworth Wildlife Research Foundation kindly provided financial assistance to this
341
project. Support was received from the La Trobe University Postgraduate Writing-Up
342
Award. This research was conducted under La Trobe University animal ethics
14
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permit number AEC05-15-W and Tasmanian Department of Primary Industries and
344
Water permit No. FA 08145.
345
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577
578
579
Table 1. Timing of migration stages of Short-tailed Shearwaters from Great Dog
Island, Tasmania.
24
Pair
Bird
ID
Sex
Arrival at
Southern
Ocean
stopover
Time
spent at
stopover
(days)
Start of
migration
Date
passing
Equator on
northbound
migration
Arrival at
nonbreeding
site
Duration of
northbound
migration
(days)
Time spent
at
nonbreeding
site (days)
Departure
from
nonbreeding
site
Date
passing
Equator on
southbound
migration
Duration of
southbound
migration
(days)
Arrival
at
breeding
site
1
8041
f
18 Mar
31
18 Apr
23 Apr
30 Apr
13
-
-
-
-
-
8066
m
11 Apr
12
22 Apr
1 May
4 May
13
146
27 Sep
6 Oct
15
11 Oct
8047
f
9 Apr
10
18 Apr
23 Apr
29 Apr
12
155
30 Sep
7 Oct
16
15 Oct
8042
m
14 Mar
37
19 Apr
24 Apr
30 Apr
12
-
-
-
-
-
8049
f
13 Mar
37
18 Apr
23 Apr
29 Apr
12
156
1 Oct
12 Oct
17
17 Oct
8053
m
29 Mar
19
16 Apr
24 Apr
29 Apr
14
157
2 Oct
14 Oct
17
18 Oct
8060
f
12 Apr
19
30 Apr
4 May
11 May
12
-
-
-
-
-
8062
m
11 Apr
13
23 Apr
1 May
7 May
15
146
29 Sep
12 Oct
19
17 Oct
8056
f
3 Mar
44
15 Apr
21 Apr
30 Apr
16
-
-
-
-
-
8054
m
17 Feb
61
15 Apr
22 Apr
30 Apr
16
158
4 Oct
8 Oct
17
20 Oct
6
8043
f
5 Apr
16
20 Apr
24 Apr
1 May
12
149
26 Sep
13 Oct
22
17 Oct
7
8050
f
8 Apr
14
21 Apr
29 Apr
4 May
14
136
17 Sep
6 Oct
24
10 Oct
8
8051
m
10 Apr
9
18 Apr
24 Apr
29 Apr
12
141
16 Sep
1 Oct
18
3 Oct
2
3
4
5*
25
9
8064
f
5 Apr
14
18 Apr
24 Apr
29 Apr
12
139
14 Sep
16 Sep
17
30 Sep
10*
8057
m
13 Apr
27
9 May
16 May
20 May
12
134
30 Sep
14 Oct
19
18 Oct
11*
8045
m
24 Feb
55
19 Apr
22 Apr
29 Apr
11
159
4 Oct
9 Oct
16
19 Oct
26
March ±
18 days
26 ± 16
days
20 April
26 April
2 May
13 ± 1.5
days
148 ± 9 days
26 Sept
7 October
18 ± 2
13 Oct
± 6 days
± 6 days
± 6 days
± 7 days
± 7.7
days
± 6 days
Mean
580
* Pairs failed to raise a chick to fledging age.
581
582
Table 2. Oceanographic characteristics within the UD kernels during different tracking periods in 2007 – 2008. Values are mean ± sd. SST: sea surface
583
temperature. SSHa: sea surface height anomaly.
Stage of Migration
Water depth (m)
SST (ºC)
Chl a
SSHa (m)
Wind Intensity (m/s)
(mg m-3)
Post-breeding
-3569 ± 1451
8.84 ± 8.33
0.34 ± 0.68
0.07 ± 0.07
1.97 ± 4.22
Northward migration
-3829 ± 1565
23.39 ± 5.96
0.18 ± 0.21
0.05 ± 0.12
0.73 ± 2.13
Nonbreeding region
-2650 ± 1948
9.6 ± 4.31
1.21 ± 1.13
0.007 ± 0.04
0.17 ± 1.0
26
Southward migration
-4061 ± 1530
24.42 ± 5.66
0.18 ± 0.30
0.08 ± 0.07
N/A
584
585
586
27
587
Fig. 1. Migration routes of 16 Short-tailed Shearwaters tracked from Great Dog Island, Tasmania.
588
589
590
Fig. 2. Utilisation distribution map for 16 non-breeding Short-tailed Shearwaters tracked from Great Dog Island, Tasmania.
Coloured polygons represent the 50, 75 and 90% density contours.
591
592
28
593
594
29
595
30