Janet R. D. Silk

Tracking of Arctic terns Sterna paradisaea reveals longest animal migration

The study of long-distance migration provides insights into the habits and performance of organisms at the limit of their physical abilities. The Arctic tern Sterna paradisaea is the epitome of such behavior; despite its small size (<125 g), banding recoveries and at-sea surveys suggest that its annual migration from boreal and high Arctic breeding grounds to the Southern Ocean may be the longest seasonal movement of any animal. Our tracking of 11 Arctic terns fitted with miniature (1.4-g) geolocators revealed that these birds do indeed travel huge distances (more than 80,000 km annually for some individuals). As well as confirming the location of the main wintering region, we also identified a previously unknown oceanic stopover area in the North Atlantic used by birds from at least two breeding populations (from Greenland and Iceland). Although birds from the same colony took one of two alternative southbound migration routes following the African or South American coast, all returned on a broadly similar, sigmoidal trajectory, crossing from east to west in the Atlantic in the region of the equatorial Intertropical Convergence Zone. Arctic terns clearly target regions of high marine productivity both as stopover and wintering areas, and exploit prevailing global wind systems to reduce flight costs on long-distance commutes.

SUMMER DISTRIBUTION AND MIGRATION OF NONBREEDING ALBATROSSES: INDIVIDUAL CONSISTENCIES AND IMPLICATIONS FOR CONSERVATION

Many birds show a surprising degree of intraspecific variability in migratory tendency and choice of wintering site. In this study, we tracked the seasonal movements of 35 nonbreeding Black-browed Albatrosses Thalassarche melanophrys from South Georgia, including 24 birds followed in two consecutive years. This revealed consistent patterns of status-related, sex-specific, and individual variation in wintering strategies, and provided the first description of the summer distribution of failed/deferring breeders. Individuals exhibited a striking degree of site fidelity, returning to the same region (southwest Africa or Australia) and showing correlated centers of distribution, as well as remarkable consistency in the chronology of their movements, in consecutive years. Nonetheless, a degree of behavioral flexibility remained, and particularly on the return migration, birds moved between, or bypassed, alternative intermediate staging sites depending on local circumstances. Initiation of the outward migration varied according to breeding status, timing of failure, and sex: deferring breeders and those that failed early departed two months before successful birds, and successful females departed 1–2 weeks earlier than males. Sex-related latitudinal variation in distribution was also apparent, with females wintering farther north within the Benguela system. Moreover, the only migrant to Australia was a male, supporting an apparent tendency for male-biased breeding dispersal inferred from genetic analyses. Distribution and timing of movements appeared in general to relate to avoidance of competition from congeners and conspecifics from other populations. From a conservation perspective, the study indicated that, for the declining Black-browed Albatross population at South Georgia, the primary focus should be toward improving the management (especially reducing bycatch levels) of fisheries in the central and eastern South Atlantic.

Seasonal sexual segregation in two Thalassarche albatross species: competitive exclusion, reproductive role specialization or foraging niche divergence?

Sexual segregation by micro– or macrohabitat is common in birds, and usually attributed to size–mediated dominance and exclusion of females by larger males, trophic niche divergence or reproductive role specialization. Our study of black–browed albatrosses, Thalassarche melanophrys, and grey–headed albatrosses, T. chrysostoma, revealed an exceptional degree of sexual segregation during incubation, with largely mutually exclusive core foraging ranges for each sex in both species. Spatial segregation was not apparent during brood–guard or post–guard chick rearing, when adults are constrained to feed close to colonies, providing no evidence for dominance–related competitive exclusion at the macrohabitat level. A comprehensive morphometric comparison indicated considerable species and sexual dimorphism in wing area and wing loading that corresponded, both within and between species, to broad–scale habitat preferences relating to wind strength. We suggest that seasonal sexual segregation in these two species is attributable to niche divergence mediated by differences in flight performance. Such sexual segregation may also have implications for conservation in relation to sex–specific overlap with commercial fisheries.

Extrinsic and intrinsic determinants of winter foraging and breeding phenology in a temperate seabird

In temperate regions, winter presents animals with a number of challenges including depressed food abundance, increased daily energy requirements, higher frequency of extreme weather events and shortened day length. Overcoming these constraints is critical for overwintering survival and scheduling of future breeding of long-lived species and is likely to be state dependent, associated with intrinsic abilities such as food acquisition rates. We examined the relationship between environmental and intrinsic factors on overwintering foraging and subsequent breeding phenology of the European shag Phalacrocorax aristotelis, a diurnal marine predator. We tested a range of hypotheses relating to overwintering foraging time and location. We found that individuals greatly increased their foraging time in winter to a peak of more than 90% of available daylight at the winter solstice. The seasonal patterns of foraging time appear to be driven by a combination of light levels and weather conditions and may be linked to the availability of the shags principal prey, the lesser sandeel Ammodytes marinus. There was no evidence that shags dispersed south in winter to increase potential foraging time. Foraging time decreased after the winter solstice and, crucially, was correlated with subsequent breeding phenology, such that individuals that spent less time foraging in February bred earlier. The relationship was much stronger in females than males, in line with their more direct control of timing of breeding. Our results demonstrate that pre-breeding intrinsic foraging ability is critical in determining breeding phenology.

Trans-equatorial migration of Short-tailed Shearwaters revealed by geolocators

Abstract Until recent decades, details of the migratory movements of seabirds remained largely unknown owing to the difficulties in following individuals at sea. Subsequent advances in biologging technology have greatly increased our knowledge of seabird migration and distribution, particularly of highly pelagic species. Short-tailed Shearwaters (Ardenna tenuirostris) (∼500 g) have been studied extensively during their breeding season but our understanding of their movements outside this period remains poor. Here, we present the first tracks of the trans-equatorial migration of Short-tailed Shearwaters from a colony on Great Dog Island, Tasmania, Australia. Data were obtained from global location sensors (GLS loggers or geolocators), which enable the estimation of location twice per day based on ambient light levels. After breeding, tracked Shearwaters flew south of the Antarctic Polar Front to a previously unknown stopover site, where they remained for several weeks, before travelling rapidly northward through the western Pacific Ocean to coastal waters off Japan. Short-tailed Shearwaters spent the bulk of the northern hemisphere summer, either in this region or further north in the Bering Sea, before returning south through the central Pacific to their breeding sites. Our results, for the first time, describe in detail the complete migration of this long-lived seabird, reveal individual variation in timing and distribution, and describe the environmental characteristics of their key non-breeding habitats.

Proximate drivers of spatial segregation in non-breeding albatrosses.

Many animals partition resources to avoid competition, and in colonially-breeding species this often leads to divergent space or habitat use. During the non-breeding season, foraging constraints are relaxed, yet the patterns and drivers of segregation both between and within populations are poorly understood. We modelled habitat preference to examine how extrinsic (habitat availability and intra-specific competition) and intrinsic factors (population, sex and breeding outcome) influence the distributions of non-breeding grey-headed albatrosses Thalassarche chrysostoma tracked from two major populations, South Georgia (Atlantic Ocean) and the Prince Edward Islands (Indian Ocean). Spatial segregation was greater than expected, reflecting distinct seasonal differences in habitat selection and accessibility, and avoidance of intra-specific competition with local breeders. Previously failed birds segregated spatially from successful birds during summer, when they used less productive waters, suggesting a link between breeding outcome and subsequent habitat selection. In contrast, we found weak evidence of sexual segregation, which did not reflect a difference in habitat use. Our results indicate that the large-scale spatial structuring of albatross distributions results from interactions between extrinsic and intrinsic factors, with important implications for population dynamics. As habitat preferences differed substantially between colonies, populations should be considered independently when identifying critical areas for protection.