Movements, winter distribution and activity patterns of Falkland and brown skuas: insights from loggers and isotopes

Abstract

In the first published study of the wintering ranges and activity patterns of skuas from any colony, we combined tracking (geolocator) and stable isotope analysis in a comparison of migration behaviour of brown skuas Catharacta lonnbergi and Falkland skuas C. antarctica from South Georgia and the Falkland Islands, respectively. Brown skuas, particularly failed breeders, departed and returned to the colony much earlier than Falkland skuas, and 2 of 3 brown skuas performed a pre-laying exodus. During winter, brown skuas were distributed widely over deep, oceanic water within the Argentine Basin (37 to 52°S) between the Antarctic Polar Front and the northern sub-tropical Front. Falkland skuas, by comparison, wintered mainly in subantarctic waters around the central Patagonian shelf-break (40 to 52°S). Much greater overlap existed among core areas within than between species, and sex did not influence distribution. The partial inter-specific spatial segregation was also reflected in a divergence in activity patterns, with brown skuas in flight for a greater proportion, and more time on average, during both daylight and darkness. Both species of skua spent far more time on the water than do foraging albatrosses, and there was limited overlap between their nonbreeding distributions and those of large procellariids from the same archipelagos. Stable isotope signatures of brown skua feathers indicated that distributions of tracked birds were typical of most or all of the breeding population, and were consistent from year to year. None was characteristic of species that winter on adjacent continental shelves or off south-west Europe. Isotope values also suggested a mixed diet for brown skuas of zooplankton, low trophic-level squid and fish, with little or no reliance on seabird predation or fisheries.

Phylogeography of the southern skua complex—rapid colonization of the southern hemisphere during a glacial period and reticulate evolution

“…TMRCA estimates date around 150,000 yBP but a larger sample size is needed to confirm this. Furthermore, the geographic position of the Falklands, and the overlap in nonbreeding distribution of Falkland Skua with Brown Skua (Phillips et al, 2007) suggests a higher likelihood of genetic exchange with other taxa, potentially hampering the exact disclosure of historic processes. However, glaciation of Antarctica was near its maximum at 150,000 yBP whilst the Falklands were probably never heavily glaciated (McDowall, 2005).…”

Consistency in migration strategies and habitat preferences of brown skuas over two winters, a decade apart

“…Although habitat use of brown skuas was 393 described previously in terms of differences in bathymetry, sea surface temperature and 394 chlorophyll a concentrations between core and peripheral areas (Phillips et al 2007), this is 395 13 the first study to present a robust model of habitat preference, accounting for availability. The 396 loggers deployed in summer 2011/12 had no deleterious effect on adult survival; indeed, 397 return rates were higher, but not statistically significant, for birds with devices than for control 398 birds.…”

“…The low 295 10 return rate of instrumented birds was attributed to the accretion of goose barnacles Lepas 296 spp. to the self-amalgamating tape wrapped around the logger causing an increase in load 297 on the leg, and is discussed in Phillips et al (2007). Of 25 birds fitted with a logger in 298 2011/12, 22 returned to breed in the following season, from which 19 loggers were retrieved 299 (three loggers were lost).…”

“…Departure date was 130 identified as that immediately prior to a directed movement away from South Georgia and 131 consecutive nights thereafter that were spent largely on the water, whereas return date was 132 that preceding the first night spent dry (on land). As skuas with eggs or chicks at South 133Georgia never forage at sea, after the initial return from migration, subsequent periods of 134 consecutive days with intervening nights spent largely on the water were classified as pre-135 laying trips (Phillips et al 2007, Carneiro et al 2014. 136…”

“…To date, 102 non-breeding ranges of brown skuas have been inferred from the distribution of just six birds 103 tracked from South Georgia during the winter of 2002 using global location sensors (GLS); 104 these birds dispersed over deep, subantarctic and mixed subantarctic-subtropical waters 105 within the Argentine Basin (Phillips et al 2007). However, stable isotope analyses of 106 feathers from a larger sample suggested that some individuals may use continental shelf or 107 shelf-slope waters ).…”