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 degrees 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 degrees 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.