Inter-colony variation in diet and reproductive performance of great skuas Catharacta skua

Abstract

Sustained population growth of great skuas Catharacta skua during the current century has been attributed largely to a high abundance of sandeels, principally Ammodytes marinus, and a plentiful supply of discards from whitefish trawlers in the vicinity of breeding colonies A new colony of great skuas was establ~shed at St Kilda, Outer Hebrides, in 1963 and has since grown rapidly despite an apparent lack of sandeels in the waters surrounding the archipelago and the presence of only a small whitefish fishery. This paper presents an analysis of diet and breeding ecology of great skuas at Hirta, St Kilda, over 3 seasons, 1994 to 1996, with comparative data from Foula, Shetland, the largest and one of the oldest of the British colonies. At St Kilda, breeding adults, nonbreeders and chicks all fed extensively upon other seabirds (44 to 65'\L of pellets over the 3 yr), and to a lesser extent on goose barnacles Lepassp. (18 to 30":, of pellets) and fish (16 to 30'X of pellets), with sandeel virtually absent from the diet (0.2 to 0.3% of pellets). In contrast, at Foula only 4 to 1 2 % of pellets were of other seabirds, with sandeels and d~scarded fish together const~tuting the vast majority ( 4 to 3 6 "~ and 55 to 88% of pellets, respectively). Despite the low availability of sandeels and discards at St k l d a , egg volumes, chick body condition and annual productivity were at least as high as at Foula, and territorial attendance of breeding adults was much higher, indicating considerably lower foraging effort during chick-rearing. Great skuas at St Kilda are therefore able to make effective use of other seabirds as a food resource during the breeding season, in sharp contrast to Foula where adult mortality increased and chick growth rates and breeding success plummeted during the marked decline in sandeel abundance in the late 1980s.

Temporal and spatial variation in the diet of a marine top predator—links with commercial fisheries

“…Previous work has highlighted inter-colony differences in diet composition of great skuas (Phillips et al 1997, Votier et al 2007 which may relate to differences in the availability of the main prey (sandeels, discards and other birds), intra-specific competition or differences in foraging behaviour. A significant positive correlation between whiting otoliths in skua pellets and fish biomass esti- 367: 223-232, 2008 mates (from ICES-FishMap) within 30 km of focal colonies indicates that, for this fish at least, intercolony variation in consumption relates to spatial differences in fishing catches, which presumably relates to differences in whiting availability.…”

Impacts of fisheries on seabird communities

“…If other feeding opportunities remained unchanged, reductions in discarding might result in reductions in numbers of great skuas, herring gulls and lesser black-backed gulls while having little direct negative effect on populations of northern fulmars, northern gannets or great blackbacked gulls. However, increased predation by great skuas as a consequence of diet switching when discard supplies are low might impact a number of other seabird species on which great skuas may feed, including black-legged kittiwakes, Atlantic puffins Fratercula arctica, European storm petrels Hydrobates pelagicus, Leach's petrels Oceanodroma leucorhoa, red-throated divers Gavia stellata, common eiders Somateria mollissima, great blackbacked gulls, and Arctic skuas Stercorarius parasiticus (Phillips et al, 1997(Phillips et al, , 1998, although variation in sandeel abundance may be a greater influence than variation in discard availability for predation rates of great skuas on other seabirds (Hamer et al, 1991;Ratcliffe et al, 1998aRatcliffe et al, , b, 2002Catry and Furness, 1999;Ratcliffe and Furness, 1999). Probably bird killing by great skuas will be a function of both sandeel and discard abundance (Caldow and Furness, 2001).…”

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

“…Skuas (family Sterorcorariidae; taxonomy follows BirdLife International) are a group of highly opportunistic predator-scavengers (Furness 1987, Phillips et al 1997, 2004a. During the breeding season, some species or populations catch few or no prey by themselves at sea, which is perhaps why skuas are sometimes excluded from comparative analyses of seabird ecology and life history (e.g.…”