1. Great skuas Catharacta skua are the only member of the genus Catharacta in the northern hemisphere. The UK holds around 60% of the World population with 8,000 breeding pairs. 2. Sustained population growth of the great skua in Scotland during the last century appears attributable to an abundant supply of discards (from commercial fisheries) and sandeels Ammodytes marinus. In addition great skuas may scavenge or predate other seabirds and their chicks when other prey is in short supply. 3. Following a decline in sandeels in the 1980s there is some evidence that an increase in predation of seabirds by great skuas may be affecting seabird populations. Future measures to reduce the amounts of fish discarded may result in a further increase in predation by great skuas. This highlights the need to quantify the current impact of great skua predation on seabird populations, and monitor any future change. 4. Accurate assessment of great skua diet is fundamental to this type of research. Many studies have used pellets of indigestible prey to assess diet in skuas and gulls, but have not quantified this technique. Captive great skuas were fed a range of fish and birds to try and understand more fully how pellets reflect diet. Feeding trials showed that skuas fed on a diet of birds produced more pellets than when feeding on fish. Fish species strongly influenced the number of pellets produced as well as the proportion and size of otoliths recovered. The numbers of pellets cast also varied significantly among differing species of bird meals. Field trials revealed that only a small proportion of pellets produced are being sampled. This study highlights the need to carefully validate the use of pellets to assess diet, particularly in a species of conservation concern. 5. The diet of great skuas was estimated based on five different techniques (pellets, prey remains, spontaneous regurgitates, observed feeds and water off-loading) and the results compared. The diet composition based on five sampling techniques in a single year generally showed a good correlation with one another. However comparing the proportion of the three main prey types estimated by four sampling techniques over three years revealed a significant interactive effect of year and sampling technique on the diet composition. While estimates of diet using different sampling techniques may be broadly comparable, technique dependent biases mean that the advantages and disadvantages of each sampling technique need to be borne in mind before conducting diet studies. A small proportion of great skuas breeding at Hermaness, Shetland exhibit distinct dietary specialisation, feeding almost exclusively upon seabird prey. Around half of these "bird-specialists" defend feeding territories within a section of seabird colony, the remainder foraging away from breeding territories. "Bird-specialists" retained their feeding habit and, if present, territory, between years. Time-budgets revealed that "bird-specialists" with feeding territories spent less time foraging than "bird-specialists" without a feeding territory or skuas feeding predominantly on fish. Results of radiotracking great skuas for the first time suggest that "bird-specialists" have smaller home ranges than "others". In all years "bird-specialists" show similar productivity to "others", but earlier hatching dates (a good measure of quality in great skuas). While we do not know whether high quality skuas feed on seabirds or that feeding on seabirds advances laying date, hatching early is likely to confer an advantage to "bird- specialists". Non-specialist great skuas experienced a reduction in clutch volume and chick condition during 1999, compared with 1998 - presumably due to a reduction in food availability. "Bird-specialists" did not experience a similar decline in clutch volume and chick condition between years, and showed higher clutch volume and chick condition than "others" in 1999. In addition to changes in clutch volume and chick condition, adult non-specialists showed reduced annual survival, compared with "bird-specialists" over the same period. These results suggest that "bird-specialists" not only have earlier hatching dates in all years, but in certain years also gain an advantage in terms of improved chick condition and adult survival that may have implications for lifetime reproductive success (LRS). Apparent fitness benefits derived from specialising in bird predation may have conservation implications for seabirds colonies in Shetland
