Conservation implications of variation in diet and dietary specialisation in great skuas

Votier, Stephen C. (2001) Conservation implications of variation in diet and dietary specialisation in great skuas. PhD thesis, University of Glasgow.

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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.

6. 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.

7. Using a bio-energetics model it is estimated that the great skua population at Hermaness requires 428.9 x 10^ kJ during 1999. 80% of the energy demanded was required by breeding adults with less than 5% being required by breeding adult “bird-specialists”. Combining data on energy requirements and prey composition we estimate that great skuas at Hermaness feed on 69.3 tonnes of fish, 6.9 tonnes of seabirds, 2,5 tonnes of rabbit Oryctolagus cuniculus and 0.6 tonnes of goose barnacles Lepas sp. The number of seabirds estimated to have been consumed by great skuas was over 11,000, the majority being auks (Alcidae) (71%). While “bird-specialists” made up only 5% of the population they consumed 30% of the seabirds. While the number of seabirds being taken by great skuas is large, the impact on populations is not clear. Despite the large numbers consumed, the impact by great skuas is much less severe than has been reported at other smaller skua colonies. Declines in discarded whitefish may result in an increase in seabird predation, but to an unknown extent.

8. In light of likely changes in food availability and concern about the effect that this might have on great skuas preying on other seabird species, we compare the diet of great skuas at two Shetland colonies in three different years. We also compare the proportion of the great skua population which specialise in feeding upon seabirds at three colonies and among three years. While the main component of great skua diet is variable both within colonies and among years, clear trends in a switch away from discarded fish species have not been observed. Similarly while fish species vaiy among years and between colonies, there are no clear trends in a switch to larger discards. The species of bird prey in the diet varied between “bird-specialists” and non-specialists, between colonies and among years. A decrease in the proportion of black-legged kittiwakes tridactyla in the diet was compensated for by an increase in either auks (Alcidae) or northern fulmars Fulmarus glacialis. A consistent proportion of “bird-specialist” pairs at study plots suggested no increases among three years. The proportion of “bird-specialist” pairs appears to be related to the size of the great skua colony.

9. Future study requires continued monitoring of great skua diets and further investigation into factors regulating numbers of “bird-specialists” at all major great skua colonies. Integrating data on great skua diet and predation with studies of diet and reproductive performance of other seabird populations may indicate the effect of changes in discarding policy more fully.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Ecology, conservation biology, skuas, Catharacta, nutrition.
Subjects: Q Science > QL Zoology
Colleges/Schools: College of Science and Engineering
Supervisor's Name: Supervisor, not known
Date of Award: 2001
Depositing User: Enlighten Team
Unique ID: glathesis:2001-73172
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jul 2022 10:34
Thesis DOI: 10.5525/gla.thesis.73172
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