In the 1998-2002 Seabird 2000 census, Shetland held 19% of the British and Irish breeding population of European Shags Phalacrocorax aristotelis (32,300 apparently occupied nests - AON), and the three largest colonies in Shetland (Fair Isle, Sumburgh Head and Foula) together held 44% of the total for the county. Subsequent monitoring at these colonies recorded substantial decreases in population size in 2004-05,2008 and 2011-13. This paper describes European Shag population and breeding dynamics at these three colonies for the period 2001-15, using annual monitoring data for six demographic parameters. Demographic changes were characterised by major reductions in breeding population size (AON), timing of breeding getting later, and considerable reductions in the percentages of nests that progressed to incubation and to hatching, and in overall breeding success. The 2004 and 2005 breeding seasons were exceptionally poor in Shetland but also at colonies elsewhere in north and east Scotland, apparently reflecting large-scale scarcity of sandeel Ammodytes prey. There was no such Scotland-wide (or Shetland-wide) uniformity in these breeding parameters in 2008 or 2011-13, when local food availability within foraging ranges of colonies appeared to be the main driver of European Shag breeding performance in Shetland. At Fair Isle, Sumburgh Head and Foula, breeding was markedly early in 2014 and 2015, and percentage incubation, percentage hatching and overall breeding success were all high. However, population sizes at these colonies remained low in 2014-15, with a combined deficit of c. 2,600 AON in comparison with the Seabird 2000 census figures. Possible mechanisms driving this situation, for example persistent non-breeding, emigration, or high mortality are evaluated. Given the high colony fidelity of European Shags once established as breeders, it is assumed these 'missing' birds are dead. Support for this assertion comes from the Shetland beached bird survey which indicated high mortality in late winter in 2003, 2011 and 2014, in the latter two years associated with prolonged gales. European Shag has recently been added to the UK Red list because of severe population decline, and continued (indeed enhanced) monitoring and ringing of the species is to be encouraged.

European Shags Phalacrocorax aristotelis (hereafter 'Shag') breed colonially along rocky coasts of the northeast Atlantic and the Mediterranean Sea. Although they build substantial nests, monitoring changes in breeding numbers and their breeding success is complicated by their asynchronous, lengthy and often variable breeding season, such that even a well-timed single count of nests is likely to record only about 75% of nesting attempts in a given year (Harris & Forbes 1987). In some years a substantial proportion of adults do not breed, which can result in an abrupt drop in nest numbers relative to previous years, and non-breeding can persist for several successive seasons (Aebischer 1986; Aebischer & Wanless 1992; Harris & Wanless 1996; Wanless & Harris 2004). Abrupt drops in counts of nests have also been recorded following oil spills (Heubeck 1997), weather-related winter mortality events (Harris & Wanless 1996; Harris et al. 1998; Frederiksen et al. 2008), or mortality events during the breeding season (Coulson et al. 1968).

The Seabird 2000 census recorded 32,306 apparently occupied nests (AON) of Shag in Britain and Ireland in 1998-2002, an estimated 40-45% of the world population (Wanless & Harris 2004). The largest concentration was in Shetland, which held 19% (6,147 AON) of the British and Irish total. The three largest colonies in Shetland, at the remote islands of Foula (2,277 AON; then the largest colony in Britain and Ireland, and one of the largest in the world) and Fair Isle (663 AON), and at Sumburgh Head at the southern tip of Mainland Shetland (270 AON), together held 44% of the Shetland total (Figure 1). At all three colonies, however, numbers had declined between the 1985-88 Seabird Colony Register and Seabird 2000 censuses, by 5% at Foula, 40% at Fair Isle, and 44% at Sumburgh Head.

Shag breeding success has been monitored at Fair Isle since 1986, Sumburgh Head since 1988, and Foula since 1997, and by 2000 each colony had averaged relatively high values of 1.42 ± 0.06 SE, 1.20 ± 0.08 SE, and 1.13 ± 0.22 SE chicks fledged per AON, respectively (Mavor et al. 2002). There was relatively low success at Fair Isle and Sumburgh Head in 1990, when the local abundance of Lesser Sandeels Ammodytes marinus was low (Walsh et al. 1991; Wright & Bailey 1993; Monaghan et al. 1997), and at Sumburgh Head and Foula in 2000, when nests were washed away by heavy seas in mid June (Mavor et al. 2001). Otherwise, Shags in Shetland seemed less susceptible to the food-related breeding failures experienced by species such as Black-legged Kittiwake Rissa tridactyla and Arctic Tern Sterna paradisaea (Monaghan et al. 1989; Harris & Wanless 1990; Monaghan et al. 1997; Heubeck et al. 1999), and by 2000 no major non-breeding events had been recorded at Fair Isle, Sumburgh Head or Foula.

This situation changed dramatically in 2004, when Shags largely deserted Foula during the breeding season (Mavor et al. 2005). In 2008, a similar crash in breeding numbers and a major breeding failure occurred at Fair Isle (Shaw et al. 2008). Then, in 2011, extensive non-breeding first became evident at Sumburgh Head (Heubeck & Mellor 2012; JNCC 2014).

Accordingly, this paper documents Shag population and breeding dynamics at these three Shetland colonies in 2001-15 using annual monitoring data for six demographic parameters, namely breeding population size (counts of AON), breeding phenology (median date chicks were first recorded at nests), the percentage of nest-building pairs that laid (percentage incubation, used as a measure of non-breeding), the percentage of incubating nests at which young were recorded (percentage hatched), the percentage of nests at which young were recorded from which at least one chick fledged (percentage fledged), and breeding success (chicks fledged per incubating nests). We quantify, test and compare temporal patterns of change in these parameters between and within the three colonies, make comparisons with Shag population changes recorded elsewhere in Shetland, and examine beached bird survey data from Shetland for evidence of abnormal mortality during this period.

Seabird monitoring on Fair Isle was funded by Fair Isle Bird Observatory Trust, with contributions from the Joint Nature Conservation Committee; thanks to all Fair Isle Bird Observatory staff members and volunteers in 2001-15 for data collection. SOTEAG monitoring is funded by the Sullom Voe Association Ltd; thanks to all who assisted with boat surveys, especially Billy Fox, Paul Harvey, Roger Riddington, Newton Harper and George Lamont Williamson. Afra Skene, Jonathan Swale and Glen Tyler (Scottish Natural Heritage), and Paul Harvey (Shetland Biological Records Centre) provided details of counts on Noss and Foula, while Rory Tallack and Andy Webb helped produce the figures. Drafts of the manuscript were greatly improved by comments from Mike Harris, Sarah Wanless and Andy Webb, and by referees Francis Daunt and Svein-Håkon Lorentsen.

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