Population and breeding dynamics of European Shags Phalacrocorax aristotelis at three major colonies in Shetland, 2001-15

Abstract

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.