Blog post by Dr Allan Perkins, Senior Conservation Scientist, RSPB Centre for Conservation Science.
Arctic skuas are spectacular birds, pirates among our seabird communities. Having spent the winter, off Namibia and South Africa, they return each spring to nest on the coastal heaths and clifftops of northern and western Scotland.
They are a kleptoparasite. This means they steal most of their food from other birds - involving aerial chases with aerobatic twists and turns until their victim drops its food. In UK waters, smaller seabirds such as terns, kittiwakes and auks act as the skuas’ main ‘hosts’, and most skuas nest close these species’ breeding colonies.
Their actual food is small fish, especially sandeels, and skuas exploit the ability of other seabirds to catch these below the sea’s surface.
Arctic skua adult (pale phase).
Scotland lies at the southern edge of the Arctic skua’s global breeding range, with Shetland and Orkney their stronghold, but with other colonies in Sutherland, Caithness, and the Hebrides as far south as Jura.
The last national seabird census (Seabird 2000) recorded 2136 Apparently Occupied Territories (AOTs), compared with 3388 AOTs in the previous 1985–88 census. Since Seabird 2000, numbers have continued declining, and Arctic skuas now on the Red list of Birds of Conservation Concern, with breeding records now sought by the Rare Breeding Birds Panel.
The study...
We wanted to assess the relative impacts on breeding Arctic skuas of ‘bottom-up’ pressures caused by food shortage, and ‘top-down’ pressures from predation. Findings from the study have just been published in the Journal of Animal Ecology.
Using regional and national datasets, notably the Seabird 2000 census and the UK Seabird Monitoring Programme (SMP), we collated data on Arctic skuas and five other species for 1992–2015 from colonies throughout Orkney and Shetland, and also Handa island in northwest Scotland.
These included four major species exploited by Arctic skuas – kittiwake, guillemot, puffin and Arctic tern – whose population size and breeding success we used as an index of food availability around Arctic skua colonies.
Arctic terns are one of four seabird ‘hosts’ that Arctic skuas in Scotland frequently kleptoparasitise (steal food from).
For predation pressure, our measure was the local density of great skuas (or bonxies). These are another pirate of the seas, and a large, powerful predator weighing in at around 1.2–1.6 Kg compared with just 0.3–0.6 Kg for the Arctic skua.
Although not as agile as Arctic skuas, great skuas also steal food from other seabirds. However, they also frequently kill and eat the seabirds themselves, including chicks, fledglings and sometimes even the adult Arctic skuas.
Great skuas also scavenge bird and animal carcasses as well as waste fish discarded from trawlers - utilising a variety of food resources has helped increase their population size in Scotland.
In the 1985–88 census, 7645 AOTs were recorded, but this increased to 9635 AOTs in the Seabird 2000 census, including colonising new breeding areas in western Scotland and Ireland. Although in many ways this is regarded as a conservation success story (our islands hold around 57% of the global population), several studies during the 1990s and 2000s revealed that great skuas were negatively impacting other seabirds at some colonies.
Adult great skua
We used digital mapping and statistical models to quantify changes in population size and breeding success of Arctic skuas, their hosts, and great skuas. We also used it to determine the relative effects of host breeding success and great skua density on Arctic skua breeding success and population trends.
We compared these trends and effects between different types of Arctic skua colony, categorised by the local abundance of cliff-nesting hosts (i.e. kittiwakes, guillemots and puffins).
What did the study find?
Between 1992 and 2015, Arctic skuas declined by 81%, and their average breeding success fell sharply, from around 0.9 chicks per pair in the early 1990s to 0.3 chicks per pair by the 2010s.
It seems clear that reduced breeding success is driving the population decline of Arctic skuas, and the two were strongly correlated, although large fluctuations between consecutive years at some sites suggested other factors at play. These may include variation in adult survival, or large scale non-breeding in years with exceptionally poor food availability.
Amongst the hosts, average Arctic tern, kittiwake and puffin colony sizes declined by approximately 90% each, and guillemots by 42%. We should stress that although indicative of the severity of declines, these figures are not derived from complete censuses of the entire populations in Orkney and Shetland and should not be used in that context – ie they are from regularly monitored colonies that hold only a proportion of the whole population.
Breeding success of hosts also declined significantly, indicative of large-scale drivers of food shortage in most years since 2000, linked to climate change. A large body of research points to increasing sea temperatures and oceanographic changes making the waters around Orkney and Shetland less hospitable for sandeels.
Unsurprisingly, Arctic skuas fledged more chicks in years when their hosts had good breeding success – ie in years when sandeels were abundant.
However, their population trends and breeding success were negatively associated with great skua density, and variation between colony types suggested that Arctic skuas were most sensitive to top-down predation pressures at colonies where great skuas had increased the most. Great skuas had increased by a colony average of 75%, but this masked substantial variation. The largest colonies showed declines while other sites had been colonised, such that overall great skua numbers had remained stable.
It seems that great skuas have to some extent redistributed themselves within Orkney and Shetland, with populations declining at the super-colonies of Hoy and Foula, but increasing rapidly on other islands including those with relatively small cliff-nesting seabird colonies (hundreds to low thousands of birds) such as Mousa, Fetlar, Rousay and Papa Westray.
These islands also saw some of the largest declines in Arctic skuas, and it would appear that great skuas here are adding top-down pressures onto a species that is already facing significant bottom-up pressures due to lack of food. At very large seabird colonies such as Fair Isle, Hermaness and Handa, great skuas have also increased, but Arctic skuas at these types of colonies appear to be more sensitive to bottom-up pressures (lack of food) than to predation pressure from great skuas.
How you can help?
The good news is you can help future projects by participating in seabird monitoring. See our blog post on it, and get involved…