Saturday, 1 September 2018

Shetland Skuas

The population of Arctic Skuas has declined by 81% according to the latest report by the British Ecological Society.

The National Seabird 2000  census recorded 2,136 Arctic and 9634 Gt Skuas with 90% of the population in Orkney and Shetland

Full report

Locations of Arctic skua colonies used in analyses of population trend (n = 33 sites) and annual productivity (underlined, n = 20 sites). Dotted line = southernmost extent of Arctic skua breeding range during Seabird 2000. Overall percentage change in Apparently Occupied Territories (AOTs) between the first and last year surveyed (1992 and 2008–2015) is given for each site (see Supporting Information Table S1 for further details). Symbol sizes are scaled to AOT counts in 1992 (four categories: 0–12, 16–26, 30–66, 101–159). Symbol shape denotes colony type, classified by cliff‐nesting seabird host abundance within 5 km of colony during the Seabird 2000 census: squares = type 1 colonies (>10,000 pairs), circles = type 2 colonies (1,000–10,000 pairs), triangles = type 3 colonies (<1,000 pairs)

Arctic Skuas declined at all the study colonies from 1061 ATO 's (1992) to just 200 in (2015), indicating the largest declines took place in the north with the largest productivity in Handa in NW Scotland.

Attacks on Kittiwakes which had only come down onto a loch to bathe  

Arctic Skua nest next to colonies of seabirds either Arctic Terns, or cliff dwelling birds that usually feed on Sandeels - Puffin, Kittiwake, Guillemot and Razorbill. These skuas rely on stealing food from these birds, this is called Kleptopasrasitism

An Arctic Tern attacks an Arctic Skua

Gt Skuas increased at most sites except Foula where a 24% decline occurred. Its odd that the greatest declines in Arctic Skuas occurred where Gt Skua population were small. Food availability and density of Gt Skua were associated with breeding success of Arctic Skua

Gt Skuas are larger and more powerful that Arctic Skuas , they rely on harassing large birds such as gannet for a free meal

Gt Skua seen off by a Fulmar

The world's Gt Skua population is around 16,000 with 57% being in Scotland (Bird life International 2018) especially in Shetland and Orkney. Around 9,634 ATO (Apparently Occupied Territories) can be found in the UK (2004)

They are very aggressive towards humans and animals that stray onto their breeding grounds, making a head on attack and sometime hitting the intruder with their feet.

Gt Skua population in Shetland has fluctuated with numbers up and down at various throughout the isle. Gannet numbers are increasing so you would think that there would be more fish brought back to young gannets and therefore more opportunities for GT Skuas . Noss for instance has around 25000 Gannets double that of 1970

On Foula in 2015 a lot of cannibalistic predation of well grown chicks was again high, although a census count of 1,846 AOT was 11% higher than 2007

In Shetland, 4 colonies at Hermaness, Noss, Mousa and Fair Isle had 1,747 ATO's in 2013, an increase of 27% since  2007. But on Foula a decrease of 28% occurred between 2000 & 2007 (2,293 down to 1,657 ATO's)

In Orkney, a complete census revealed a 23% decrease between 2000 - 2010 (1,710 ATO's)

Discards from fishing boats used to be an important part of the Gt Skuas diet in Shetland but with the reduction of discards they have now started to either concentrate on attacking Gannet for fish, eating birds such as Puffin or resorting to cannibalism .

Having just taken part in the 2018 Seabird census I am very interested in seeing the current figures

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