Great skuasIUCN Red List status: Least Concern
There are an estimated 16,000 breeding pairs of Great skuas in the world, all of whom breed in the northern hemisphere, mainly in Iceland, the Faroe Islands and Scotland.
An estimated 60% of this global population breed in Great Britain, specifically Scotland. In other words, conservation of the Scottish breeding populations is critical to the whole species.
Since the 1970s, Great skua numbers in Britain increased by up to 70%, primarily as a result of an increased availability of discards from commercial fisheries.
However, the last decade of extremely poor breeding seasons (2009 notwithstanding) for Scottish seabirds in general, and for Great skuas specifically, are a cause for concern. It is not yet clear the effect this had had on numbers of Great skuas, but there are indications that overall they have declined.
Conservationists also worry about the effect that Great skuas have on the other seabirds on which they prey (and increasingly so, as the availability of discards has now reduced again, causing skuas to eat more birds).
This is especially worrisome when Great skuas are breeding close to globally importnat colonies. An example of this is occurring on St. Kilda, where it is believed that the bonxies are having a serious impact on Leach's storm-petrels. Remarkably, the highly adaptable skuas have even learnt to hunt the storm-petrels at night.
In Great skua terms, Handa Island is a conservation success story: a regional stronghold for this nationally important species; the largest breeding colony on the west coast of Scotland, and probably a source population for other colonies in the area. The population has continued to increase slightly in recent years though it is, we believe, now approaching the carrying capacity of the island.
Moreover, against the background of declines and poor breeding seasons in Scottish colonies since 2003, especially those in the Northern Isles (where the most bonxies traditionally occur), the Handa Great skua colony has been relatively productive - with the mean number of chicks fledged per pair remaining consistently above the national average.
On Handa, Great skuas have a diverse diet (see research page for more details), though it is dominated by fish and birds.
We have found that the proportion of birds in their diet has slightly decreased in recent years, although so has the size and breeding success of most of Handa's other seabird colonies, including common guillemot, razorbill and kittiwake.
However, these seabird declines are typical of colonies across Britain since 2003, and while predation by Great skuas surely adds to the problem, observations of abandoned eggs and starving chicks on the breeding ledges of these other species point to the same major underlying causes as elsewhere - namely a collapse in the availability of fish, due to a combination of overfishing and rising sea temperature.
Arctic skuasIUCN Red List status: Least Concern
Globally, as a species, Arctic skuas are not threatened, with possibly half a million birds breeding on the Russian tundra and in several countries of northern Europe.
However the local picture in northern Scotland, the British stronghold for this bird, is very different, and there is genuine concern among UK conservationists over its alarmingly rapid decline.
The latest preliminary analysis of UK seabird colony monitoring data for the JNCC shows a decline in breeding pairs by 35% between 1986-2007. It is likely that the total UK population is now no higher than 1500 breeding pairs, and possibly much less.
Moreover, in 2007 and 2008, many of the largest colonies (in Shetland and Orkney) failed altogether. 2009 has been better, but they will need a series of good seasons to halt the decline.
It should be remembered that the usual "victims" of the Arctic skua's piracy, such as kittiwakes and auks, have been struggling to find food themselves and for their chicks, meaning there is also less food for the skuas. In this interdependent food chain, the reduced availability of fish at critical times of the breeding cycle is also at the root of the decline of the kleptoparasites and the higher predators.
Nevertheless, it has been observed that many of the most severe declines in Arctic skuas in the UK have occurred where they are breeding adjacent to Great skua colonies - who are known to prey on Arctic skua eggs and chicks, and are more likely to exploit this food source when less fish is available.
On Handa, the relatively small Arctic skua breeding population peaked around the year 2000 and has declined since then to about 20 pairs.
Breeding from 2005-8 was poor, with very few chicks surviving. While sad to see, this provides an interesting case study, we have found that predation by Great skuas has been a major factor - especially predation of fledglings as they practise their weak flight around the territory during their first few airborne days.
That said, in 2009 they appear to have launched a fightback, with their most successful breeding season in five years. This follows a local increase in density of Arctic skua territories, and a resurgence of effective "cooperative defence" against the invading bonxies - a tactic which had not been seen for a few years, but which appears to be an important strategy.
We will be watching and learning over the next few years, to see whether and how the Arctic skuas can build on the success of 2009, and recover from the recent desperate times.