Guillemot
Watching
The Guillemot is one of three auks you are likely to see on and around Skomer. The other two there are the Puffins and the Razorbill. Guillemots occupy cliff ledges right around the island, and they also breed on the nearby islands of Skokholm, Midland, Ramsey and Grassholm, although in much smaller numbers than they do on Skomer. They also occur on some mainland cliffs in Pembrokeshire, most notably Elegug Stacks on the Castlemartin Peninsula and Stackpole Head. The Skomer Guillemot colony is the largest and most important in Wales, and larger than most others in England, the birds being best seen at High Cliff and on The Wick.
Look carefully among the Guillemot colonies and you may be fortunate enough to see a variant form known as the ‘bridled’ Guillemot. This has a white eye-ring from which a white line runs towards the nape. About one per cent of the Guillemots on Skomer are ‘bridled’, the proportion rising the further north you travel, reaching over 50% in Iceland.
Populations
At the present time the Guillemots population on Skomer and probably throughout the Irish Sea is doing better than at any time in the last 100 years. Until recently the story of Guillemot population changes had been a depressing one. We do not know how many bred on Skomer at the beginning of the 20th century, but we know from photographs taken at the time that the population was very large, possibly in the order of 20,000 pairs. However, sometime between then and the 1940s numbers started to decrease. When Ronald Lockley photographed the Wick in 1934 Guillemots bred continuously along the main fault running the length of the cliff. By 1946 numbers had declined and there were several gaps in this colony.
Threats
Guillemots are often the main victims during oil pollution incidents, and it is possible these declines were the result of the practice of washing tanks at sea, and the shipping toll during the Second World War. More recently oil tanker disasters like the Sea Empress have caused many casualties.
Annual counts of Guillemots on Skomer were started in 1963, and have been made ever since. These show that numbers continues to decline through the 1960s and reached their lowest level in 1970 following the ‘Irish Seabird Wreck’. In that year less than 2000 pairs bred on the island. Numbers have generally been on the increase since, with a particularly rapid rate of increase recorded between 1990 and 2000. At the beginning of the current century the population stood at nearly 14,000 individuals, and numbers seemed to be levelling out. In 2005, however, a massive increase of 39% occurred, with 19,711 birds counted. It is possible that this increase in numbers was due to an influx of birds from North Sea colonies, as birds in this region generally failed to rear any chicks in the preceding year (2004) due to a shortage of food. The population in 2008 was 17,088. Although Guillemots are normally very site-faithful and generally do not move between colonies, some may desert a site if they experience continued breeding failure.
The 2004 North Sea breeding failure was thought to be a result of warmer winter sea temperatures inhibiting the production of zooplankton (tiny marine animals) which had a knock-on effect further up the food chain. Zooplankton predators such as sandeels and other small fish - the staple diet of many seabirds - also failed to reproduce, creating the catastrophic food shortage.
Although Skomer escaped this particular seabird catastrophe, as our birds rely on different fish populations, it is perhaps changes in ocean dynamics as a result of climate change that poses the greatest threat to Guillemots and other seabirds in the 21st century.
Migration and Breeding
Guillemots disperse out to sea after breeding, with some occasionally returning too the cliffs from late autumn. Fom April onwards visits are more regular and the first birds start to lay in early May. Guillemots lay a single, large egg. Almost every one is different in its colour and pattern, and this enables the birds to recognize their own egg in the dense groups in which they breed. If the first egg is lost a second may be laid. The chick hatches after 32 days and spends a further three weeks or so at the colony, being fed mainly on Sprats and to a lesser extent, sandeels. One of the most amazing things about Guillemot chicks is that they ‘fledge’ when only part grown and flightless. The chicks leave the colony in late June or early July, departing at dusk. They literally hurl themselves off their cliff ledge and parachute down to the sea where the male is waiting, then quickly swim out to sea, presumably to avoid predatory gulls. The male then looks after the chicks for a further four to six weeks, after which it becomes independent. By early August the last Guillemots have left the cliffs and the ledges seem strangely hushed after three months of hectic activity.
Research
Since the early 1970s Guillemots on Skomer have been the subject of a long term study. This has had several objectives, including and investigation of their social behaviour, breeding biology and population dynamics. These factors are all closely interrelated. Guillemots typically live shoulder to shoulder with their neighbours, and in areas, such as the seaward end of the Wick, they breed at incredible densities – 30 pairs per square metre! Such high density living requires complex social relationships, but also provides a major benefit. By breeding in dense groups Guillemots are able to deter Herring Gulls, Great Black-backed Gulls and Ravens, the three most important predators of Guillemot eggs and chicks on Skomer. The proportion of Guillemot pairs successfully raising a chick is highest (90%) where the groups are most dense and the lowest where they are sparse (20%). Overall, about 70% of Guillemots manage to rear a chick each year. Or, to put it another way the average breeding success is 0.70 chicks per pair each year.
Compared with song birds like the Blue Tits or Blackbirds, the output of young by each pair of Guillemots seems very low, so how is the population able to maintain its numbers? The answer lies in the long lifespan of the adults. By marking Skomer birds with colour rings so that each bird is individually recognisable it has been possible to estimate their survival from year to year, and to calculate how long they live. These studies are helped by the fact that Guillemots tend to breed in exactly the same spot year after year. One colour-ringed bird bred on the same tiny ledge on south side of the island for some twenty years. Although Guillemots return to the colony by the time they are three years old most do not breed until they are six years of age, while some do not commence until they are nine. On average it appears that they live for a further ten years or so once they start breeding.
Just how old Guillemots are when they start breeding is another question the research has tried to answer. For several years now several hundred Guillemots chicks have been ringed on Skomer each June, and in subsequent years teams of observers have searched for them. The young first come back in their second or third summer, and from the large groups of birds, referred to as ‘clubs’, seen near the foot of the cliffs at High Cliff and at the seaward end of the Wick. In their fourth year these young birds start visiting the breeding ledges, often returning to within a few feet of where they were hatched themselves! It looks as though breeding first takes place in the fifth year, although this part of the study has not continued long enough to be sure of this.
These studies are helping us to understand the lives of individual Guillemots and also how their populations work. As Britain's most numerous seabird, the Guillemot is a barometer of the health of the wider marine environment, and it is through long-term studies such as these that we can detect any changes that may be taking place.