Abstract The Common Guillemot Uria aalge population on Skomer Island, Pembrokeshire, comprised around 100,000 pairs prior to the Second World War, after which it rapidly declined to the point that by 1972, when the study described here started, it had reached about 2,000 pairs. The aim of the study was to under-stand the population biology of the Guillemot and what was causing the decline. Chronic oil pollution was considered a significant factor. From 1980, the popula-tion began to increase and has since continued to do so at 5% per annum; in 2022, around 21,000 pairs were present on the island. Estimates of adult and immature survival, based on annual resightings of colour-ringed individuals, together with measures of reproductive success, have helped to quantify factors causing the pop-ulation increase. The effects of oil pollution and climate change on survival have also been quantified. Since 1972, the median egg-laying date of Guillemots on Skomer has advanced by over two weeks. This effect is likely to be due to climate change, but so far with no apparent negative effects. An important focus of this study is the reliability and repeatability of the methods employed to monitor Guillemots and identifying the limitations of other monitoring methods.

As an undergraduate, I was invited to join Ian Prestt’s team, who were studying Grey Herons Ardea cinerea,and spent several happy weeks in 1970 at the Monks Wood Experimental Station, Cambridgeshire. I revelled in the station’s scientific atmosphere and enjoyed talking to its researchers. Towards the end of my visit, I mentioned to John Parslow that I would be keen to assist with other projects. A few months later, he suggested that I might help with a project on Common Guillemots Uria aalge. This was another fish-eater at the top of the food chain that, like the Grey Heron, might have been suffering from pesticide poisoning. 

The day after my last university examination, in May 1972, my father drove me to Wales to start several weeks of Guillemot observations on Skomer Island, Pembrokeshire, following Parslow’s instructions. Only later did I recognise just how perceptive John’s approach was in rejecting some of the crude methods employed by earlier Guillemot researchers. Most previous researchers had estimated breeding success by climbing onto the breeding ledges, flushing all the adult Guillemots and counting their eggs and chicks (e.g. Tuck 1961). John Parslow’s common-sense approach was to observe Guillemots from a hide and obtain an estimate of breeding success from birds undisturbed by human activity.

Those few weeks on Skomer in 1972 set the course for the rest of my life. I have been back there every year but two since then to continue that study of Guillemots. Skomer Island, off the western tip of south Wales, is one of the UK’s most important seabird colonies (Saunders & Sutcliffe 2017; Alexander 2021). My aim here is to provide an overview of the rather varied dimensions – ecological, behavioural, conservation and political – entailed in conducting a study that has so far spanned half a century.

Some background
In the 1960s and early 1970s, Guillemots, and two other auk species – Razorbill Alca torda and Puffin Fratercula arctica – were thought to be in decline in southern Britain. This perception, and the decline itself, was reinforced by the mass mortality of tens of thousands of Guillemots caused by the Torrey Canyon oil disaster of 1967 and the Irish Seabird Wreck of 1969 (Bourne et al. 1967; Cowan 1968; Holdgate 1971). It seems remarkable now but, in the early 1970s, no one had given much thought to how to monitor seabird populations and, consequently, how to monitor the impact that such mass mortality had on breeding populations. Chris Perrins, at the Edward Grey Institute (EGI) in Oxford, and the Institute’s director, David Lack, were keen to have research students on the Pembrokeshire islands of Skomer and Skokholm to study all three auks in order to better understand their population biology. I was selected to study Guillemots on Skomer. My PhD began in 1973, and I was fortunate to have had a head start with my 1972 field season.

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202. Common Guillemots Uria aalge at their breeding colony, Skomer, Pembrokeshire, June 2012. 

Tim Birkhead 

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203. Skomer, looking east. 

Crown Copyright Visit Wales

With a broad remit to study the Guillemot’s population biology, I had, in fact, free rein to do pretty much whatever I wanted. My approach, based initially on Parslow’s instructions, evolving very much through trial and error, was three-pronged: (i) to develop a method of monitoring Guillemot numbers; (ii) to discover how Skomer’s Guillemot population worked and what was causing the decline; and (iii) to pursue my fascination for Guillemot social behaviour and extra-pair behaviour (aka ‘sperm competition’), in particular.

Guillemots are extraordinary. Although less immediately visually appealing than some other seabirds, their social life at the colony is complex and fascinating. Some seabirds spend much of their breeding season alone in a burrow, whereas Guillemots typically breed out in the open, interacting continually with surrounding friends and family. I soon realised that the key to understanding Guillemot biology at the colony was to work out why and how they bred in such close physical contact – at an average of about 20 pairs per m2 – with their conspecific neighbours.

Breeding habitat on the cliffs and offshore islands where seabirds breed is limited to earth burrows, rock crevices and open ledges, typically occupied by Puffins, Razorbills and Guillemots, respectively. Burrows and crevices provide protection from predatory gulls and corvids in search of eggs or chicks, whereas open ledges do not. Unless, like Guillemots, individuals lay their eggs together and defend them as a group, on narrow cliff ledges or on broad ledges or, as is the case in the Farne Islands, Northumberland, on the tops of stacks. Bunching together, breeding synchronously, and sitting tight in the face of a predator is how Guillemots successfully produce chicks. In the 1800s, this intransigence earned them the sobriquet ‘foolish Guillemot’. But they were ‘foolish’ only in the eyes of human predators. As a defence against gulls and corvids, sitting tight in a dense group is extremely effective.

Between the 1930s and 1960s, the Skomer Guillemot population fell by about 95%, with oil pollution being a significant factor (Birkhead 2016); fortunately, this is less of a threat today (Camphuysen 2022). The huge reduction in numbers – from around 100,000 pairs to just 2,000 – had resulted in many of the breeding areas being occupied by low-density groups, as I witnessed when I started my study. As I later realised, this was a consequence of the Guillemots’ strong site-fidelity, i.e. their reluctance to move, even when there were few immediate neighbours due to the population decline. In this respect, I did sometimes think that they were ’foolish’. They could (I thought) so easily have regrouped. Presumably, in the long run, site fidelity pays off, but it didn’t look like that in the 1970s when the Guillemots in these sparse groups were especially vulnerable to losing their eggs and chicks to gulls and corvids (Birkhead 1977). 

Guillemots are difficult to census precisely because, since they breed so close together, it is impossible to count pairs or nests, as would be the case for other species. I identified the best time to count individuals, how individuals translated into pairs and established sample study plots to monitor numbers each year (Birkhead 1978a; Birkhead & Montgomerie submitted).

Breeding at high density (plate 204) means almost constant social interactions, both friendly and not so friendly. Because site fidelity is high, Guillemots might spend years (decades in some cases) with the same breeding neighbours, many of whom seem to become ‘friends’ – as indicated by their regular mutual allopreening. Fights are common, too, over breeding sites or copulation opportunities, and sometimes these drew blood (Birkhead 1978b). 

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204. High-density breeding. This is our main Guillemot study plot on the Amos, Skomer, June 2022. See also plate 209–211, where this region lies on the upper right side of those photographs. 

Klaus Nigge

High-density breeding also provides regular opportunities for extra-pair copulations. Guillemots are socially monogamous, forming long-term pair bonds. However, males in particular rarely pass up the opportunity of copulating with a neighbour’s partner, usually only when the neighbouring male is absent. Most extra-pair copulations were initiated by males and are successful only if the female co-operates. If a male witnesses an extra-pair copulation attempt on his partner, his aggression is unrestrained. Molecular methods showed that about 7% of chicks were fathered by a male other than the one that helped to rear them (Birkhead et al. 2001) – similar to some estimates for humans.

Population biology
David Lack and Chris Perrins had spent their careers investigating how bird populations work – how births, deaths, immigration and emigration determine population sizes. At its simplest level, for a population to remain stable, births must balance deaths, complicated by rates of emigration or immigration. So, all I had to do was to measure adult survival and breeding success, immigration and emigration. Easier said than done. 

Obtaining estimates of the proportion of birds that survive from one year to the next relies on marking birds in some way so that they can be recognised as individuals, then trying to resight them in subsequent years. On Skomer during the 1970s, simply catching enough Guillemots to mark was a challenge. The birds in this much-reduced population were extremely nervous, such that getting close enough to slip a noose around their foot or neck, especially in a location where one could hope to see the Guillemots again, was difficult. To mark birds, I initially used a combination of a metal BTO ring and up to three colour rings made of Darvic (a colour-fast plastic), and the code on the latter could be read through a telescope – on the rare occasions when the birds deigned to show their legs. This worked well enough, and provided a minimum adult survival estimate of around 90% from year to year (Birkhead & Hudson 1977; Hatchwell & Birkhead 1991). 

Over the course of the study, up to 2022, we have colour-ringed 849 adult Guillemots and 11,000 chicks. Our oldest bird is one ringed as a chick in 1985 – it is still alive, 37 years later.

To measure breeding success, I established study plots of 100 or so pairs where I had a reasonable chance of seeing when eggs were laid, whether and when they hatched, and if the chick survived to leave the colony. This was laborious and required long hours in a hide each day between late April and July, checking birds against my reference photographs on which each pair’s breeding site was labelled. 

Guillemots lay a single egg and, overall, during the three breeding seasons of my PhD, about 67% of pairs successfully reared a chick to fledging (considered to be the point at which the young bird departed from the breeding ledge to the sea). Significantly, breeding success was greater when Guillemots bred at higher densities and less at lower densities (Birkhead 1977).

Despite having the two key values of adult survival and breeding success, it was not clear whether these numbers could account for the observed population changes. The missing piece from the jigsaw was the survival of immature birds to breeding age. Since we found that most Guillemots did not breed until they were seven years old (although a few did so at three and some not until they were ten), it took several years to accumulate sufficient data to construct a population model and understand how Skomer’s Guillemot population ‘worked’.

Long-term study
By 1976, with my thesis complete, it was now abundantly clear that a three-year PhD study was inadequate for understanding Guillemot population dynamics, so I decided to continue. During my PhD, I had read papers on research expeditions in Arctic Russia and how researchers there created new breeding areas for Guillemots in what were apparently expanding populations. With a mere 2,000 pairs on Skomer in the early 1970s, I had fantasised about what it would be like if the Skomer population was increasing rather than decreasing. 

By the mid 1980s, it seemed that the fortunes of Skomer’s Guillemots might indeed be improving. I was a lecturer in the Zoology Department at Sheffield University by this time. In 1984, I was fortunate enough to recruit Ben Hatchwell as a PhD student, with a view to him repeating my PhD research, but now on an increasing rather than a decreasing population. I was keen that he benefited from what I had already learnt but that he could also avoid the mistakes I had made. An outstandingly able researcher, Ben convinced me by his three-year study that the Skomer Guillemots now had considerable potential for a long-term study.

The key to success for any long-term study is consistency of methodology: measuring things in the same way every year so that results are comparable over time. Thus, my assistants and I continued to measure the timing of breeding (median laying date), breeding success (proportion of pairs rearing a chick to the point where it left the colony), adult and immature survival, age of first breeding (see above) and the species of fish on which juvenile Guillemots were reared. Some minor changes in methodology were needed occasionally but comparability of data between years was a key element of the study.

During my PhD, I marked juvenile Guillemots with a ‘year class’ colour ring. This enabled me to see how old they were when they first returned to Skomer (never sooner than their second summer) but, because I couldn’t identify individuals, it was impossible to estimate the proportion that survived. From the mid 1980s, we used individually numbered Darvic rings, all of which were hand engraved and shaped to fit the Guillemot’s flattened tarsus. The rings had to be robust enough to withstand the wear and tear on the breeding ledges and they required considerable strength to open with circlip pliers and put on. Another innovation, driven by Chris Perrins, involved the BTO’s metal rings. The ‘G’ rings that had been used for decades were ‘wrap around’ and shaped so that they could rotate on the bird’s leg. This meant that the complete number could rarely be read through a telescope and often meant that part of the number was obliterated by wear. The new metal rings were of a shape that enabled the entire number to be read through a telescope (if you were close enough), and on which the number was unlikely to be abraded (plate 205). Remarkably perhaps, almost all birds ringed as chicks returned to Skomer to breed, often within a few metres of where they were reared.

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205. Guillemot rings. Left: BTO ‘Guillemot special’ metal ring, with the numbers visible on the ‘flat’ edge of the bird’s tarsus such that the ring can be read when the birds are sitting on the cliffs and the numbers aren’t worn away as the bird shuffles around on the cliff ledge; right: an engraved plastic ring. 

Tim Birkhead

Initially, resightings of ringed birds were recorded in notebooks and transferred onto index cards but, from 2007 onwards, we started using a handheld computer (personal digital assistant, PDA) to record sightings that went straight into a custom-made database created for us by Ian Stevenson (Sunadal Data Solutions). Using the PDA was infinitely more efficient because it saved transcribing data from notebooks into electronic files; it also minimised errors since the PDA would ‘ask’ if we had really seen a particular bird that we recorded if it had not been seen previously at a particular site, and so on.

When I started my PhD, I was issued with a Hertle & Reuss 25–60 x 60 telescope, with a warning from Chris Perrins that if I used it too much it might result in ruptured retinal blood vessels. It didn’t, but this was a horrible telescope and, over the following years, it was a relief to be able to afford and acquire better-quality optics.

The way that we caught Guillemots changed over the years, too. One of the most striking consequences of the increasing population size and the resulting increase in the density of breeding groups was that the entire feel of the colony shifted from one of nervousness and vulnerability to one of supreme confidence. Whereas, initially, Guillemots would sometimes leave their breeding ledges (and eggs) at the mere sight of a person on the horizon, or on hearing the alarm call of one or more of the thousands of Herring Gulls Larus argentatus that bred on the island, they would now sit tight. It thus became possible to catch birds for ringing much more easily. 

The original method of capture was to use a modified fishing rod to place a nylon noose around the bird’s neck. However, Mike Harris – whose own Guillemot studies started on the Isle of May in 1981 – discovered that a lightweight fishing rod fitted with a miniature shepherd’s crook was much less disruptive. Hooking a bird around the leg and drawing it gently closer is how we how do it now. I use the same technique to obtain samples of fish (for identification purposes) brought back to the chick (plate 206). This confirmed our visual records that most prey were sprats Sprattus sprattus (based on the sharp, serrated keel), with smaller numbers of sandeels (both Ammodytes and Hyperoplus lanceolatus) and gadids (cod-like fish in the family Gadidae), with little variation over the years (Riordan and Birkhead 2018; https://bit.ly/430RF35).

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206. A sample of fish brought back by adult Guillemots for their chicks on Skomer in 2022: clupeids (Sprats Sprattus sprattus) and three sandeels (among the four lower fish).Scale is in centimetres. Sprats mean: 9.45 cm ± 0.52 SE (n=15); the longest sandeel, probably Hyperoplus lanceolatus, is 18.7 cm.Sprats distinguished from herring Clupea harengus by the sprat’s prominent post-pelvic scutes (https://fishbase.mnhn.fr/search.php).

Tim Birkhead

For many years, my field assistants and I conducted two successive dawn-to-dusk watches (in four-hour shifts) to record the frequency with which Guillemot chicks are fed, thus providing an estimate of food availability. It sounds simple enough: we selected an area of cliff with a precisely known number of chicks and then recorded the number of feeds delivered to that area over a set time-period. The number of food deliveries divided by the number of chicks present provided an estimate of the number of feeds per chick per 24 hours. We performed this arduous undertaking for many years until one season I was suspicious of what seemed like an extremely low feeding rate. The following year, I conducted a double-blind test in which, unbeknown to each other, two assistants recorded the number of food deliveries to the same group of Guillemot chicks at exactly the same time. The results were shocking, with one observer recording twice the feeding rate of the other! Of course, I should have checked earlier, but there had never previously been sufficient funds for a second assistant. This depressing disparity in data highlights the need for a scientific approach: methods must be repeatable. For a task that required close attention, I had been naive in assuming that all assistants would be equally observant or assiduous. The several years of data from these feeding watches were dumped and never used. In a few instances like this, I have had to throw away hard-won data that failed the repeatability test.

The Guillemot’s egg
Look at almost any book on birds’ eggs and a Guillemot’s egg will feature prominently (plate 207). Large, brightly and variedly coloured and an extraordinary ‘pyriform’ (pear-like) shape, the Guillemot’s egg has long been an object of desire and mystery. Why all those different colours? Why that peculiar shape? These questions have fascinated biologists for well over a century. 

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207. Variation in Guillemot eggs from Skomer – a ‘haphazard’ sample, photographed under licence. 

Tim Birkhead and Jamie Thompson

A common statement among those who collected Guillemot eggs in the first half of the twentieth century was that no two Guillemot eggs were alike. The variation is indeed extraordinary, although it can seem much reduced when examining museum specimens as their colours fade over time, but the ground colour of freshly laid Guillemot eggs ranges from white, through vivid pale green and blue through to dark green. It was the variability in ground colour and maculation that excited egg-collectors, who sought out the rarest types, such as red eggs (Birkhead & Montgomerie 2018). 

The adaptive significance of the variability in colour was discovered by the Swiss ethologist Beat Tschanz. In the 1950s, he recognised that such individual variation allowed parent Guillemots to find and incubate only their own egg, and to retrieve it if it rolled away from their breeding site (Birkhead et al. 2021). The variation in egg colour and pattern was one of the several adaptations to high-density breeding (Tschanz 1959, 1990).

And the shape? Whenever I caught a glimpse of a Guillemot egg during my PhD, usually when an incubating bird stood up to shake its wings, I wondered whether the two widely believed explanations for the Guillemot’s pyriform egg shape could possibly be true. The first of these explanations, originally reported in 1831, was that the pyriform shape allowed the Guillemot’s egg to spin on its axis so that it was unlikely to roll off the ledge when nudged or knocked. This notion was based on blown, empty Guillemot eggshells, which one can certainly persuade to spin like a top on their side. However, so too can empty eggshells of any shape or species. Spinning like a top is not an explanation for the Guillemot’s pyriform egg.

The second idea I refer to as ‘rolling in an arc’. A Guillemot’s egg placed on a smooth, gently sloping surface generally rolls in an arc rather than in a straight line. In the 1940s, it was suggested that this was the explanation for the egg’s pyriform shape since rolling in an arc would also reduce the risk of an egg rolling off a ledge. In the late 1970s, one of Tschanz’s students spent three years rolling Guillemot eggs and the much less pointed eggs of Razorbills on natural ledges to test this idea. He found little evidence for the ‘rolling in an arc’ explanation. Natural Guillemot ledges are rarely smooth and the arc described by a rolling egg is often much greater than the width of ledge on which the egg is laid. Most tellingly of all, Guillemots typically incubate facing the cliff wall with the blunt end of the egg facing that same way. This means that if the egg was to roll, it would do so outwards, towards the cliff edge. If the pyriform egg had evolved to minimise the risk of loss in this way, it would make more sense to orient and incubate the egg with its blunt end facing away from the cliff edge so that the egg rolled inwards to safety.

I spent a lot of time thinking about Guillemot egg shape without actually doing much about it. I was spurred into decisive action after seeing the erroneous ‘spinning like a top’ story broadcast to millions of television viewers in 2012. 

It seemed to me that the most plausible explanation for the pyriform egg shape was indeed likely to be linked in some way with reducing the chances of losing the egg from its precarious site. Guillemots make no nest and incubate their single egg on bare rock. My best hope of answering the question was to consider the different ‘selection pressures’ Guillemot eggs have to deal with. I initially considered two ideas. The first focused on the fact that Guillemot ledges are invariably covered in faeces. After rain, these ledges can be extremely dirty places. Towards the end of incubation, eggs are often liberally coated in ‘muck’ (plate 208). However, if you look at Guillemot eggs, the large end of the egg – the end through which the chick emerges – is often relatively clean, since the pyriform shape elevates much of the blunt end of the egg above the substrate. 

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208. A muck-covered Guillemot egg: such eggs usually hatch. 

Tim Birkhead

The other possibility I considered was that a pyriform shape confers extra strength on an egg, protecting it from the various knocks that are inevitable among birds breeding at high density and with no nest. But, in fact, a long, pyriform egg is weaker rather than stronger – a sphere is the strongest shape of all. Guillemots compensate for an inherently weak egg shape by producing the thickest eggshell relative to its size of any egg. So, strength was not the explanation for the pyriform egg shape,

The explanation for the pyriform shape came about almost by accident. I was down on a cliff ledge holding a Guillemot egg and a Razorbill egg in each hand, waiting for my assistant to measure them. As I waited, I tried placing the Guillemot egg onto a steeply sloping piece of rock beside me and was amazed when it simply remained where I’d put it. I then tried placing the Razorbill egg in exactly the same spot and it slipped off. I tried repeatedly with both eggs, with the same result each time. I realised in a flash that the long flat edge of the Guillemot’s egg towards the point means that, with more of the egg in contact with the substrate, especially on a slope, the resulting friction enabled the egg to remain in place. 

My colleagues and I went on to conduct numerous experimental tests with Guillemot and Razorbill eggs that confirmed that initial result (Birkhead et al. 2018). A pyriform egg is simply more stable and less likely to move in the first place than a more rounded egg like that of a Razorbill (see also https://bit.ly/3njkzw9).

I also realised that the Guillemots routinely breed on sloping ledges, whereas Razorbills almost never do so; and Guillemots typically breed very close to conspecifics while Razorbills are, in contrast, always well spaced out. As I mentioned earlier, the secret to Guillemot breeding success is breeding in close contact with their neighbours. The inherent stability of a pyriform egg allows Guillemots to incubate on almost any substrate, sloping or level, thereby allowing them to breed in extremely close proximity to conspecifics (Birkhead et al.2018).

Funding and politics 
Keeping a study like this one going for 50 years requires a lot of effort. Funding is a major issue. Once the potential of a long-term study on Skomer became apparent in the 1980s, I was fortunate that the Countryside Council for Wales (CCW), who owned and then managed Skomer, were extremely supportive. They provided a modest but adequate annual budget to enable me to employ a field assistant and cover the necessary costs of field research. Throughout the 1980s, 1990s and part of the 2000s, CCW prided itself on permitting access to Skomer by both the public and scientific researchers.

In 2014, however, CCW was absorbed by a new quango, Natural Resources Wales (NRW), who promptly terminated my Guillemot funding. Their reasons were that the Skomer Guillemot population was increasing and hence not of concern, and that the Joint Nature Conservancy Council (JNCC) already undertook some Guillemot (and other seabird monitoring on Skomer) through their Seabird Monitoring Programme (SMP), which started in 1986 (see: www.bto.org/our-science/projects/seabird-monitoring-programme/about-smp).

There was dismay and disbelief among the UK seabird research community at NRW’s decision to pull the funding for the Skomer Guillemot study. Following NRW’s decision, a conference was organised in Cardiff at short notice in April 2014 to demonstrate the necessity and value of accurate, reliable, long-term seabird monitoring. Contributors came from across the UK. A key supporter was Iolo Williams, who opened the meeting. Representatives from NRW attended, too, but were reluctant to speak directly to any of the other participants. Sadly, despite the overwhelming case for continuing the study, NRW did not reverse their decision. 

NRW’s decision was particularly unfortunate because, a few years previously, it had become apparent that some aspects of the monitoring of Guillemot productivity on Skomer failed to meet the normal scientific standards of accuracy and reliability. This became clear because of the unique situation on Skomer in which two groups (my team and the SMP observer, collecting data for the JNCC scheme) collected the same data at the same time. Just as with my feeding watches discussed above, this provided a rare opportunity to compare the results from two different methodologies. Since the SMP observers are required to collect data on multiple species in a relatively limited space of time, a less demanding, short-cut method is used (Walsh et al. 1995). This involved counting adults and chicks then dividing the number of chicks by the number of adults on particular ledges to obtain an index of breeding success (Walsh et al. 1995). That method seems logical but assumes that the ratio between the number of adult birds and chicks is constant over time. It isn’t, because the Skomer Guillemot population is increasing (see below) and the number of adults on the study ledges increased more rapidly over time than the number of chicks since non-breeding adults attend the colony prior to breeding themselves. The result was an apparent year-on-year decline in breeding success. In contrast, our data, collected using a different, more intensive methodology (Birkhead & Nettleship 1980), showed no overall change in breeding success (fig. 1).

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Fig. 1. Common Guillemot Uria aalge breeding success on Skomer over time. Values for 1973–76 are from the author’s PhD and those for 1985–87 from Ben Hatchwell’s PhD. Part of the overall increase in breeding success from the 1970s and 1980s to later is due to the increase in breeding density (see Votier et al. 2009). The low breeding success in 1985 was the result of the Bridgeness oil spill that year (see Hatchwell 1991). The low value in 2021 was due to severe storms during the breeding season (see text).

Most monitoring work on birds proceeds on the assumption that the methodology is sound and executed appropriately, and that there is no need to check the accuracy of the results through double-blind testing. In addition to the differing methods in the data collection, and despite a succession of well-intentioned and conscientious assistants, an absence of any quality control over many years resulted in the SMP methodology being allowed to drift away from how it should have been conducted, until the results ceased to be accurate or meaningful (Birkhead 2022). Notwithstanding, NRW continued to claim that the methodology employed by their Guillemot monitors was robust and that, as was written in a letter from Carl Sergeant of the Welsh Government to William Powell, Chair of the Petitions Committee, on 14th October 2014, ‘the long-term increase in Guillemot numbers at Skomer Island, and the fact that this species will continue to be monitored under the JNCC contract, reassures me that there will be no loss of data or information about these birds.’ For me, this kind of political misinformation and stonewalling came as a rude shock. As a scientist, I was used to objective evidence and discussion, but this was different. 

This error in the SMP’s Guillemot productivity measure was no one’s fault; but, with hindsight, someone should have checked sooner that the SMP’s short-cut method of estimating Guillemot productivity was sound. What was strange was that no one at JNCC expressed any surprise or concern about the contradiction between an apparent ongoing decline in breeding success among Skomer’s Guillemots while the population itself was rapidly increasing. 

It is to the credit of the conservation managers of the Wildlife Trust of South and West Wales (WTSWW), who are now in charge of Skomer, that they convinced JNCC to abandon use of the short-cut method of estimating Guillemot productivity (on Skomer at least) and replace it in 2022 with the full-scale method that we had been using since the 1970s.

My criticism of the SMP sounds harsh, disloyal even; but, as a scientist, I believe honesty to be the best policy. Funds are tight but population monitoring is not a lowly, tick-box exercise, and there is no point in doing it if the results are inaccurate. The slow drift away from the SMP-recommended methodology on Skomer provides an important lesson for monitoring in general. As events on Skomer demonstrate, reliable monitoring requires proper investment, careful management, careful training and supervision, and quality-control checks, especially in situations where field assistants change from year to year.

The SMP, previously coordinated by JNCC alongside over 20 organisational partners, has undergone a recent review and, as a result, has been coordinated by the BTO since July 2022, in partnership with JNCC and associate partner, the RSPB. Alongside these organisations, an advisory group has also been put in place, consisting of 24 organisations all working together to take the scheme forwards. With BTO now leading on the coordination, various important tasks – including developing a sampling strategy, a review of the SMP methodological handbook, a review of reporting methodology and the formation of an engagement plan (including participant training) – are all under way.

The continuity of funding is a major hurdle for all long-term studies. This is especially acute as most scientific research grants are usually for three or five years. Moreover, the emphasis of the Skomer Guillemot study was not always suitable for such short-term funding (although we have received Research Council grants in the past), which tends to focus on shorter-term results.

To counter NRW’s withdrawal of funding, someone at my university suggested crowdfunding. This approach was new at that time, and I was sceptical, but a fortuitous coincidence led me down the crowdfunding path. Shortly after the NRW decision, I was asked to write a commentary for the journal Nature on the problems faced by long-term studies. I soon realised that when this was published the international publicity could launch a crowdfunding campaign (Birkhead 2014). And that is what happened, although not without a massive amount of effort. 

I contacted everyone I could think of and generated as much media interest as possible. It was worth it for, within two weeks, I had reached my target of one year’s funding. As their first crowdfunding venture, the university was delighted, and the support I received from them was (and continues to be) remarkable. Since then, I have continued to publicise the Guillemot project through talks, using any speaker’s fees that I receive to support the project. In addition, I have sought and received donations from philanthropists with an interest in conservation. My aim is to secure sufficient funding such that the project can continue in perpetuity.

What is funding needed for? Primarily for a field assistant’s salary for the duration of the field season. In addition, there are costs for accommodation on Skomer, travel to and from the island, purchase of engraved rings, renewal of climbing and safety equipment, as well as the occasional replacement of a laptop, tablet, telescope or tripod. The charges for all these items increase year on year.

There are always people willing to act as field assistants, but relatively few seem – on interview – to have a realistic idea of the dedication required. The best assistants have generally been those with some kind of academic background (such as a PhD) who develop a commitment to the study by coming back over several years. Finding someone effective who returns in successive seasons has many advantages.

What have we discovered?
My initial goal in the 1970s was to understand how Skomer’s Guillemot population worked and why it had been declining over the previous three or four decades. Then, after numbers started to increase in the 1980s (fig. 2, plates 209 & 210), the question became, what is driving the increase? 

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Fig. 2. Changes in number of breeding pairs of Guillemots on Skomer based on whole-island counts (see Birkhead & Montgomerie submitted). 

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209 & 210. The Amos, Skomer, in June 1966, when the Skomer Guillemot population was near its lowest; and in 2022, when numbers were around 21,000 pairs. Note the large area of grass on the right-hand side of the Amos in 1966 that was gradually removed by the action of loafing Guillemots and erosion as the population expanded. David Saunders

guillemots 009.jpg

Tim Birkhead

Our routine monitoring of adult (fig. 3) and immature survival, together with our annual measures of breeding success, allowed us to model the Skomer Guillemot population dynamics. Since each year provides only a single data point, it took 30 years to accumulate sufficient information to do this. That model compared the observed changes in numbers with the predicted changes based on our measures of survival and breeding success. The close match between the two showed not only that we had a reasonable idea of how the population worked, but that the changes in numbers were intrinsically driven (Meade et al. 2012). That is, there was no need to invoke high levels of immigration from elsewhere (as had once been thought to drive large population changes, e.g. Tuck 1961). Indeed, as sightings of colour-ringed individuals show, immigration and emigration to and from Skomer seem to occur at a low level, even though there is some exchange between colonies.

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Fig. 3. Estimates of Guillemot annual adult survival on Skomer. While survival has remained similar over the entire 36-year period (mean: 94% ± 0.69 SE), the year-to-year variation is accounted for by climate effects (North Atlantic Oscillation) and oil pollution (Votier et al. 2005; 2008). Courtesy K. Laurenson.

Over the duration of the study, two main factors have been shown to affect the survival rate of Skomer’s Guillemots: oil pollution and climatic effects. Four major oiling incidents have occurred during the study: (i) the Aegean Sea in northwest Spain in December 1992, (ii) the Sea Empress in Wales in February 1996, (iii) the Erika in northern France in December 1999, and (iv) the Prestige in Galicia in December 2002, all of which had a negative impact on both the numbers and the survival of Guillemots from Skomer. 

In addition to the effect of oiling incidents, the overwinter survival rate of adult breeding Guillemots was negatively affected by the North Atlantic Oscillation (NAO), that is, by warmer, wetter, windier winters (Votier et al. 2005). The survival rate of immature non-breeding Guillemots was less affected by either oiling incidents or NAO than was the case with adults, presumably because immature birds are more widely dispersed during the winter months. 

However, a surprising result to emerge from our analyses was that recruitment into the Skomer breeding population doubled following an oil spill. In other words, the losses of adult birds caused by oil pollution were offset by compensatory recruitment of young birds. This in turn meant that – apart from the Sea Empressincident, which resulted a 16% decline in Guillemot numbers – the negative impact of oiling incidents was poorly reflected in counts of adult birds on the cliffs. That is, counts of birds at the colony underestimated the effect of oil pollution. The effects of all four oiling incidents were very clear from our measures of adult survival, which were calculated using the return rate of ringed birds, demonstrating that, compared with counts, adult survival is a much more sensitive index of the health of the population (Votier et al. 2005, 2008).

One of the most striking results of our study has been the general advance in the timing of breeding. On average, Guillemots on Skomer now breed over two weeks earlier than they did in the 1970s, albeit with large year-to-year variation (fig. 4; see also Votier et al. 2009). Similar advances in the timing of breeding have been recorded in other bird species in the UK and have been attributed to climate change (Pearce-Higgins 2021). So far, the advance in the timing of breeding among Guillemots on Skomer does not seem to have had any negative effects, but it does have important implications for the timing of census counts (Birkhead & Montgomerie submitted).

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Fig. 4. Median laying dates for first laid eggs over 36 years on Skomer. The red line is linear regression ±95% CI (shaded region). Laying dates were obtained using the ‘full scale method’ described in Birkhead & Nettleship (1980).

Climate change has resulted in more extreme weather events, especially winter storms, and these can cause mass mortality events (‘wrecks’). The so-called Irish Seabird Wreck of 1969, before the present study started, had a noticeable negative effect on Skomer’s Guillemot numbers (Holdgate 1971). Similarly, the impact of a wreck in 2014 (Morley et al. 2016) was reflected in the noticeable increase in mortality of adult Guillemots on Skomer (fig. 3). A further climate change effect is the increased severity of weather during, rather than outside of, the breeding season. Guillemot breeding success in 2021 was lower than in any of the preceding 30 years (fig. 1). This was due to two major storms in May 2021, which resulted in the direct loss of many Guillemot eggs and reduced hatching success in those eggs that survived.

Discussion and concluding remarks
Globally, seabird numbers have declined by 70% since the 1950s (Paleczny et al. 2015). Within the UK, more northerly populations have declined more than those in the south. Different factors, including overfishing, fishing bycatch, plastic, oil pollution, introduced animals and climate change have all contributed to these declines. Climate change has had a greater effect on seabirds than almost any other group of birds (Pearce-Higgins 2021). This is thought to be due to shifts in the distribution and availability of prey, such that some populations fail to breed at all in some years, or, in others, birds produce chicks that subsequently starve to death. There are other climate change effects too. A sudden rise in sea temperatures off the west coast of North America between 2014 and 2016 resulted in the death of around one million Common Guillemots, thought to be due to a lack of fish that they depend on, since most birds starved to death (Piatt et al. 2020). In the Arctic, the Common Guillemot’s congener, the Brünnich’s Guillemot U. lomvia, has suffered both directly and indirectly as a result of higher air temperatures at its breeding colonies, through, for example, an increase in mosquitoes that cause debilitating blood loss (Gaston et al. 2002; see also Choy et al. 2021). 

In the UK and elsewhere, ongoing climate change has driven the quest for ‘green energy’ and a major solution has been offshore windfarms. In 2020/21, plans to place windfarms in the southern Irish Sea raised concerns for seabirds on Skomer and elsewhere in that region. Plans for the world’s largest offshore windfarm in the North Sea have raised serious concerns for the 500,000 pairs of seabirds breeding on Flamborough Head, Yorkshire. Martin Harper of the RSPB said: ‘The Government has accepted that the expansion of offshore wind turbines in this part of the North Sea will be damaging to seabird numbers in the surrounding area and is putting its faith in an unproven compensation scheme…’ It seems likely that the development of windfarms in the vicinity of Skomer will also have a negative effect on seabird numbers. This alone is sufficient reason to continue to undertake rigorous monitoring of numbers, survival and breeding performance.

The emergence of the HPAI (Highly Pathogenic Avian Influenza) virus in 2021/22 has had a devastating effect on some seabird populations (Cunningham et al. 2022; Pearce-Higgins 2022). Although, at the time of writing, HPAI has not affected the Skomer Guillemot population, HPAI has also shone a spotlight on the need for reliable monitoring (https://bit.ly/3HUTM0n).

Once, while I was trying to generate some publicity for the Skomer Guillemot project, someone I knew said to me: ‘You know, your Guillemot study isn’t as important as you seem to think it is.’ I was shocked, but it made me realise that he had missed the point: the Skomer Guillemot project is a single, small example representative of a much larger issue. I am also well aware that other long-term studies of birds or mammals have generated much more in the way of scientific publications than ours. But that has not been the primary aim of the Skomer Guillemot project, although the study has generated over 40 peer-reviewed publications in scientific journals and provided opportunities and training for PhD students and post-doctoral researchers (see https://bit.ly/41LIBOq). Having established how the Skomer Guillemot population ‘works’, the impact that environmental perturbations and climate change have on it, and understanding the evolution of egg shape, not just in Guillemots, but in birds in general, my objectives switched. The focus is now on ensuring that the annual monitoring of numbers and breeding parameters is conducted in a consistent, reliable and meaningful manner. 

Half a century of watching Guillemots at their breeding colonies has given me wonderful insight into the intricacies of their social life, breeding biology and evolution. Besides watching Guillemots, however, I have also observed people. Indeed, it has been hard to overlook the humans within the social hierarchy of those responsible for monitoring seabirds. The enthusiasm of volunteers and field assistants has not changed over time; but among those towards the top of the hierarchy, changes have occurred. The difference, for example, between CCW’s empathetic and constructive approach and the less-than-enthusiastic attitude of their successors is disappointing. This change is partly driven by economics, but also by political priorities that together probably account for the reluctance to admit fault, where denial is deemed better than losing face. 

The political situation I have described is not unique to the UK. Encouragingly, however, some of those involved in monitoring and conservation are prepared to challenge policy-makers and acknowledge failures. In a broad-ranging review, Bill Sutherland has highlighted the widespread shortcomings of many conservation measures, including monitoring. A key issue is that ‘conservation managers tend not to use scientific evidence to support their decision-making [and] instead mainly rely on personal experience, anecdotes and the advice of colleagues’ (Sutherland 2022). Despite efforts to rectify this situation, progress is slow. In describing the conservation of birds in the internationally important Wadden Sea, in the Netherlands, Theunis Piersma has highlighted the difference in approach between scientists and other organisations (Piersma 2018). The parallels with UK seabird monitoring are striking, and a key message in the accounts of both Piersma and Sutherland is that monitoring should be subject to the same kind of scientific scrutiny and peer review as scientific research.

I agree: if monitoring is to have any genuine value, more honesty and transparency – facilitated by scientific scrutiny and oversight – are essential. Fortunately, this may now occur. Having assumed the organisational lead role in the Seabird Monitoring Programme, the BTO recognises the value of long-term seabird studies, and crucially, the need for improvements in the SMP methodologies (Pearce-Higgins 2022; Pearce-Higgins et al. 2023)

Ecological studies of birds and other animals spanning four or more decades are relatively few, but their role in measuring and tracking environmental responses to climate and disease in a meaningful way is more important than ever (Sheldon et al. 2022). My (naive?) hope is that the population of Guillemots on Skomer will eventually reach or exceed the levels that they were at before the start of the Second World War, in the region of 100,000 pairs. Even if the current rate of increase was to continue, this will not be until about 2050. 

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211. The author, with Kirsty Laurenson, catching Guillemot chicks for ringing, Skomer, summer 2022.

Klaus Nigge

Over 50 years, many people have contributed to this study. In particular, I would like to thank my DPhil supervisors, Chris Perrins and Euan Dunn, the late John Parslow, Mike Alexander, David Saunders (who made the first counts of Guillemots on Skomer in the 1960s), Steve and Anna Sutcliffe, numerous field assistants but especially Julie Riordan, Jamie Thompson, Steve Votier and Sherry Wilson, who helped over multiple seasons. Thanks also to the various Skomer wardens and their partners, the Wildlife Trust for South and West Wales (WTSWW), notably Lisa Morgan and Lizzie Wilberforce. Special thanks to Ben Hatchwell, who has been involved with the project for the last 40 years. I am grateful to Kirsty Laurenson for help with survival values and to Theunis Piersma for valuable discussion. Euan Dunn, Ben Hatchwell and Bob Montgomerie made constructive comments on the manuscript for which I am grateful. Financial support was provided by the Countryside Council for Wales (CCW), the Joint Nature Conservancy Council (JNCC), the Natural Environment Research Council (NERC) and the University of Sheffield. I am hugely indebted to David Meadows, Ceris Morris and Louise Shaw of Sheffield University’s Campaigns and Alumni Relations Department, who helped to launch my crowdfunding in 2014 (see www.justgiving.com/fundraising/guillemotsskomerand to all those people and organisations that have contributed to the project.

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Tim Birkhead, Department of Biosciences, University of Sheffield, Sheffield S10 2TN; e-mail: [email protected]

Tim Birkhead is emeritus professor of Zoology at the University of Sheffield. His main research has been on promiscuity in birds and the shape of birds’ eggs. He is committed to the public understanding of science and has written a number of academic and popular books.



































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