The status of in Britain & Ireland in 2003/04 Sarah Wanless, Stuart Murray and Michael P. Harris ABSTRACT In 2003/04 there were about 261,000 occupied nest-sites of Northern Gannets Morus bassanus, in 24 colonies, in Britain & Ireland. Three new colonies had been established since the last census in 1994/95. Although the British and Irish population was 12% higher than in 1994/95, the average rate of increase (1.2% per annum) over this period was appreciably lower than during the previous decade.

e probably know more about the There are relatively few breeding colonies and breeding distribution and abundance most are on offshore islands or stacks. Probably Wof seabirds than of any other avian because of their spectacular locations, gan- group (Croxall et al. 1984; Brooke 2004), and, netries hold a particular fascination for undoubtedly, the Northern Gannet Morus bas- ornithologists and there is a well-established sanus (hereafter ‘Gannet’) is one of the seabirds tradition of visiting colonies and making about which we know most. It is the largest, and counts. Censuses of the entire world population arguably the most spectacular species in the were made in the 1900s, 1939, 1949, 1969/70, North Atlantic. Its breeding range extends from 1984/85 and 1994/95 with the result that Brittany north to Iceland and east to the changes in numbers at individual colonies and Murman coast of Russia, and from the Gulf of in the world population are better known than St Lawrence to Labrador in the west Atlantic. for any other relatively common bird (Gurney 1913; Fisher & Vevers 1943, 1944; Wanless 1987; Lloyd et al. 1991; 250000 Murray & Wanless 1997). The available historical information indicates that numbers in Britain & Ireland declined during much of the nineteenth century, mainly 150000 owing to excessive harvesting of eggs and chicks for human con- sumption. Following protection, however, numbers had started to Apparently occupied nests Apparently recover by the end of the century. 50000 Initially, the rate of increase was 1900 1950 2000 slow but, from about 1939 Year onwards, it gained momentum Fig. 1. The long-term increase in the British and Irish population of and for much of the second half Northern Gannets Morus bassanus, 1904-2004. of the twentieth century the

280 © British Birds 98 • June 2005 • 280-294 Northern Gannets in Britain & Ireland in 2003/04 /SNH/CEH Stuart Murray 148. Stac Lee, St Kilda,Western Isles, 21st June 2004.This spectacular, 180-m-high sea rock holds more than 13,000 pairs of Northern Gannets Morus bassanus, half of which are crammed onto the summit table. world population increased at an average rate of two not counted were those on Ortac and Les approximately 2% per annum (p.a.). The Etacs in the Channel Islands. Detailed changes increase in Gannet numbers is thus best consid- at individual gannetries are presented elsewhere ered not as a population explosion, but rather (Wanless et al. 2005a,b). In this paper, our aim as a long-term recovery from intense persecu- is to present an overview of the total popula- tion. This recovery occurred during a period tion, to describe three new colonies (two in when feeding conditions were favourable, with and an embryonic one in Wales) that good stocks of small- to medium-sized pelagic have been founded recently, and to highlight the shoaling fish such as mackerel Scomber scom- slowing down of the rate of increase of the total brus, herring Clupea harengus and sandeels population. Ammodytes spp. (Nelson 2002). These long- term trends in the British and Irish population Counts are summarised in fig. 1. Most British and Irish Gannet colonies are on Recent censuses of British and Irish colonies steep cliffs on offshore islands. Gannets are have been carried out every ten years and, since large and conspicuous and this facilitates the one was scheduled for 2004/05, counts of gan- rapid discovery of new colonies. Since Seabird netries were of relatively low priority during 2000, colonies have been founded on Sule Seabird 2000 (Mitchell et al. 2004). Nonetheless, Skerry and Westray, both in , and on St during the course of this comprehensive inven- Margaret’s Island, Pembrokeshire (fig. 2). tory of British and Irish seabirds, carried out Counting nests at all but the smallest gannetries between 1998 and 2002, counts were made of is a formidable undertaking, hence the larger 14 of the 21 extant colonies. The gannetries colonies tend to be counted infrequently and covered tended to be the smaller, more acces- usually only during dedicated surveys. sible ones and results from these indicated that Aerial surveys of ten of the major colonies numbers were still increasing (Wanless & Harris were carried out in 2004. Such surveys require a 2004). As planned, a co-ordinated survey of slow, twin-engine aircraft; an experienced pilot; Gannets was carried out mainly in 2004, and at least two photographers with a reasonable this paper presents counts for 22 of the 24 tolerance of airsickness; and good weather. The colonies for which counts were attempted. The ideal time for a survey is between mid May and

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the aerial photographs. Hermaness Sule Sule The 2004 survey was Stack Skerry carried out using similar Noss methods to those used in Westray 1984/85 and 1994/95 (Murray & Wanless 1986, 1997). Each gannetry was Flannan Isles photographed several times St Kilda and from several different Troup Head angles, using 35-mm reflex cameras fitted with 50-mm lenses and using 64 or 100 ASA colour-slide film, to Bass Rock ensure complete coverage. Ailsa Craig The resulting colour trans- parencies were used to form a mosaic of the colony to avoid missing areas and Scare Rocks double-counting. In general, Clare Bempton Island the same count-section boundaries were adopted as Ireland’s Eye in the previous two cen- suses. Little The only practical Skellig Great Saltee counting unit is the ‘appar- Grassholm ently occupied site’ (AOS), a Bull St Margaret’s Rock Island site occupied by one or two Gannets, irrespective of whether nest material is Les Etacs present. Slides were pro- Ortac Channel Islands jected onto a suitable surface and each AOS Fig. 2. The locations of Northern Gannet Morus bassanus colonies marked off as it was in Britain & Ireland in 2004. counted. Sites clearly occu- mid June, but covering all the colonies within pied by nonbreeders were excluded wherever this time frame is dependent on favourable possible. Counts were made by one or two weather conditions, in particular excellent visi- observers and the figures presented here are the bility, low wind speeds and a high cloud-base. means of these counts. Even when photographs Surveys must also fit in with military activities; are of high standard, there is variation both for example, St Kilda, Scare Rocks and Sule among counts made by the same observer and Stack are all subject to various restrictions. among those made by different observers In general, breeding Gannets are unaffected (Harris & Lloyd 1977). Within-observer differ- by slow aircraft whereas most nonbreeding ences in 2004 were generally less than those birds fly off. The usual procedure is, therefore, between observers, with values for the latter of to wait until these birds have left before starting the order of 5-10%. These values were similar to to take photographs. At the Bass Rock and those recorded in the 1984/85 and 1994/95 Grassholm, however, the nonbreeders remain surveys (Murray & Wanless 1986, 1997; Wanless stubbornly on the ground. Consequently, aerial 1987). photographs of these two colonies include both Direct field counts of apparently occupied the breeding and the nonbreeding components nests (AON) were made at nine colonies. On of the population. Visits were therefore made to Foula, AOS were counted directly, while at Sule the Bass Rock on 17th July 2004 and to Skerry both a direct field count and a count Grassholm on 5th September 2004 to establish from a land-based photograph were made. the boundaries of the breeding areas shown by Details of methodology and totals for all the

282 British Birds 98 • June 2005 • 280-294 Northern Gannets in Britain & Ireland in 2003/04

Table 1. The most recent counts of gannetries in Britain & Ireland. Counts from photographs were apparently occupied sites (AOS), most field counts were of apparently occupied nests (AON).

Colony AOS/AON Date of count Method Source % p.a. change since last count

Scotland Bass Rock 48,065 21 June 2004 Aerial Wanless et al. (2005a) 1.9 photography Troup Head 1,547 1 July 2004 Field count Mavor et al. (2005) 20.5 Fair Isle 1,875 13 June 2004 Field count D. Shaw/Fair Isle Bird 8.6 Observatory (in litt.) Noss 8,652 8-14 June, Field count Marshall & Thomas (2003) 1.9 9-30 July 2003 Hermaness 15,633 26 June, Field count Duffield (2003) 3.0 22-26 July 2003 Foula 919 3 July 2004 Field count S. Gear (in litt.) 8.7 Westray 14 21-22 June 2004 Field count K. Judd/RSPB (in litt.) Sule Skerry 57 8 July 2004 Ground D. Budworth/ photograph A. Blackburn (in litt.) 4,618 26 May 2004 Aerial Wanless et al. (2005a) -0.6 photography Sula Sgeir 9,225 26 May 2004 Aerial Wanless et al. (2005a) -1.2 photography Flannan Isles 2,760 26 May 2004 Aerial Wanless et al. (2005a) 6.7 photography St Kilda 59,622 21 June 2004 Aerial Wanless et al. (2005a) -0.1 photography Ailsa Craig 27,130 6 June 2004 Aerial Wanless et al. (2005a) -2.0 photography Scare Rocks 2,394 6 June 2004 Aerial Wanless et al. (2005a) 2.3 photography Total 182,511 0.6 Ireland Ireland’s Eye 285 13-16 June 2004 Field count O. Merne (in litt.) 20.3 Great Saltee 2,446 7-8 June 2004 Field count O. Merne (in litt.) 6.9 Little Skellig 29,683 17 June 2004 Aerial A. Walsh/S. Murray (in litt.) 1.2 photography Bull Rock 3,694 17 June 2004 Aerial A. Walsh/S. Murray (in litt.) 7.4 photography Clare Island 3 July 2004 Field count S. Callaghan (in litt.) 0.0 Total 36,111 2.0 Wales Grassholm 32,094 12 July 2004 Aerial C. M. Perrins/ 1.6 photography Wanless et al. (2005b) St Margaret’s 1 22 May 2004 Field count S. Sutcliffe (in litt.) Island Total 32,095 1.6 England Bempton 3,940 29 July 2004 Field count T. Charlton/RSPB (in litt.) 9.2 Channel Ortac 2,500 late May 1999 Aerial J. Hooper/La Société 3.6 Islands photography Guernesiaise (in litt.) Les Etacs 3,450 late May 1999 Aerial J. Hooper/La Société 0.4 photography Guernesiaise (in litt.) Total 5,950

Total population 260,607 1.2

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Boreray and its two stacks, Stac an Armin and Stac Lee (32,333, 13,369 and 13,920 AOS, respec- tively). Overall there had been vir- tually no change in numbers since 1994. In contrast, the second- largest colony, on the Bass Rock, East Lothian, had increased by 21% since 1994, to 48,065 AOS. The third-ranked gannetry, on Ailsa Craig, Kyle and Carrick, had fared less well and, despite a slight increase in the extent of the colony, numbers (27,130 AOS) were 13.4% lower than in 1995, suggesting an average rate of decrease of 2% p.a. The 1995 count was made from photographs taken relatively late in the season (5th August) and it is possible that there could have been proportion- ally more immatures attending the colony than during the 2004 count, which was made two months earlier (6th June). However, detailed comparison of the two sets of photographs showed a marked decrease in the density of occupied sites and it /SNH/CEH seems likely that numbers had genuinely declined. Counts at this colony made from the sea between the 1940s and 1970s showed peri- Stuart Murray odic, inexplicable fluctuations in 149. Overhead view of Stac Lee, St Kilda,Western Isles, on 21st June 2004, showing the southwest half of the summit table with about 5,000 numbers of occupied sites (Nelson pairs of breeding Northern Gannets Morus bassanus. 2002). Although there have been no recent records of population colonies are given in table 1. Unless otherwise crashes, it is possible that one occurred in 2004. stated, totals for 2004 are compared with those Clearly, a repeat census is required in the near from the 1994/95 survey. Following the pre- future to confirm the status of this colony. The vious convention, overall population estimates two larger colonies, at Hermaness were obtained by combining colony counts, (15,633 AON) and on Noss (8,652 AON), were even if different counting units had been used, counted in 2003 and the totals indicate a con- and expressing these as AOS. tinuing increase since the mid 1990s, although the total for Hermaness was slightly lower than Scotland the count of 16,386 AON made in 1999. Scotland remains the Gannet’s stronghold, both There had been a decline of 11.6% on Sula in terms of colonies (14 in 2004) and in terms Sgeir (9,225 AOS), Western Isles, probably due of population size (182,511 AOS). However, to a reduction in overall nesting density, since overall numbers had increased only by 6% since the extent of the colony appeared unchanged 1994/95, an annual average rate of 0.6% p.a. since 1994. Young gannets have been harvested The 2004 survey showed that St Kilda, at this colony since at least the sixteenth century Western Isles, remains the world’s largest colony and this practice still continues, but under of Gannets with birds breeding on the island of licence. The Men of Ness, from the Isle of Lewis,

284 British Birds 98 • June 2005 • 280-294 Northern Gannets in Britain & Ireland in 2003/04 spend a week or so on the rock in late summer, killing and salting young gannets (or ‘gugas’) to be taken back for distribution to friends and for sale (Beatty 1992). The possible effect of this ‘hunt’ is discussed later. On Sule Stack, Orkney, numbers (4,618 AOS) have shown little change over the last 30 years and it seems likely that the colony is full up, since few apparently suitable areas remain unoccupied. All the five smaller Scottish colonies that were extant in 1994/95 showed substantial increases: Flannan Islands, Western Isles (92%); Scare Rocks, Galloway (23%); Fair Isle and Foula, both in Shetland (127% and 130% respectively); and Troup Head, Northeast Scotland (200%). New colonies have been founded on two islands in Orkney since the 1994/95 survey. At Sule Skerry, Gannets /SNH/CEH were periodically recorded ashore on the west side of the island between 1992 and 2001 Stuart Murray (Sule Skerry Ringing Group in 150. Boreray, St Kilda,Western Isles, 21st June 2004, the most challenging litt.), and in 2002 birds were gannetry in Britain & Ireland to survey.The 32,000 pairs of Northern ‘loafing’ in the Stack Geo area Gannets Morus bassanus nest in groups scattered over more than a among nesting Common kilometre of complex cliff architecture, from sea level to the summit Guillemots Uria aalge. The first of this 400-m-high island. breeding record was in 2003, when 15 nests Am Balg, Sutherland, for example in 1996 and with either eggs or chicks were recorded on 1997 (W. Boyd Wallis pers. comm., Scottish Bird 13th July (Blackburn & Budworth 2004). The Report for 1997) but breeding has not been mean count of 57 AOS in 2004 was made from proved. No birds were seen here on 28th May a photograph taken from the land on 8th July 2004 (pers. obs.). A pair bred on Rockall, 300 km by D. Budworth. The gannetry on sheer cliffs on west of St Kilda, in 1992 (Belaoussoff 1993) but the Noup, Westray, was first noted in 2003, this remote rock was not checked in 2004. when five nests were found, again among Common Guillemots. At least two eggs were Ireland laid and one chick probably fledged (RSPB pers. There are five gannetries in Ireland. In 2004 comm.). In 2004, there were 14 nests in three these contained a total of 36,111 AOS, an discrete groups. increase of 20% since the 1994/95 survey, at an In the past, one or two pairs of Gannets have average rate of 2.0% p.a. bred on two of the Shiant Islands in The Minch, The largest colony, on Little Skellig, Co. between the Outer and mainland Kerry, had 29,683 AOS in 2004, a 12% increase Scotland (Murray & Wanless 1997), but no at 1.2% p.a. since the last count, in 1994. birds were seen ashore in 2004 (pers. obs.). Gannets are occasionally recorded ashore on continued on page 289

British Birds 98 • June 2005 • 280-294 285 Northern Gannets in Britain & Ireland in 2003/04 /SNH/CEH Stuart Murray 151. Roareim, Flannan Isles,Western Isles, 26th May 2004.A view from the south showing the whole gannetry. In 2004 there were 2,760 AOS, nearly double the 1994 count. /SNH/CEH Stuart Murray 152. The colony of Northern Gannets Morus bassanus on the Bass Rock, East Lothian, viewed from the northwest, 21st June 2004.The doubling in size of this colony, to 48,065 AOS since 1985, makes this Britain & Ireland’s second-largest colony.The bird-free hollow below the summit is a long-abandoned garden.The area immediately behind the garden, and to the left of the summit, is occupied mainly by nonbreeding birds. This colony is increasing more rapidly than expected for its size, and could soon overtake St Kilda as the largest gannetry in the east Atlantic.

286 British Birds 98 • June 2005 • 280-294 Northern Gannets in Britain & Ireland in 2003/04 /SNH/CEH Stuart Murray 153. The southern part of the gannetry around the helipad and at Sula Sgeir,Western Isles, on 26th May 2004. Approximately 2,000 young Northern Gannets Morus bassanus are harvested at this colony each year for human consumption and numbers here declined by more than 11% between 1994 and 2004. Alyn Walsh 154. Numbers of nests at the Irish colony on Bull Rock, Co. Cork, doubled between 1994 and 2004 after 25 years of relative stability.This is probably at least partly owing to a lessening of disturbance following the automation of the lighthouse in the late 1980s.

British Birds 98 • June 2005 • 280-294 287 Northern Gannets in Britain & Ireland in 2003/04 /CCW/CEH Christopher Perrins 155. A view showing most of the 32,000 pairs of Northern Gannets Morus bassanus on Grassholm, Pembrokeshire, on 12th July 2004.The colony continues to expand along its landward edge, particularly in the area shown in the bottom centre of this picture.A visit on 5th September 2004 revealed that all the birds on The Tump (the large isolated rock in the background), as well as those in the isolated groups at the extreme left and along the fringe of the colony, were nonbreeders.A land visit to delimit the breeding area was essential before the formidable task of counting nesting pairs from the aerial photographs could begin. Jez Blackburn 156. Few ornithologists have landed on Sule Stack, Orkney.This is the view of the north rock as seen by the Sule Skerry Ringing Group on a visit on 20th July 2003. Counts suggest that this colony may be full.

288 British Birds 98 • June 2005 • 280-294 Northern Gannets in Britain & Ireland in 2003/04 Dave Budworth 157. The newly founded gannetry on Sule Skerry, Orkney, 8th July 2004. Northern Gannets Morus bassanus were first recorded breeding among Common Guillemots Uria aalge on Sule Skerry in 2003. The nearby colony of Sule Stack is visible in the background. continued from page 285 increase (9.2% p.a.) since 1994.

Numbers on Bull Rock, Co. Cork, showed little Channel Islands change between 1970 and 1994. However, the Neither of the Channel Islands gannetries, on 2004 count of 3,694 AOS indicated an approxi- Ortac and Les Etacs, was counted in 2003 or mate doubling in size since 1994, mainly due to 2004. The most recent estimates, in 1999, indi- a large expansion of the colony near the light- cated totals of 2,500 and 3,450 AOS respectively house. Numbers on Great Saltee, Co. Wexford for the two colonies, compared with 2,098 and (2,446 AON), and Ireland’s Eye, Co. Dublin (285 3,380 in 1994, increases of 19% (3.6% p.a.) and AON), continued to increase. Three nests were 2% (0.4% p.a.). Numbers appear to be recorded on a small stack at Clare Island, Co. increasing at both colonies (J. Hooper in litt.). Mayo. There appears to be no room for further Therefore, we used the previous rates of expansion here since this colony has never held increase to calculate a provisional total popula- more than 4 pairs since it was founded in 1978. tion for the Channel Islands of 6,500 AOS in 2004. Wales The gannetry on Grassholm, Pembrokeshire, Discussion has long been the only colony in Wales. In 2004, The survey showed that in 2004 there were 24 there were 32,094 AOS, an increase of 16% gannetries in Britain & Ireland (including the (1.6% p.a.) since 1994, and 4.6% (0.7% p.a.) lone pair on St Margaret’s Island as a colony), since 1999 (Murray 2000). In 2004, breeding and that the total population was approximately was also recorded at St Margaret’s Island, 261,000 AOS. The estimate of 259,000 AOS for Caldey, Pembrokeshire. Since the mid 1990s, the population in 1999 (Wanless & Harris 2004) Gannets have occasionally been recorded on must now be considered too high, since it was stacks here, among Great Cormorants Phalacro- based mainly on the totals for smaller colonies, corax carbo. In 2003, a pair built a nest among which were counted during Seabird 2000, and Herring Gulls Larus argentatus, and in 2004 an the erroneous assumption that the larger egg was laid, though no chick was reared (S. colonies, most of which were not counted, had Sutcliffe in litt). continued to increase at the 1984/85 to 1994/95 rates. Combining the 2004 figures for Britain & England Ireland with the latest available counts for other There were 3,940 AON at Bempton, East York- countries (table 2) suggests an east Atlantic shire, the only colony in England, a 140% population of about 314,000 AOS and a world

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Table 2. The most recent estimates (AON/AOS) of the world population of the Northern Gannet Morus bassanus.

Count Year % east Atlantic % world population population Britain 225,046* 2003-04 71.7 53.9 This survey Ireland 36,111 2004 11.5 8.6 This survey Rouzic, France 17,000 2004 5.4 4.1 Siorat & Bentz (2004) Norway 4,500 2002-03 1.4 1.1 R. T. Barrett (in litt.) Helgoland, Germany 190 2004 0.1 <0.1 O. Hueppop (in litt.) Iceland 28,536 1999 9.1 6.8 A. Gardarsson (in litt.) Mykines, Faeroe Islands 2,340 1995 0.7 0.6 B. Olsen (in litt.) Russia 35 1998 <0.1 <0.1 J. V. Krasnov per R. T. Barrett (in litt.) Total east Atlantic 313,758 100.0 75.1 St Lawrence, Canada 79,321 2004 19 G. Chapdelaine/ Canadian Wildlife Service (in litt.) Newfoundland, Canada 24,500 2004 5.7 J. W. Chardine/ Canadian Wildlife Service (in litt.) Total west Atlantic 103,026 24.9 Total world population 417,579 100 * includes an estimate of 6,500 AOS for the Channel Islands (see text)

4 population of about 418,000 AOS, with Britain & Ireland holding 83% and 62.5% of these 3 totals respectively. Although the British and Irish 2 population of the Gannet Bass Rock increased by 12% since 1994/95, the average rate of increase of 1 1.2% p.a. was markedly lower than the 2.0% p.a. recorded 0 between 1969/70 and 1984/85 Log increase and the 2.2% p.a. recorded between 1984/85 and 1994/95 -1 Sule Stack (Wanless 1987; Lloyd et al. 1991; Wanless & Harris 2004). The -2 latest counts of colonies in Sula Sgeir Norway, the French colony on Rouzic in the Sept-Iles, Nor- -3 2 4 6 8 10 12 mandy, and the German colony Log colony size in 1969 on Helgoland (table 2) also suggest that the rates of increase Fig. 3. Log-log plot of the rates of change in numbers of Northern Gannets Morus bassanus at British and Irish gannetries between 1969 at these colonies, and hence in and 2004, in relation to colony size in 1969.The majority of colonies the whole east Atlantic popula- lie close to the line, indicating density-dependent population regulation. tion, are slowing down. The point representing Sula Sgeir falls well below the line, which Two recent studies have inves- suggests a much lower rate of increase than would be expected. In contrast, the point for the Bass Rock lies well above the line, tigated how rates of population indicating that it has increased at a much faster rate than predicted change at gannetries vary as their for its size (see text for details and discussion). population size increases (Lewis

290 British Birds 98 • June 2005 • 280-294 Northern Gannets in Britain & Ireland in 2003/04 Karen Judd Karen 158. The gannetry on the Noup,Westray, Orkney, was established in 2003 when three pairs of Northern Gannets Morus bassanus were reported breeding among Common Guillemots Uria aalge. In 2004 the colony had increased to 14 nests and this picture taken in August 2004 shows a group of five nests with chicks. et al. 2001; Moss et al. 2002). New gannetries rather than the increase expected for its size. must obviously be founded by birds that were This is the only colony where harvesting of raised at another colony. After colonisation, chicks for human consumption is still per- numbers typically increase rapidly with much mitted. Licence returns indicate that 33,690 of the initial increase being fuelled by immigra- young were taken between 1985 and 2001. It tion (Nelson 2002). As the colony grows, seems likely that additional chicks must die however, net immigration becomes less impor- through falling off the ledges or being unable to tant (Moss et al. 2002). Comparing the rate at get back to their nests during the intense and which the number of sites at a colony changes prolonged disturbance that must inevitably in relation to the size of the colony shows the occur during the hunt (Beatty 1992). An assess- classic pattern expected if density-dependent ment of this kill on the Gannet population on competition for some resource is the main Sula Sgeir is hampered by the lack of any infor- factor determining population size (Lewis et al. mation on breeding success or adult survival at 2001; Moss et al. 2002). Hence, when we plot this colony. However, assuming that Gannets on the average rate of change between the counts Sula Sgeir have a similar breeding success to for the 1969/70 Operation Seafarer survey and those elsewhere in Scotland (0.66-0.78 chicks the 2003/04 survey of gannetries in Britain & per pair; Nelson 2002; Mavor et al. 2004), the Ireland (fig. 3), we see a significant negative harvest represents about 30% of the annual relationship between colony growth rate and production of chicks. Moreover, since the colony size. licence returns have remained constant, the Most of the gannetries lie relatively close to proportion of chicks harvested will be the line but one colony, Sula Sgeir, stands out as increasing as the population declines. There are showing a substantial decrease in numbers no other obvious colony-specific threats to the

British Birds 98 • June 2005 • 280-294 291 Northern Gannets in Britain & Ireland in 2003/04 Oscar Merne 159. The top of the main (original) colony on Great Saltee, Co.Wexford, on 8th June 2004 showing where birds have removed the Sea Campion Silene uniflora for nest-building. Oscar Merne 160. Looking down at the Southern Slope colony on Great Saltee, Co.Wexford, on 8th June 2004. The colony extends over the cliff edge.This is where the recent major spread has occurred and there is plenty of room for more pairs.

292 British Birds 98 • June 2005 • 280-294 Northern Gannets in Britain & Ireland in 2003/04

Gannets of Sula Sgeir so the guga hunt appears if adult mortality does increase, then we would to be the most likely cause of the lack of growth expect a negative effect on population size. of this colony. The 2003/04 survey indicated that Scotland The gannetry on Sule Stack has also shown remains the stronghold of the Gannet, with the little change during the last 20-30 years, but largest colony still being on St Kilda. However, here there appears to be a shortage of space. In numbers there are more or less stable, whereas contrast, the Bass Rock colony has increased those on the Bass Rock, the second-largest faster than predicted for its size (4.8% p.a. over colony, are currently increasing at 1.9% p.a. If the last 35 years), perhaps indicating that condi- this pattern continues over the next 5-10 years, tions for Gannets have been particularly then the Bass Rock will overtake St Kilda to favourable in the North Sea. This increase become the largest east Atlantic colony. cannot continue for too much longer, however, Nonetheless, the award for the largest colony in as there is now limited room for expansion. The the North Atlantic seems likely soon to be won comparison between the Bass Rock and Sula by Bonaventure Island, Quebec, which had Sgeir is stark: in 1969, the counts differed by 53,635 pairs in 2004 and had increased by a only 13 AON, while in 2004 the former had staggering 45% (7.7% p.a.) since 1999 (Gilles over five times as many AOS as the latter. Chapdelaine/Canadian Wildlife Service in litt.). In conclusion, results from the 2003/04 The relentless rise in the numbers of the survey of Gannets in Britain & Ireland showed Gannet is a classic among long-term ornitho- that the population continued to increase, but logical studies. In Britain & Ireland there were there are signs that the seemingly relentless rise thought to be around 47,000 nests in the eight in numbers could be coming to an end. At known gannetries in 1904 (Fisher & Vevers present, it is unclear how much this is due to 1944); by 2004, there were 5-6 times as many the larger colonies being close to their carrying occupied nest-sites and three times as many capacity, either because of lack of breeding sites gannetries. The rate of increase now appears to (although this seems unlikely in the case of St be slowing down. Given the importance of this Kilda, Ailsa Craig and Grassholm) or because of population in a world context, monitoring competition for food during the breeding future changes and establishing the causes of season. Lewis et al. (2001) showed that, theoret- population change merit a high priority. ically, density-dependent disturbance of fish shoals by diving Gannets was sufficient to sig- Acknowledgments nificantly reduce feeding success close to the We sincerely thank the many people who took part in this colony, thereby generating appropriate condi- survey, particularly those whose counts are acknowledged in table 1. Pilots George Cormack and Robin Kilroy made tions for intraspecific competition for food. To the aerial surveys possible and Stuart Benn, Jill Harden, date, the Gannet appears either to have been Tony Mainwood and Matt Parsons provided photographic unaffected by, or even to have benefited from, support for the Scottish flights. Christopher Perrins took the aerial photographs of Grassholm, and Simon Avery the changes that have occurred in the seas and Ian Bullock helped to arrange the colony visit. Malcolm around Britain & Ireland. However, there is Collie and Denise Wright provided computing assistance. increasing concern that climate change is The aerial surveys of Scottish colonies and Grassholm were undertaken under contracts with Scottish Natural affecting the east Atlantic ecosystem and Heritage and the Countryside Commission for Wales, causing major changes to the food-web, partic- respectively, and we thank Rhys Bullman and Catherine ularly in the North Sea (Edwards & Richardson Gray for facilitating this work.The Irish National Parks and 2004; Richardson & Schoeman 2004). While Wildlife Service Departments of Environment, Heritage and Local Government financed the flights to Little Skellig Gannets were one of the few seabirds not to and Bull Rock. suffer major breeding failure in northern Britain in 2004, it is possible that conditions for References them are becoming less favourable. The species Beatty, J. 1992. Sula: the seabird hunters of Lewis. Michael could also be adversely affected by changes in Joseph, London. discarding practices by fishing vessels (Votier et Belaoussoff, S. 1993. Northern Gannet and Common Guillemot nesting on Rockall. Brit. Birds 86: 16. al. 2004). An analysis of Gannet ringing recov- Blackburn, A., & Budworth, D. 2004. A new gannetry for eries for Britain & Ireland over the last 40 years the UK. Seabird Group Newsletter 96: 10. found evidence for an increase in adult mor- Brooke, M. L. 2004.The food consumption of the world’s seabirds. Proc. Roy. Soc. Lond. B 271: S246-S248. tality over the last ten years (Wanless et al. in Croxall, J. P., Evans, P.G. H., & Schreiber, R.W. (eds.) 1984. press). The reason for this remains unclear but Status and Conservation of the World’s Seabirds.Vol. 2.

British Birds 98 • June 2005 • 280-294 293 Northern Gannets in Britain & Ireland in 2003/04 Jez Blackburn 161. Northern Gannets Morus bassanus and Common Guillemots Uria aalge on Sule Stack, Orkney.

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