The importance of the United Kingdom for wintering gulls: population estimates and conservation requirements

Abstract

Capsule The UK supported over 3.8 million wintering gulls in 2003/04–2005/06.

Aims To provide new winter population estimates for Great Britain, its constituent countries, Northern Ireland, Channel Islands and Isle of Man, for five species of gull.

Methods Observers undertook counts of gulls at roosts across three winters. Population estimates were produced by combining counts from key inland and coastal sites and estimates derived from stratified random sampling of inland tetrads and coastal stretches.

Results Over 3.8 million gulls were estimated to winter in the UK and its near-shore coastal waters, a total that, due to more comprehensive methods, adds appreciably to previous estimates. Results inform conservation listings and required site-based conservation measures. Twenty sites held internationally important numbers of individual species and 37 internationally important assemblages and provisionally could qualify as Special Protection Areas for gulls alone. Forty-nine sites held nationally important numbers of particular species and provisionally could be notified as Sites/Areas of Special Scientific Interest solely for their importance for gulls.

Conclusions Given the importance of the UK for wintering gulls and the declining status of some species, there is a clear requirement to consider measures and information needs that would provide for their conservation.

Gulls have not, on the whole, been traditional targets for conservation in many countries including the UK, because of the problems that they may cause, to humans (Benton et al. 1983, Horton et al. 1983, Monaghan et al. 1985, Ferns & Mudge 2000, Allan 2002, Sodhi 2002, Calladine et al. 2005, Dolbeer & Wright 2008) and other wildlife and their habitats (Harris & Wanless 1997, Finney et al. 2001, 2003, Guillemette & Brousseau 2001, Oro et al. 2005), and thus the perception that they are pests (Oro & Martínez-Abraín 2007). Nevertheless, there remains a need to assess the conservation status of these species in their own right, while statutory obligations require consideration of the measures and information needs that would provide for their conservation.

As with other bird species, the status of gulls has been assessed both at European and UK levels. Due to recent population declines, likely largely driven by changes in the availability of food, e.g. from fishery discards (Camphuysen & Garthe 2000), even some widespread gull species are now included in conservation listings (Table 1).

Table 1. The statuses in existing conservation listings of the principal species considered in this article.

Species	UKBAP^1	BoCC^2	BiE2/BiEU^3	AEWA^4
Black-headed Gull		Amber	Non-SPEC^E	B2
Common Gull		Amber	SPEC 2	B2
Lesser Black-backed Gull		Amber	Non-SPEC^E
Herring Gull	argenteus ssp.	Red	Non-SPEC^E	B2
Great Black-backed Gull		Amber	Non-SPEC^E
^1 UKBAP: UK Biodiversity Action Plan species (http://www.ukbap.org.uk/NewPriorityList.aspx).
^2 BoCC: Birds of Conservation Concern (Eaton et al. 2009).
^3 BiE2: Birds in Europe (BirdLife International 2004a)/BiEU: Birds in the European Union (BirdLife International 2004b). SPEC 2 = species with populations concentrated in Europe and with an unfavourable conservation status; Non-SPEC^E = species with populations concentrated in Europe but with a favourable conservation status.
^4 African–Eurasian Waterbird Agreement (AEWA). B2 = species' populations listed in Column B, Category 2, i.e. populations numbering more than around 100 000 individuals and considered to be in need of special attention as a result of (c) showing significant long-term decline (http://www.unep-aewa.org/documents/agreement_text/table1-overview.htm).

The UK holds considerable proportions of the European breeding populations of gull species (BirdLife International 2004a), most notably 65% of the world population of the graellsii subspecies of Lesser Black-backed Gull Larus fuscus (Mitchell et al. 2004). Consequently, a total of 33 Special Protection Areas (SPAs) classified in the UK under the EU Birds Directive include breeding gulls as features. The UK is also well recognized as an important wintering area for Europe's breeding gull populations (Stanley et al. 1981, Coulson et al. 1984a, 1984b, Horton et al. 1984, MacKinnon & Coulson 1987, Wernham et al. 2002). Nevertheless – and in notable contrast to other waterbirds – gulls have not, to date, been included as qualifying species of UK SPAs on the basis of their winter (or ‘non-breeding’ season) numbers, largely owing to a lack of data (Stroud et al. 2001). There is thus a current need to assess the protection afforded winter populations, both in the UK and elsewhere in Europe – a need that is recognized by a recent review of the UK SPA network (Musgrove et al. 2011b) and recommended also in the UK Biodiversity Action Plan for Herring Gull (http://jncc.defra.gov.uk/_speciespages/2362.pdf). However, to facilitate this, it is essential that surveys exist to provide reliable estimates of gull populations in the non-breeding season that will provide the context for identifying important sites.

In this article, we report the results of a survey undertaken between 2003/04 and 2005/06 to quantify the importance of the UK for the five principal species of gull that over-winter in the country's terrestrial areas and near-shore waters – Black-headed Gull Chroicocephalus ridibundus, Common Gull Larus canus, Lesser Black-backed Gull, Herring Gull L. argentatus and Great Black-backed Gull L. marinus – and to provide information useful to the species' conservation. The potential use of the population estimates produced from the survey and the counts themselves in informing conservation listings and site-based conservation measures is discussed. Specifically, we identify a provisional list of sites of international or national importance for gulls, based on the counts from the survey and thresholds derived from the new population estimates, which provisionally could qualify for classification as SPAs (or under other protection regimes) for their gull populations alone. A site may be considered for SPA status if the populations of birds that they support surpass certain thresholds – for the migratory species considered here, sites are considered if the species' numbers regularly exceed 1% of the species' biogeographic population size (Stage 1.2 of the selection process) or if more than 20 000 waterbirds occur there regularly (Stage 1.3 of the selection process) (Stroud et al. 2001). Sites regularly supporting 1% of species' British or all-Ireland populations may also be notified, respectively, as Sites of Special Scientific Interest (SSSI) in Britain or Areas of Special Scientific Interest (ASSI) in Northern Ireland. Knowledge gaps – notably the further data requirements for statutory site identification – that hinder present ability to meet gull conservation requirements are highlighted.

METHODS

Geographical scope and field methods

The 2003/04–2005/06 Winter Gull Roost Survey covered all four constituent countries of the UK (England, Scotland, Wales and Northern Ireland), and the Crown Dependencies of the Channel Islands and Isle of Man. The population estimates produced are representative of those birds wintering within these countries' land areas and their near-shore coastal waters. The surveys excluded birds which may have roosted offshore, not visible from land, but still within UK territorial waters (i.e. 12 nautical miles from shore). This geographical scope equates to that previously used for other UK or Great Britain (‘Britain’) bird population estimates and the thresholds used for identifying sites worthy of statutory protection (Stroud et al. 2001). While many gulls may potentially over-winter further offshore within the UK offshore marine area (i.e. within British Fishery Limits and the UK Continental Shelf Designated Area), knowledge of their numbers is limited, although European Seabirds At Sea (ESAS) data have been used to estimate numbers in the North Sea as a whole (Skov et al. 1995, 2007) and more recently to identify areas within the British Fishery Limit that might qualify as possible marine SPAs (Kober et al. 2010).

Population estimates were generated from one-off counts of survey sites undertaken between December and February in one of the winters of 2003/04, 2004/05 or 2005/06. The field methodology followed that used by previous decadal winter gull surveys (Hickling 1954, 1967, 1977, Bowes et al. 1984, Burton et al. 2003) with volunteer observers arriving at sites 2–3 hours before dark, first counting any birds already settled and then counting further birds arriving to roost at dusk (Hickling 1977). At larger roosts, several observers counted birds arriving simultaneously on different flight-lines. Counts at individual roosts may have underestimated overall numbers if many birds arrived after dark; however, movements to and from roosts after dusk are thought to be limited (Austin et al. 2003).

Identification of individual species was sometimes not possible, and so numbers of unidentified ‘small’ gulls (Black-headed and Common Gulls) and ‘large’ gulls (Lesser Black-backed, Herring and Great Black-backed Gulls) were provided; where assignment to size class was not possible, birds were classified as ‘unidentified’. It was assumed that numbers of scarce gull species (Little Gull Hydrocoloeus minutus, Mediterranean Gull Larus melanocephalus, Ring-billed Gull L. delawarensis, Caspian Gull L. cachinnans, Yellow-legged Gull L. michahellis, Iceland Gull L. glaucoides, Glaucous Gull L. hyperboreus) and the largely maritime Black-legged Kittiwake Rissa tridactyla in these groupings were negligible.

Survey design

Counts at roosts provide the most convenient means to estimate total winter gull populations. In contrast to the day when birds may be distributed widely across varied foraging habitats, roosting gulls are usually restricted to wetland habitats (typically large inland water bodies or coastal near-shore waters). However, although sizeable proportions of species' populations may be counted at a relatively few key sites, the summed counts from these sites only provide minimum estimates of the overall populations of gulls wintering in the country. In order to obtain more complete estimates (with confidence limits), the 2003/04–2005/06 survey also sampled other areas away from these sites, both inland and on the coast. Survey sites were thus split into four categories: inland and coastal key sites, random inland tetrads and random coastal stretches.

Key sites

A total of 482 sites (271 inland, 211 coastal) were identified prior to the survey as being important for gulls on the basis that previous Winter Gull Roost Surveys or recent local bird reports had shown that they had held at least 1000 roosting gulls. Key sites were targeted in the first winter of the survey, in particular the weekend of 17–18 January 2004 (to minimize double-counting and so that counts were undertaken during the same month as the International Waterbird Census: http://www.wetlands.org/Whatwedo/Biodiversitywaterbirds/InternationalWaterbirdCensusIWC/tabid/773/Default.aspx, and could thus feed into this international scheme). Key sites not covered in the first winter were targeted in the subsequent two winters to aim to ensure that all sites were covered once. As the survey as a whole was spread across three winters, it is acknowledged that some birds may have been double-counted or alternatively missed due to movements between sites.

Random inland tetrads

A sample of random inland tetrads (a standard sampling unit of 2 × 2 km) was selected from a stratification based on freshwater cover data derived from the Centre for Ecology and Hydrology Landclass 2000 database (CEH2000: Fuller et al. 2002) and coastal proximity. Use of this stratification aimed to minimize the magnitude of confidence limits attached to the resulting population estimates while ensuring that the full range of UK habitats used by gulls was surveyed.

The CEH2000 data cover the UK at 1-km resolution. Freshwater cover data were summarized to a tetrad resolution in an ArcView Geographic Information System (GIS: ESRI 2003) project and reclassified according to percentage water cover into ‘no water’ (0%), ‘low water’ (≤ 5%) and ‘high water’ (> 5%). It was thought likely that the numbers of gulls on tetrads in close proximity to the coast would be consistently different to tetrads further inland. Thus tetrads were further classified by coastal proximity using an arbitrary 1-km buffer to the landward side of the coast. All tetrads that clipped this buffer were classified as ‘coastal’ while those which did not were classified as ‘inland’.

The freshwater cover classification was superimposed on tetrads not in close proximity to the coast to give three ‘inland’ strata, thus giving a four-strata classification for the purposes of targeting sampling effort – inland high water, inland low water, inland no water and coastal. Tetrads encompassing inland key sites were excluded from the selection of random inland tetrads and subsequent extrapolation from sample tetrads surveyed. In total, a sample of 701 random inland tetrads was selected from the stratification for surveying. Counts of these sites were spread across the three winters of the survey.

Random coastal stretches

Sample random coastal stretches were selected once the majority of coastal key sites had been covered and their boundaries mapped and thus mostly took place in 2004/05 or 2005/06. Points were selected at a regular interval along the coastline, to give a pseudorandom sample of 933 coastal stretches. Observers were asked to choose a suitable vantage point as close as possible to central grid references to undertake counts and were provided with maps to record the boundaries of their chosen count sections. For the purposes of analyses, the coast outwith surveyed key sites was divided into two strata – one for unsurveyed key coastal sites and one for the remaining coast where gull densities were expected to be lower.

The strata of random inland tetrads and random coastal stretches were also split into 13 regions – north and west Scotland, east Scotland, southwest Scotland, northwest England, northeast England, Wales, the Midlands, East Anglia, southwest England, southeast England, Northern Ireland, Isle of Man and Channel Islands (Fig. 1). These regions were based on those used in previous decadal surveys and provided a further stratification that broadly reflected north–south and east–west clines in species' distributions across the UK (Lack 1986).

Figure 1. Stratification used to generate population estimates of wintering gulls in Great Britain, its constituent countries, Northern Ireland, the Channel Islands and Isle of Man, for the five principal species of gull that winter in the UK. Dark grey = tetrads in the inland high water strata; light grey = tetrads in the inland low water strata; white = tetrads in the inland no water strata. Grey lines indicate regional boundaries used in the stratification: 1 = north and west Scotland; 2 = east Scotland; 3 = southwest Scotland; 4 = northwest England; 5 = northeast England; 6 = Wales; 7 = midlands; 8 = East Anglia; 9 = southwest England; 10 = southeast England; 11 = Northern Ireland; 12 = Isle of Man; 13 = Channel Islands. Black indicates inland and coastal key sites.

Data analysis

Only counts undertaken between December and February were used to calculate population estimates. It is assumed throughout that counts at different sites were mutually exclusive. Targeting a single weekend in January 2004 for coverage of key sites promoted synchronicity of these counts and avoided repeat counting of the same birds at the most important sites. At sites counted more than once, the count nearest to the Saturday of the target weekend was treated as definitive, all others as supplementary.

Population sizes for each of the five principal species were estimated using bootstrap techniques similar to those used successfully for waterbirds (Rehfisch et al. 2003, Jackson et al. 2006, Austin et al. 2007). With 999 repetitions, separate estimates were made of the total population size in Britain (and its constituent countries England, Scotland and Wales), Northern Ireland, the Channel Islands and Isle of Man. Each of these overall estimates was obtained by summation of the total number of individuals recorded across all key sites and estimates for each stratum represented in each region contributing or equating to the country or dependency in question. The latter were derived for each stratum by taking a random sample with replacement from the survey data (random inland tetrads and random coastal stretches) for the given stratum until the cumulative land area (for inland strata) or coastal length (for coastal strata) equated to the total for the entire country or dependency assigned to that stratum outwith the key sites. With each repetition, an overall estimate for Britain was obtained by summing the estimates for England, Scotland and Wales. The 500th, 25th and 974th ascendant-ordered estimates were used to estimate, respectively, median, lower and upper 95% confidence limits for the population.

Data frequently included counts of unidentified gulls. During each bootstrap repetition, a unique estimate was thus made for each region of the proportion of positively identified gulls known to belong to the species in question. Each estimate of this proportion was obtained by drawing a random sample with replacement of 100 (arbitrarily chosen as a large number relative to the average number of samples representing each stratum) from all key and random sample sites within the appropriate region, summing across species and calculating the proportion of this total belonging to the species in question. This was done separately for inland and coastal sites but otherwise without regard for strata. These estimates were then used to derive adjusted counts (C_adj) to include the total positively identified as the species in question (C_species) and an expected number of the species that had been recorded as either ‘small gulls’ (E_small) or ‘unidentified’ (E_unidentified) in the case of Black-headed and Common Gull, or as ‘large gulls’ or ‘unidentified’ (E_large) for the other three principal species, i.e.:

for Black-headed and Common Gull: C_adj = C_species + E_small + E_unidentified

and for Herring, Lesser Black-backed and Great Black-backed Gull: C_adj = C + E_large + E_unidentified

where E_small, E_large and E_unidentified are estimated from the sample with replacement as follows:

E_small = C_small × Σ C_species / Σ C_small (counts for all positively identified small gulls)

E_large = C_large × Σ C_species / Σ C_large (counts for all positively identified large gulls)

E_unidentified = C_unidentified × (Σ C_species + Σ E_small) / Σ C_all (counts for all gulls identified to species or size-class)

or E_unidentified = C_unidentified × (Σ C_species + Σ E_large) / Σ C_all (counts for all gulls identified to species or size-class).

Population estimates were only calculated for the five principal species that winter in the UK. Raw totals are presented for other species, although given the small numbers counted, it was not appropriate to provide extrapolated estimates.

The population estimates calculated for Britain were used to calculate thresholds – rounded 1% levels of the estimates – for identifying sites of national importance for each species. Sites where the counts surpassed these national thresholds (or equivalent all-Ireland thresholds for sites in Northern Ireland: Crowe 2005) or international thresholds (Wetlands International 2012), on at least one occasion, are listed. This list is based on the raw counts of individual species and not estimated numbers incorporating counts of ‘small’, ‘large’ or ‘unidentified’ gulls, as their use might lead to inclusion of sites which do not in actuality pass thresholds. It should also be noted that the advised 1% threshold of international importance for Herring Gull relates solely to the argenteus subspecies. Similarly, the 1% international importance threshold for Lesser Black-backed Gull is based on the graellsii subspecies only (following protocols used in Holt et al. 2012 in each case). In addition, we list those sites which held at least 20 000 roosting gulls and thus which are of international importance on the basis of total numbers (Stroud et al. 2001). Estuaries were treated as single discrete sites in this analysis (to match the delimitation of existing protected sites), by summing counts from subsites.

RESULTS

Population estimates

Over the three winters encompassed by the survey, 435 (90%) of 482 identified key sites, 520 (74%) of 701 random inland tetrads and 505 (54%) of 933 random coastal stretches were surveyed, and (excluding supplementary counts) a total of 2 440 681 gulls was counted (Table 2). Population estimates produced from these counts are provided in Table 3.

Table 2.  Raw count totals of individual gull species, ‘small gulls’, ‘large gulls’ and ‘unidentified’ gulls used to produce population estimates.

Species	Inland	Coastal	Total
Black-headed Gull Chroicocephalus ridibundus	641 395	534 616	1 176 011
Common Gull Larus canus	212 568	198 457	411 025
‘Small gulls’	189 086	126 042	315 128
Lesser Black-backed Gull Larus fuscus	48 219	13 576	61 795
Herring Gull Larus argentatus	82 331	211 211	293 542
Great Black-backed Gull Larus marinus	14 967	17 880	32 847
‘Large gulls’	33 193	32 550	65 743
‘Unidentified’	485	82 304	82 789
Mediterranean Gull Larus melanocephalus	20	258	278
Little Gull Hydrocoloeus minutus	0	11	11
Ring-billed Gull Larus delawarensis	0	3	3
Caspian Gull Larus cachinnans	2	3	5
Yellow-legged Gull Larus michahellis	59	51	110
Iceland Gull Larus glaucoides	12	16	28
Glaucous Gull Larus hyperboreus	8	17	25
Black-legged Kittiwake Rissa tridactyla	0	1 341	1 341
Total	1 222 345	1 218 336	2 440 681

Table 3. Population estimates (with 95% confidence limits in parentheses) for the five principal wintering gull species in Great Britain, its constituent countries, Northern Ireland, the Channel Islands and Isle of Man in 2003/04–2005/06 and in Great Britain and the UK in 1993 (after Burton et al. 2003).

Survey / country	Black-headed Gull	Common Gull	Lesser Black-backed Gull	Herring Gull	Great Black-backed Gull
2003/04 -2005/06
England	1 854 876 (1 796 286–1 922 835)	469 863 (449 287–491 278)	114 369 (107 742–120 437)	362 821 (346 588–379 444)	53 361 (48 884–58 325)
Scotland	199 682 (188 437–211 796)	200 296 (185 410–215 034)	6 510 (5 742–7 294)	273 058 (252 574–293 613)	18 113 (16 751–19 653)
Wales	100 836 (87 741–116 973)	25 133 (21 686–29 121)	3 838 (3 321–4 453)	93 613 (75 238–116 885)	4 365 (3 897–4 852)
Great Britain	2 155 147 (2 093 327–2 225 476)	695 833 (669 581–721 158)	124 654 (118 055–131 148)	729 801 (696 424–762 731)	75 860 (71 209–80 704)
Northern Ireland	44 336 (37 813–51 822)	9 559 (7 464–11 988)	459 (280–552)	13 559 (10 797–16 776)	750 (588–926)
UK (Britain and Northern Ireland)	2 199 483	705 392	125 113	743 360	76 610
Channel Islands	7 565 (5 476–9 438)	7 702 (4 402–12 102)	14 (8–22)	10 828 (7 743–14 594)	732 (479–1 047)
Isle of Man	1 753 (598–3 159)	35 (15–60)	7 (3–12)	10 106 (8 630–11 542)	566 (461–680)
1993
Great Britain	1 682 385	429 331	60 830	376 775	43 108
UK	1 697 797	430 927	60 830	378 748	43 156

Black-headed Gull was by far the most abundant species, with total estimated numbers exceeding those of the other four principal species combined. Most (86%) of the estimated Britain population of 2 155 147 was found in England, 9% in Scotland and 5% in Wales.

The population estimates for Common Gull and Herring Gull were similar, though their distributions differed. Of 695 833 Common Gulls in Britain, 68% were found in England, 29% in Scotland and 4% in Wales. In comparison, Scotland and Wales, respectively, held 37% and 13% of the 729 801 Herring Gulls in Britain.

Around 92% of the 124 654 Lesser Black-backed Gulls wintering in Britain were estimated to occur in England and just 5% and 3% in Scotland and Wales respectively. England likewise held the majority (70%) of the 75 860 British Great Black-backed Gulls, Scotland and Wales supporting just 24% and 6%, respectively.

The numbers of gulls wintering in Northern Ireland, the Channel Islands and Isle of Man are comparatively small, although 44 336 Black-headed Gulls were estimated to occur in Northern Ireland and > 10 000 Herring Gulls in each area.

Site importance

Thresholds for identifying sites of national importance in Britain (derived from the population estimates presented here) are presented in Table 4, together with equivalent all-Ireland thresholds for sites in Northern Ireland, thresholds for identifying sites of international importance and associated biogeographic population estimates. Sites where numbers of individual species surpassed national or international thresholds, on at least one occasion, are listed in Appendix 1.

Table 4. National and international thresholds (and associated biogeographic population estimates) for the five principal wintering gull species in Great Britain and Northern Ireland used in the identification of important sites.

	1% GB threshold^a	All-Ireland threshold^b	1% international threshold^c	Biogeographic population(s)^c
Black-headed Gull	22 000^d	1 000	42 100^d	3 700 000–4 800 000 (W Europe population)
Common Gull	7 000	500	16 400	1 200 000–2 250 000 (NW and C Europe population)
Lesser Black-backed Gull	1 200	500	5 500^e	530 000–570 000 (subspecies graellsii) 325 000–440 000 (subspecies intermedius)
Herring Gull	7 300	500	10 200^f	1 300 000–3 100 000 (subspecies argentatus) 990 000–1 050 000 (subspecies argenteus)
Great Black-backed Gull	760	500	4 200	330 000–540 000 (N and W Europe population)
^a Thresholds for Great Britain are derived from the new population estimates in Table 3 and are rounded following the conventions of Musgrove et al. (2011a): for populations of 10 001–100 000, thresholds are rounded to the nearest 10; for populations of over 100 000, thresholds are rounded to the nearest 100; for populations of over 1 000 000, thresholds are rounded to the nearest 1 000.
^b There are no all-Ireland population estimates for wintering gulls and thus no 1% thresholds. For Northern Ireland, we thus use the threshold values used to identify important sites presented by Crowe (2005).
^c International importance thresholds and biogeographic population estimates are taken from Wetlands International (2012).
^d For all populations of 2 000 000 or more individuals, the 1% international threshold is set at 20 000, as all sites which regularly hold 20 000 or more waterbirds of any species qualify as wetlands of international importance under Ramsar Criterion 5 (Wetlands International 2012); 20 000 is thus also the maximum national importance threshold (Holt et al. 2012).
^e Threshold for Larus fuscus graellsii used here following Holt et al. (2012).
^f Threshold for Larus argentatus argenteus used here following Holt et al. (2012).

On the basis of the new population estimates and survey counts, 20 sites were identified as holding internationally important numbers of one or more species of gull and a further 29 held nationally important numbers of one or more species. In total, there were 28 individual cases where international thresholds were surpassed and 74 where national thresholds were surpassed (62 in Britain, 12 in Northern Ireland). More sites were identified by this analysis as being nationally important for Lesser Black-backed Gull than for any other species.

A total of 37 sites also held assemblages of at least 20 000 gulls (Appendix 2). The most important single site was The Wash in eastern England, which held an overall total of 124 907 gulls. Bewl Water, Sussex was the most important inland site, with a peak (supplementary count) of 103 021 gulls in January 2005.

All of these sites were identified as key sites prior to the survey.

DISCUSSION

Comparison with previous national and European population estimates

The population estimates reported here add appreciably – between 30% and 102% for the UK as a whole – to the previous 1993 national estimates (Table 3; Burton et al. 2003). The 1993 survey, and those that preceded it, only comprised counts of known roosts and as some roosts were inevitably missed, particularly on the coast and in less populated regions, and no allowance was made for this, these surveys only provided minimum population estimates. Given that earlier analysis of data from key sites from the 2003/04–2005/06 and previous surveys also indicated that there have been recent declines in the numbers of some species (Banks et al. 2009, see below), the increase in population estimates would thus appear to be largely due to the sampling of areas outwith key sites. Therefore, the 2003/04–2005/06 survey has provided the most complete population estimates of winter gull populations in Britain, Northern Ireland, the Channel Islands and Isle of Man to date.

Present European estimates – which include totals for the UK from the 1993 survey – suggest that the UK holds 53%, 47%, 47%, 47% and 29% of the respective European populations of Black-headed Gull (3 200 000 birds), Common Gull (910 000), Lesser Black-backed Gull (130 000), Herring Gull (800 000) and Great Black-backed Gull (150 000) (Fig. 2; BirdLife International 2004a). These are certainly underestimates, however, and do not reflect the totals of 175 530 Common Gulls, 15 315 Lesser Black-backed Gulls, 971 700 Herring Gulls and 299 900 Great Black-backed Gulls that have been estimated to winter in the North Sea (Skov et al. 1995, 2007). Inclusion of the 2003/04–2005/06 survey results would suggest that the UK holds more significant proportions of the species' European populations – 60%, 60%, 64%, 64% and 42%, respectively, for Black-headed Gull, Common Gull, Lesser Black-backed Gull, Herring Gull and Great Black-backed Gull. However, it should be noted that the figures from the 2003/04–2005/06 UK survey represent by far the most comprehensive population estimates for wintering gulls reported for any country, with the main source of information for most other European countries being the daytime waterbird counts undertaken as part of the International Waterbird Census, which are likely to underestimate the total numbers of these species (see discussion of knowledge gaps below).

Figure 2. The proportions of the European winter populations of Black-headed Gull (BH), Common Gull (CM), Lesser Black-backed Gull (LB), Herring Gull (HG) and Great Black-backed Gull (GB) held by the UK, incorporating data from the 1993 UK survey (black columns; BirdLife International 2004a), and the increase on these figures suggested by the 2003/04–2005/06 UK survey (grey columns).

The winter populations estimated for the UK also represent considerable percentages of species' respective biogeographic populations (see Table 4): Black-headed Gull – 52%, Common Gull – 41%, Lesser Black-backed Gull – 13% (of the combined graellsii and intermedius populations), Herring Gull – 23% (of the combined argentatus and argenteus populations) and Great Black-backed Gull – 18%.

The importance of the UK for wintering gulls

The new winter population estimates add to the improved knowledge of gull populations obtained by the most recent national breeding census, Seabird 2000 (Mitchell et al. 2004) and have been incorporated into a recent review of British wintering waterbird population estimates (Musgrove et al. 2011a). By comparing breeding and wintering gull estimates it is possible to consider the likely scale of movements into and out of the UK at the end of the breeding season. In making such comparisons, it should be recognized that estimates of breeding season numbers exclude an unknown proportion of non-breeding birds, so the scale of difference between breeding and wintering estimates is likely to be exaggerated. This may be lessened for species such as Lesser Black-backed Gull because many immature birds do not return to the UK until breeding age (Wernham et al. 2002), but is more of an issue for other species because immature birds may summer near breeding colonies.

While 127 907 apparently occupied nests of Black-headed Gulls (i.e. around 256 000 breeding adults) were recorded in Britain by Seabird 2000 (Mitchell et al. 2004), the new winter estimate of over 2.1 million birds suggests mass immigration over winter. Mackinnon and Coulson (1987) estimated that 71% of wintering Black-headed Gulls in Britain and Ireland originated from continental Europe, the influx being greatest in southeast England (Horton et al. 1984); the new population estimates suggest that the proportion might be higher.

The distribution of breeding Common Gulls in the UK is strongly biased to the north, with nearly 100% of apparently occupied nests in Britain recorded by Seabird 2000 in Scotland (Mitchell et al. 2004). There are limited post-breeding movements of native breeders and immature birds, mostly within northern Britain and Ireland. As with Black-headed Gull, there are mass movements of continental breeders to the UK, particularly to eastern Britain (Wernham et al. 2002). Consequently, estimated winter numbers are greatest in England (68% of the Britain total estimate), with 29% estimated in Scotland and 4% in Wales.

Many Lesser Black-backed Gulls breeding in the UK and on the continent move to the Atlantic coasts of southern Europe and Africa to winter (Wernham et al. 2002). Although there is a reported growing tendency for this species to winter further north (Baker 1980, Wernham et al. 2002) and numbers wintering in England (and to a lesser extent Scotland, Wales and Northern Ireland) have increased greatly since the first winter gull roost survey in 1953 (Banks et al. 2009), differences between breeding and wintering estimates suggest that there is still considerable migration away from the UK. Seabird 2000 recorded 111 835 apparently occupied nests in Britain (i.e. approximately 224 000 breeding adults; Mitchell et al. 2004), in comparison to the new winter estimate of 124 654 for Britain. Given that winter numbers in the UK are also bolstered by immigration of graellsii and intermedius subspecies of Lesser Black-backed Gull from Iceland, the Faeroes, Scandinavia and the continent (Wernham et al. 2002), there would still appear to be much emigration from Britain.

Herring Gulls that breed in the UK are largely resident and the winter population thus comprises both the breeding argenteus population and additional argentatus birds entering the country from northern Europe. The majority of breeding Herring Gulls in Britain occur in Scotland (50%) and England (40%), with 142 942 apparently occupied nests (approximately 286 000 breeding adults) recorded in Britain as a whole by Seabird 2000 (Mitchell et al. 2004). Most ringing recoveries of immigrant wintering Herring Gulls have been reported from the east of Britain (Wernham et al. 2002), although wintering estimates for Wales (93 613) are in excess of breeding estimates there (nearly 14 000 apparently occupied nests; Mitchell et al. 2004), suggesting that there is also movement into western areas. There is some migration from northwest Britain into Northern Ireland (Wernham et al. 2002), and this coupled with movements within Ireland contributes to an estimate of 13 559 Herring Gulls wintering in the province.

The breeding Great Black-backed Gull population in the UK is largely sedentary (Wernham et al. 2002). Within Britain, 85% of apparently occupied nests recorded by Seabird 2000 were in Scotland (Mitchell et al. 2004), yet only 24% of the total winter population was found in that country, probably explained largely by southward movements of birds from Scotland and westward migration of gulls from Norway and Russia into England (Wernham et al. 2002). The 17 394 apparently occupied nests recorded in Britain by Seabird 2000 (Mitchell et al. 2004) equates to approximately 35 000 breeding adults. While this breeding season estimate excludes an unknown proportion of non-breeding immature and adult birds, the wintering estimate of 75 860 suggests an influx of Great Black-backed Gulls into the country.

Potential conservation measures and associated knowledge gaps

The results of the 2003/04–2005/06 Winter Gull Roost Survey not only highlight the importance of the UK for wintering gulls, but are also of use in informing conservation listings and site-based conservation measures. Nevertheless, the results highlight several knowledge gaps that should be addressed to better address conservation needs.

Informing conservation listings

The statuses in existing conservation listings of the principal species considered here are summarized in Table 1. Information from the 2003/04–2005/06 Winter Gull Roost Survey has already been used to inform the UK Birds of Conservation Concern lists (Eaton et al. 2009). All five commonly occurring wintering gull species are included on either the Red or Amber lists of Birds of Conservation Concern in the UK. This is primarily due to the status of their breeding populations in this country, although for Herring (Red), Black-headed and Great Black-backed Gulls (Amber), also because of declines in their winter populations, as shown by comparison of counts from the 2003/04–2005/06 and previous national Winter Gull Roost Surveys (Banks et al. 2009). The Common Gull is listed as a Category 2 ‘Species of European Conservation Concern’ owing to its ‘Depleted’ status (resulting from a moderate population decline) and the concentration of its global breeding population in Europe (BirdLife International 2004a). While Black-headed Gull, Lesser Black-backed Gull, Herring Gull and Great Black-backed Gull are categorized as of ‘Secure’ status and thus not listed as Species of European Conservation Concern, their global breeding populations are also recognized as concentrated in Europe.

Informing potential site-based conservation measures

The results of the survey are of most particular value in providing the information required for site-based conservation measures. On the basis of the new population estimates and survey counts, 20 sites were identified as holding internationally important numbers of individual gull species and 17 of these sites and a further 20 other sites held assemblages of at least 20 000 gulls and so provisionally could thus qualify for classification as SPAs for their gull populations alone. Together these sites supported a minimum of 30.3%, 48.3%, 35.5%, 16.3% and 11.7% of the estimated UK populations of Black-headed, Common, Lesser Black-backed, Herring and Great Black-backed Gulls, respectively. Just 21 of these 40 provisionally internationally important sites are presently classified as SPAs for other species and only five of these include breeding gull species or a breeding seabird assemblage featuring gulls as qualifying species (Appendices 1 & 2).

Based on thresholds derived from the new British population estimates and Irish thresholds (Crowe 2005), 49 sites held nationally important numbers of particular species and thus also qualify for notification, respectively, as SSSI or ASSI solely for their importance for gulls. At present, 34 of these are either SSSI/ASSI, or SPAs, which are themselves underpinned by component SSSI/ASSI. However, in only eight cases do these SSSI/ASSI include breeding gulls or a breeding seabird assemblage including gulls as qualifying species, while just one includes an assemblage of non-breeding birds that includes gulls (Appendices 1 & 2).

In justifying the selection and legal protection of important sites, demonstration of regularity of use is important (Stroud et al. 2001) and with the present frequency of monitoring of gull roosts, this would be difficult (Burton et al. 2007). Non-breeding waterbird species in the UK are monitored by monthly Wetland Bird Survey (WeBS) counts and, for the purposes of site assessments (Stroud et al. 2001), the number of individuals that a site regularly supports is defined as being the mean of the most recent five years' peak annual counts (Holt et al. 2012). ‘Five-year peak mean’ values are generated for gulls from these counts too and while non-breeding gulls have been included as qualifying features of some sites designated in the UK under the Ramsar Convention (http://www.ramsar.org/) based on these values, they have not to date been included as qualifying features of SPAs. Nevertheless, due to the methods employed, it is likely that many important sites would be overlooked by using WeBS counts alone (Burton et al. 2007). Maxima of only 224 169 Black-headed Gulls, 52 115 Common Gulls, 34 907 Lesser Black-backed Gulls, 106 832 Herring Gulls and 9,463 Great Black-backed Gulls were counted by WeBS in Britain in 2010/11 (Holt et al. 2012) – i.e. just 10%, 7%, 28%, 15% and 12% of the species' respective new population estimates. This is, first, because these counts are usually made in the daytime when many gulls may be feeding away from monitored wetland sites and, second, as counts of gulls are optional in this scheme. A better alternative would thus be to undertake annual surveys of key roosts (Burton et al. 2007), supplemented by more comprehensive occasional surveys to update population estimates.

Knowledge gaps concerning population estimates

The population estimates reported are representative of the numbers of gulls using terrestrial area and near-shore coastal water habitats, the standard geographical scope for defining UK or Britain population estimates (Stroud et al. 2001). Nevertheless, the estimates exclude the many birds that may occur further offshore within the UK offshore marine area, inclusion of which would provide a better understanding of the national importance of areas both on- and off-shore.

There also remains a need for better quality information on both breeding and wintering gull populations internationally. Information on breeding populations, used to derive international importance thresholds (Wetlands International 2012) and designate sites, varies in quality between species. This is due both to the variable accuracy of information on sizes of some breeding populations and also because the proportion of non-breeding birds in populations is unknown. Given the increase in winter population estimates for the UK resulting from the 2003/04–2005/06 survey, European winter population estimates should also be revised and consideration given both to the completeness of estimates for other European countries and numbers of gulls occurring far offshore.

CONCLUSIONS

Given the importance of the UK for wintering gulls and their present conservation status (Banks et al. 2009), there is a clear requirement to consider the measures and information needs that would provide for their better conservation. Both the African–Eurasian Waterbird Agreement (AEWA) and the EU Birds Directive require member states to maintain the favourable conservation status of gulls (Table 1). As indicated above, one potential conservation measure that could be implemented would be to list wintering gulls as qualifying features of UK protected sites, and there is a legal obligation under the EU Birds Directive to classify SPAs for gulls in the non-breeding season as well as the breeding season. This need has been recognized by the 2001 UK SPA Review (Stroud et al. 2001) and is being addressed in a current review of the UK network of terrestrial and coastal SPAs (Musgrove et al. 2011b).

ACKNOWLEDGEMENTS

We are particularly indebted to the many dedicated observers who took part in the 2003/04–2005/06 survey and the BTO regional representatives who helped with survey coordination. Mark Rehfisch helped to initiate the survey and, together with Steve Holloway and Mike Armitage, with initial planning. Heidi Mellan, Emma Glaister, Viv Hiom and Maria Knight helped with survey administration and data handling. The 2003/04–2005/06 Winter Gull Roost Survey was funded by the Countryside Council for Wales, English Nature (now Natural England), the Environment and Heritage Service (now the Northern Ireland Environment Agency), the Joint Nature Conservation Committee, Scottish Natural Heritage and Northumbrian Water Ltd and we are grateful to David Stroud, Nigel Buxton, Stuart Pudney, Andy Musgrove, John Quinn and an anonymous referee for their useful comments. Land cover data were provided by CEH.

Appendix 1. Sites in Great Britain and Northern Ireland supporting gull numbers exceeding the 1% national (italics) or international (bold) importance thresholds for at least one species during Winter Gull Roost Survey counts in 2003/04–2005/06.

Count site	County	Designation^a	BH^b	CM^b	LB^b	HG^b	GB^b
Bartley Reservoir	West Midlands				1 200
Belfast Lough	Antrim	SPA	11 055	553		854
Belvide Reservoir	Staffordshire	SSSI			3 000
Bewl Bridge	Sussex		69 000	75 000
Blashford Lakes (part of Avon Valley (Bickton to Christchurch) SSSI)	Hampshire, Wiltshire	SSSI			1 350
Blithfield Reservoir	Staffordshire	SSSI			2 620
Brogborough No. 1 and Stewartby Clay Pits	Bedfordshire						1 242
Burghfield Gravel Pits and Theale Gravel Pits	Berkshire				6 800
Chelmarsh Reservoir	Shropshire				3 500
Chew Valley Lake	Avon	SPA	29 800	18 200	7 015
Chingford Reservoirs	Greater London, Essex	SSSI	25 600	9 460
Coquet Island	Northumberland	SPA^c,d					980
Cropston Reservoir and Swithland Reservoir (overlaps with Bradgate Park and Cropston Reservoir SSSI)	Leicestershire	SSSI			2 050
Dungeness	Kent	SPA^c,d					1 404
Eye Brook Reservoir	Leicestershire	SSSI		16 100
Firth of Forth/Firth of Forth Islands	Central, Fife, Lothian	SPA^c,d	26 835	14 647		12 313
Grafham Water	Cambridgeshire	SSSI					1 050
Hallington Reservoir	Northumberland			25 000
Haweswater Reservoir	Cumbria			27 986
Hoveringham	Nottinghamshire						1 600
Humber Estuary	Lincolnshire, Yorkshire	SPA	34 118	31 134			2 387
Hurleston Reservoir	Cheshire		20 000
Inner Thames Estuary (overlaps with Thames Estuary and Marshes SPA)	Essex, Greater London, Kent	SPA	43 602		1 898		857
Llys-y-Fran Reservoir	Dyfed						1 500
Loch of Skene	Grampian	SPA^e		17 284
Lound Gravel Pit (part of Sutton and Lound Gravel Pits SSSI)	Nottinghamshire	SSSI					1 176
Lough Foyle	Derry	SPA	1 300	1 050
Lough Neagh and Lough Beg	Antrim, Armagh, Down, Londonderry, Tyrone	SPA^c,d	1 821
Lower Derwent Valley	Yorkshire	SPA					1 041
Minsmere-Walberswick	Suffolk	SPA		8 600
North Norfolk Coast	Norfolk	SPA^c,d	20 075	9 925
Ogston Reservoir	Derbyshire	SSSI					900
Outer Ards	Down	SPA	1 003	1 110		924
Queen Mary Reservoir	Surrey				6 656	8 279
Ribble and Alt Estuaries	Lancashire, Merseyside	SPA^c,d	21 491			19 592
Roadford Reservoir	Devon				3 978
Roughrigg Reservoir	Strathclyde					15 144
Rutland Water	Leicestershire	SPA	21 000	12 080
Severn Estuary	Gloucestershire, Avon, Somerset, Gwent, East Glamorgan	SPA^d	20 080		6 713
South Cerney	Gloucestershire, Wiltshire	SSSI			5 800
South West London Waterbodies (including Wraysbury, Staines, Island Barn, Queen Elizabeth II, Knight and Bessborough Reservoirs)	Surrey	SPA	24 187		1 758
Stanford Reservoir	Leicestershire, Northamptonshire			8 110
Strangford Lough	Down	SPA	3 177	504		1 755
The Wash	Lincolnshire, Norfolk	SPA^d	25 657	7 794		13 189
Tophill Low Reservoir	Yorkshire	SSSI		11 150
Ullswater	Cumbria			11 470
Upper Solway Flats and Marshes	Cumbria, Dumfries and Galloway	SPA		12 486
Westwater	Borders	SPA		10 050
Westwood Great Pool	Hereford and Worcester	SSSI			3 800
^a Count site overlaps wholly or partly with a SPA or SSSI. Note: all non-marine SPAs in the UK are underpinned by component SSSI or, in Northern Ireland, ASSI.
^b Counts are those used for the estimation of species' populations unless superseded by a supplementary count undertaken between November and March, 2003/04–2005/06. Note, counts do not include estimates derived from numbers of ‘small’, ‘large’ or ‘unidentified’ gulls and thus in some cases are minima; some coastal sites were also incompletely counted. BH = Black-headed Gull; CM = Common Gull; LB = Lesser Black-backed Gull; HG = Herring Gull; GB = Great Black-backed Gull.
^c SPA qualifying features include breeding gull species or a breeding seabird assemblage featuring gulls.
^d Qualifying species of a component SSSI include breeding gull species.
^e Qualifying species of a component SSSI include non-breeding gull species.

Appendix 2. Sites in Great Britain and Northern Ireland which held at least 20 000 gulls during Winter Gull Roost Survey counts in 2003/04–2005/06.

Count site	County	Designation^a	Peak total count^b
The Wash	Lincolnshire, Norfolk	SPA^d	124 907
Bewl Bridge	Sussex		103 021
Humber Estuary	Lincolnshire, Yorkshire	SPA	72 188
Chew Valley Lake	Avon	SPA	58 428
Firth of Forth/Firth of Forth Islands	Central, Fife, Lothian	SPA^c,d	57 196
Severn Estuary	Gloucestershire, Avon, Somerset, Gwent, East Glamorgan	SPA^d	56 622
Inner Thames Estuary (overlaps with Thames Estuary and Marshes SPA)	Essex, Greater London, Kent	SPA	50 998
Ribble and Alt Estuaries	Lancashire, Merseyside	SPA^c,d	47 160
Queen Mary Reservoir	Surrey		43 716
North Norfolk Coast	Norfolk	SPA^c,d	42 996
Queen Mother Reservoir	Berkshire		40 500
Chingford Reservoirs	Greater London, Essex	SSSI	36 883
Rutland Water	Leicestershire	SPA	33 501
Mersey Estuary	Cheshire, Merseyside	SPA	32 606
Upper Solway Flats and Marshes	Cumbria, Dumfries and Galloway	SPA	32 322
Hallington Reservoir	Northumberland		32 070
South West London Waterbodies (including Wraysbury, Staines, Island Barn, Queen Elizabeth II, Knight and Bessborough Reservoirs)	Surrey	SPA	30 821
Draycote Water	Warwickshire		29 965
Eye Brook Reservoir	Leicestershire	SSSI	29 404
Haweswater Reservoir	Cumbria		28 880
Ouse Washes (including Mepal Gravel Pit)	Cambridgeshire, Norfolk	SPA	28 877
Ullswater	Cumbria		27 260
Northumbria Coast	Durham, Northumberland, Tyne and Wear	SPA	26 894
Stour and Orwell Estuaries	Essex, Suffolk	SPA	26 613
Lower Derwent Valley	Yorkshire	SPA	26 082
Morecambe Bay	Cumbria, Lancashire	SPA^c,d,e	24 769
Lackford	Suffolk	SSSI	24 698
Poole Harbour	Dorset	SPA^c,d	24 385
Chasewater (Cannock Reservoir)	Staffordshire		23 604
Fletton Brick Pit	Cambridgeshire		22 786
Ferring to Goring	Sussex		21 235
Breydon Water	Norfolk	SPA	20 999
Hurleston Reservoir	Cheshire		20 903
Strathclyde Park Loch	Strathclyde		20 837
Blithfield Reservoir	Staffordshire	SSSI	20 318
South Cerney	Gloucestershire, Wiltshire	SSSI	20 272
Portsmouth Harbour	Hampshire	SPA	20 015
^a Count site overlaps wholly or partly with a SPA or SSSI. Note: all non-marine SPAs in the UK are underpinned by component SSSI or, in Northern Ireland, ASSI.
^b Counts are those used for the estimation of species' populations unless superseded by a supplementary count undertaken between November and March, 2003/04–2005/06. Note, some coastal sites were incompletely counted.
^c SPA qualifying features include breeding gull species or a breeding seabird assemblage featuring gulls.
^d Qualifying species of a component SSSI include breeding gull species.
^e Qualifying species of a component SSSI include non-breeding gull species.