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Lesser Black-backed Gull Larus fuscus

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Numerous winter visitor and passage migrant. Increasing rooftop-breeding population in urban sites

See also:  'Roof-top nesting gulls within the Birmingham boundary' by Jim Winsper


Winter status

In the West Midlands, along with many other regions of England, Lesser Black-backed Gulls became increasingly numerous winter visitors from the mid-1950s. At Blithfield Reservoir, Staffs, for example, there were 900 in October 1960 and up to 3500 during November and December 1967. Harrison et al. (1982) estimated a regional total of around 10000 by the late 1960s and 25000 - 30000 by the mid 1970s. 12000 were estimated in the roost at Draycote Water in December 1974. Numbers declined somewhat towards the end of that decade, with a total population of around 20000. Since then numbers have remained relatively stable, with Harrison & Harrison (2005) estimating the wintering population at the start of the current millennium as still around 20000. Counts at some of the larger roosts are now intermittent but data suggest an increase between 2000 and 2010 but a fall since, coinciding with the closure of some landfill sites. By 2020, although based upon intermittent, non-systematic counts, summed maxima from  principal roosts suggest a regional population of between 15000 and 20000.

The five-yearly mean maximum counts from selected sites during 1986 - 2010, as documented primarily in the Annual Reports of the West Midland Bird Club,  are presented in Table 1.

5-year Mean Draycote Tame Valley Westwood Belvide Blithfield Chasewater Bartley
1986 - 1990 2400 695 1415 1175 2000 1950 380
1991 - 1995 2840 1645 2890 1135 1990 2860 1475
1996 - 2000 1400 800 2840 2325 1850 3120 1210
2001 - 2005 2075 730 4000 1950 2350 2400 1830
2006 - 2010 1875 1330 3600 4500 inadequate data 3740 inadequate data

Table 1. Five-yearly mean maximum counts of Lesser Black-backed Gulls at selected sites in the West Midlands Region, 1986 - 2010.

Roosts of around 2000 have also been reported from Westport Lake in Staffordshire, while as many as 7000 have been estimated feeding at the landfill site at Throckmorton in Worcestershire (though the site has now ceased receiving household waste).

Origins of wintering Lesser Black-backed Gulls

The majority of Lesser Black-backs wintering in the region are of the race graellsii, which breeds in Britain and also in Iceland, the Faeroes, the Netherlands (where some intergrade with intermedius), Brittany, and Spain. In Britain as a whole, there have been recoveries of birds originating from across this range and also from Belgium, Denmark, Sweden and Finland (Wernham et al. 2002). Most individuals wintering in the West Midlands region are believed to involve British-breeding graellsii originating from Northern England and Scotland (Harrison et al. 1982). However, recoveries of birds ringed during winter include individuals returning to breeding colonies in the Netherlands and Norway (Harrison and Harrison 2005), which indicates the potential for arrivals from disparate regions of the breeding distribution, including intermedius as well as graellsii. Rock (2002, in Wernham et al. 2002) estimated that almost 60% of foreign-ringed recoveries were 'likely to be intermedius from Norway, Sweden and probably Denmark too'. It is probable that intermedius is under-recorded in the West Midlands region, though the recovery in Norway highlights a further issue : that, conceivably, graellsii may be breeding now in Norway. Paler-mantled Lesser Black-backed Gulls breeding in southern Norway are attributed to intermedius but the population of such paler-mantled birds is increasing and some are now found among colonies of the dark-mantled nominate fuscus in northern Norway. Helbig et al. (2009) wrote:  ' ... the Norwegian intermedius population increased steeply during the 1980s and early 1990s (Lorentsen 2007) and, since the late 1980s, greyish-mantled Lesser Black-backed Gulls with a L. f. intermedius appearance have been found in colonies in northern Norway which were previously occupied solely by the nominate fuscus (Strann & Vader 1992, Bustnes et al. 2006). It is not known whether these new birds originate from expanding intermedius colonies in southern Norway, or from colonies of a third subspecies, L. f. graellsii, which has increased in the North Sea area and in Ireland (Creme et al. 1997, Garthe et al. 1999), as these two subspecies are difficult to distinguish'.

Age distribution among wintering Lesser Black-backed Gulls

The wintering population in Britain involves a high proportion of adults. Prior to the 1940s, adult and younger Lesser Black-backed Gulls in Britain departed between August and October to winter along the Atlantic coasts of southern Europe and Africa (see, for example, Cramp & Simmons 1983 & Coulson 2019). During the 1940s and 1950s, increasing numbers of adults began to remain in Britain during the winter, a phenomenon attributed in large part to the availability of ample food supplies provided by landfill sites. However, the majority of first calendar-year birds continued to migrate to southern climes. Based upon data from 1959 - 1960, Barnes (1961) estimated that 80% - 90% of  LBBGs wintering in Britain were adults, with most of the remainder comprising 2W and 3W. Thus, 1W were very much in the minority. Despite the very substantial national increase in the wintering population since that time, these proportions have remained representative, including in the West Midlands. Entries in WMBC Annual Reports include that 98% of 1000 LBBGs roosting at Draycote on February 28th 1993 were adults. At the Chasewater roost during late-February 2000, Lesser Black-backed Gulls were 'mostly adults' (whereas Herring Gulls were 'mostly immatures'). More-recent confirmation comes from sample counts arranged by Jim Winsper (2020) and discussed in detail in the Ringing Report in WMBC Annual Report No. 85 (covering 2018). Counts supplied from Bartley, Belvide, Chasewater and Draycote revealed between 1% and 16% first-winter individuals, with the higher numbers confined to the earlier post-breeding period and followed by an exodus, with generally lower single-digit percentages during January and February.

The very significant increase in the numbers of LBBGs wintering in Britain and the high proportion of adults in the wintering population can lead to an impression that most adults breeding in the UK now remain here during the winter. However, comparisons between breeding and wintering numbers show that the former are larger by a factor of c. 1.8. This suggests that, although a significantly higher proportion of 1cy gulls depart, there is still considerable migration of adults away from the UK (Banks et al. 2007). From studies centred on the Severn Estuary Region (SER), Rock (2005) estimated that 22% of adults were : ' ... either returning extremely early or not migrating at all and, from preliminary analyses, it appears that overwintering may be more prevalent among urban gulls'.

Rooftop nesting

The breeding distribution of Lesser Black-backed Gulls also underwent a radical expansion during the latter part of the 20th century, with the greatest change being the increase in inland nesting, mainly on man-made structures such as rooftops (see Balmer et al. (2013) and citations therein). Rooftop breeding did not extend to the West Midlands until the mid-1980s and has been attributed to expansion from colonies in the SER (see Harrison & Harrison (2005) and Winsper (2014) for further discussion). The first instances of breeding documented in WMBC Annual Reports were at Worcester in 1986 and in central Birmingham in 1987 (though Rock (2004) notes that a single pair is considered to have bred in Worcester in 1982). Detailed assessments of breeding numbers at Worcester and Birmingham were limited until the late 1990s and early 2000s but the indications are that numbers began to increase substantially during that period. Surveys in Worcester by Peter Rock indicated 230 pairs by 2003, 540 pairs by 2006 and over 900 pairs by 2020. In Birmingham, during 2008 to 2011, a survey conducted by Jim Winsper of  'Rooftop nesting gulls within the Birmingham boundary' found at least 555  'apparently occupied nests' (AONs) of Lesser Black-backed Gulls. A pilot survey in 2018, organised in Birmingham by the BTO and DEFRA, found 1921 AONs. WMBC Reports have also documented rooftop breeding in other urban areas of the West Midlands, including Kidderminster, Solihull, Stoke, Stourport, West Bromwich and Wolverhampton. Smaller numbers of Herring Gulls have also been involved. These figures, together with other significantly sized colonies at places such as Solihull (Dean 2020), would suggest that the breeding population of Lesser Black-backed Gulls in the region had risen to over 3000 pairs (6000 individuals) by 2020. This is of the order of 25% to 30% of the currently estimated wintering population and, hence, numbers during the breeding season will now contribute significantly to the overall annual profile of numbers. 

Rooftop roosting

Most nesting activity has ceased by the end of July or early August, with adults and their offspring dispersing and few gulls then to be seen at rooftop colonies during the day. However, gulls may return to roost in the initial weeks of the post-breeding season and there is evidence that they may be joined by other individuals, producing rooftop roosts of substantial size. Other rooftop roosts occur at sites not used for breeding (while Black-headed Gulls, which are not rooftop breeders in England, may also establish rooftop roosts at this time). Some rooftop roosts disperse by the end of September while others persist and even peak in mid-winter (Deacon 2019). Banks et al. (2007) noted that studies by Shedden (1983) and Austin et al. (2003), based on traditional water-based roosts, suggested that there are only limited movements to and from roosts after dusk. However, according to Deacon (2019), gulls may arrive at rooftop roosts in the middle of the night. Nocturnal movements may vary according to season and differ between rural and urban sites. Discussing local movements of urban breeding gulls tracked by GPS, Spelt et al. (2019) noted : 'Street lighting enables them [urban gulls] to forage at night. Indeed, whereas urban gulls spend approximately 50% of their time flying during the day, they spend 35% of their time flying at night'. While gulls foraging in urban areas at night, utilising street lighting, are quite likely to be noticed (though not necessarily seen or reported by bird-watchers), gulls arriving at roosts well after nightfall are less likely to be detected. Rooftop roosts as a whole are very probably under-recorded while known sites have been little studied. Are gulls arriving at rooftop roosts well after dark relocating from traditional water-based roosts, from nearby land-based or other rooftop pre-roosts, arriving from nocturnal feeding excursions or some combination of these and other sources? At rooftop roosts which disperse at the end of September, are most gulls then relocating to traditional water-based roosts or are significant numbers locally breeding gulls which have delayed southbound migration until this time, their departure then triggering dispersal of the entire roost? If relocating to traditional roosts, then any significant upturn in numbers there during late-September may not be due exclusively to recently arriving winter visitors from outside the region. Thus, migration patterns of gulls inferred from numbers at traditional roosts could be somewhat  misleading.

Many issues relating to rooftop-nesting and rooftop-roosting gulls remain to be answered. The emergence of GPS studies offers a very useful technique to monitor the origins and movements of gulls, while the uptake of nocturnal sound-recording ('nocmig') could elucidate the scale of nocturnal movements. GPS studies are rather small-scale at present but, already, a Lesser Black-backed Gull has been detected roosting in winter on rooftops in central Birmingham (Peter Rock in litt.). 

Non-breeding gulls in summer

Large numbers of predominantly immature gulls have occurred during the summer at some landfill sites, a phenomenon described as 'recent' in 1982 by Harrison et al., who mentioned 2000 LBBGs at Packington Landfill and 3000 at Ladywalk in June 1974. Subsequently, at the (hitherto) huge landfill site at Throckmorton in Worcestershire, numbers reached 1000 during August 1999, 3000 during August 2009 and 2500 during June and July 2010. Household waste landfill ceased at this site in 2016 and the August count in 2019 was just 68 (with the annual max 450 during February) but smaller assemblies are still to be found during the summer at other landfill sites. There is little indication of such mid-summer numbers from the data published from traditional roosts but mid-summer counts at traditional roosts have always been relatively limited. While confined to a small number of sites, the gulls in these large summer assemblies will provide another significant contribution to the regional monthly distribution of Lesser Black-backed Gulls.

Changes in the seasonal profile of numbers

The changes in status and behaviour outlined above clearly have repercussions for the seasonal profile of numbers. Figure 1 shows the historical pattern from personal counts at two reservoir roosts : Blithfield during 1980 - 1985 and Bartley during 1998 - 2003. The former time-span, from Blithfield, will involve primarily winter-visiting gulls, as the species was not breeding in the region at that time and there was no significant roosting of immature gulls at the site during the summer months. The profile at Bartley dates from a time when the numbers of Lesser Black-backs breeding in the region were beginning to increase from a low base. At both roosts, Lesser Black-backed Gull numbers were low during the summer months but then increased rapidly from July to October, reaching a peak in November at Blithfield and in December at Bartley. Harrison et al. (1982) also detected a November peak based on region-wide data from 1955 to 1978. Numbers at both Blithfield and Bartley remained relatively constant through January and February. However, wintering birds starting to disperse as February advanced were no doubt offset by passage migrants beginning to move through to breeding sites elsewhere. By late March or early April numbers became relatively small. Overall, the two sets of data show a comparable profile through much of the year ('bell-shaped' when plotted from July through winter to June, which captures the natural profile of numbers at that time). However, the November peak of the earlier (Blithfield) period declined and the annual peak shifted towards mid-winter (Bartley).

Lesser Black-backed Gull numbers at two West Midlands reservoir roosts

Figure 1. The traditional monthly distribution of Lesser Black-backed Gull based upon monthly maxima at two reservoir roosts.
Bartley Reservoir, West Midlands, 1998 - 2003; Blithfield Reservoir, Staffordshire, 1980 - 1985.

Recent experience indicates more-pronounced changes in the monthly profile of numbers, with the expansion of significant breeding colonies producing larger number during the summer months and the probability of continuing changes in the phenology of winter numbers and passage migrants. However, quantification of these more-complex changes is beset by a number of problems, including :

Traditional roosts.

(1) Accurate monthly distributions can no longer be assessed from data at traditional roost sites. Counts of gulls at many of the region's most important traditional roosts have become infrequent. Published data from some sites often include only one or two reliable counts per annum (sometimes none) rather than methodical and season-long data. Data published for some roosts frequently includes numbers which are unrealistically small and clearly unrepresentative and are presumably no more than entries in casual bird lists reaching the in-trays of county recorders. The larger roosts will have a significant influence in determining the true regional monthly profile, so fragmented data from key sites can easily lead to distortion of the regional monthly distribution.
(2) During the breeding season, gulls generally roost at the nesting site. Thus, even more extensive data from a traditional roost, spanning most of the year, will under-represent the true numbers present in the region during the breeding season.
(3) At some sites the monthly profile is anomalous and clearly not representative of the region as a whole. For example, Upton Warren in Worcs attracts very large numbers of LBBGs during just two or three months of the year, with relatively small numbers during the remainder of the year. Such anomalous concentrations can lead to a single site having a disproportionate influence on the monthly profile for the region as a whole (see further comments below and Figure 2b).  Other roosts are now of relatively short duration. At Coton in N. Warks (my own study roost for many years), the roost does not assemble until late October and disperses in late January or early February (and is now in decline following closure of the local landfill site).

Diurnal gatherings.

(4) At diurnal loafing and feeding sites many off-duty and non-breeding gulls may be encountered during the summer. However, at loafing sites near to a breeding colony, there may be relatively few gulls using the site during the winter, after the colony disperses. The monthly profile at such sites will not reflect the overall regional profile. See Dean (2020) for an example from Olton Mere.
 

There is no single 'data-set' which can circumvent these issues. The best prospect of evening out imbalances may be to sum all counts, both diurnal and from roosts, for sites for which data has been published through the majority of a given year, though combining diurnal and roost counts will inevitably lead to some duplication. The four provisos above must be kept in mind but prima facie a broad indication of trends should emerge. Figure 2a has been generated by summing all counts published in the WMBC Reports for the five years 2015 to 2019 from 32 sites with data for at least ten months in any given year. (Note especially proviso 1 above  -  i.e. Fig. 2a  provides a guide to the monthly profile but the numbers scale is merely a sample of the full regional population and is a five-year sum. Although based upon fragmented, non-systematic data, peak numbers published from all principal roosts combined suggest that by 2020 the regional wintering population numbers between 15000 and 20000.)

Figure 2a. Recent monthly distribution of Lesser Black-backed Gull as indicated by summed counts from sites
tabulated in WMBC Annual Reports for 2015 to 2019 and with data for at least ten months of the year.
 

Comparing Fig. 1 with Fig. 2, the 12-month cycle still exhibits increasing numbers in the post-breeding season and declining numbers through the early spring. However, during the late autumn and winter, there is a more 'bi-modal' and less 'bell-shaped' distribution. There are proportionately much-more significant numbers through July to October; a subsequent slight decline in numbers through November and December; an increased prominence of numbers during January and February.

A change in the relative balance between winter and summer numbers is to be expected, given the significant numbers now breeding on rooftops in urban sites.  Changes in the migration strategies of local gulls and an increase in passage migrants are also likely to be involved. As early as 1983, Cramp and Simmons (BWP3) noted evidence that adults were beginning their southwards migration in August to October but starting to return north from November, with gulls from their 2W onwards reappearing in British roosts in rising numbers through December to February. This was written in 1983 but, with numbers breeding inland nationally increasing rapidly since then and breeding not starting in the West Midlands until the mid-1980s, changes in the monthly profile in the West Midlands might be expected to have accelerated more-recently.

Intriguingly, as noted above, there are certain sites which contribute significantly to the total in particular months but little in other months; they do not adhere closely to the overall annual profile. They emphasise further the complexity of the issues contributing to the overall pattern. A particular case is Upton Warren, in Worcestershire. Here, in recent years, there has been a large influx of Lesser Black-backed Gulls during September and October, reaching 2200 in October 2019, but subsequent numbers in the winter months have been relatively small. In Fig. 2b, Upton contributes 29% to the September total and 27% to the October total. In stark contrast, the percentages from November to February are between 0.7% and 2.9%. The profile for all other sites combined shows a higher total for November than for October. The reasons for such an anomalous profile are not clear.

Figure 2a. Recent monthly distribution of Lesser Black-backed Gull as indicated in Fig. 2a but with contributions from Upton Warren shown separately.
Note the significant contributions to the regional Sept and Oct totals provided by this single site

Other races

Very small numbers (generally single figures) of the race intermedius are reported in the West Midlands region each winter. This race breeds in southwest Scandinavia, Denmark and the Netherlands and migrates south and south-west to wintering grounds in western Europe and west Africa. Thus, it is to be expected in Britain and it is likely that it is under-recorded in the region. There are a number of ringing recoveries of this form. However, field identification of birds away from the breeding range is not straightforward, as there is some variation in the shade of grey of the upperparts in graellsii and very dark individuals can be found breeding in LBBG colonies in Britain. In Denmark and the Netherlands many of the breeding birds are considered to be intermediate between graellsii and intermedius. There is some debate over the validity of intermedius as a subspecies rather than simply a cline among graellsii.

One or two birds of the nominate race fuscus ('Baltic Gull') have also been reported in most years. This race breeds in the Baltic area and northern Fennoscandia. Generally, it migrates south-east to winter predominantly in the Middle East and east Africa, though 1cy individuals from Northern Norway are more likely to move west, at least initially (Helberg et al. 2009). The breeding population of nominate fuscus is declining.

The identification of fuscus away from the breeding grounds is fraught with difficulties. The old perception that nominate fuscus could be identified on the basis of very black upperparts is now known to be false, as some individuals from the range of intermedius can appear just as dark in the field. It is structurally smaller and more elongated than intermedius, on average, but individual variation makes these characters unreliable. The status, movements and identification of 'Baltic Gull' were discussed in a key paper by Lars Jonsson  (Jonsson, L. 1998. Baltic Lesser Black-backed Gull Larus fuscus fuscus - moult, ageing and identification. Birding World 11: 295 - 317). However, a proviso re suspended moult in the other races has tempered some conclusions, even in this reference. The timing of the moult cycle seemed likely to provide a more reliable route to confident identification. The nominate race moults later than intermedius and graellsii and, prior to migration, adults normally moult only the two inner-most primaries. Thus, the outer primaries are still in place and all of uniform age during August and September, when the other two races are in active wing-moult. Unfortunately, it has recently been demonstrated that occasional individuals of intermedius (and graellsii) undergo 'suspended' moult, so that they too may show a moult cycle comparable to that of fuscus. Thus, positive identification of nominate fuscus remains problematic. It remains essential, however, to determine the precise state of moult with any suspected fuscus, as an individual in active wing-moult during the late-summer / early-autumn is certainly not fuscus.

The highest probability of success now appears to reside with 2cy individuals. For discussion of the more-advanced wing and body moult of fuscus in its 2cy and the possibility of confident identification of individuals of this age, see the relevant pages on the Gull Research Organisation webpages at: http://www.israel.gull-research.org/02cyapril.html and the references there to the relevant paper on 'Primary moult of Baltic Gull during the first 15 months (in Dutch Birding 28: 158 - 161) by Hannu Koskinen & Visa Rauste. Even further limitations emerged subsequently. The following appears on the website of BBRC, which now adjudicates claims of nominate fuscus (see http://www.bbrc.org.uk/subspecies-information/skuas-to-terns):

'Winters (2006) went on to propose a tighter definition of which second and third calendar year birds might be acceptable but this position was subsequently reviewed by Altenburg et al. (2011) who concluded that only second calendar year birds in April to June (and perhaps  into the summer) can be safely identified.'

Thus, the identifcation of fuscus away from the breeding grounds remains a considerable challenge. Ringing recoveries provide the surest route and offer some support for fuscus being a rare visitor to Britain, as several colour-ringed  fuscus have now been observed, as have individuals meeting the 2cy criteria mentioned above.

Given the difficulties in identification, it is considered that most historical claims of 'Baltic Gull' from the West Midlands Region will certainly prove inadequate and it is probable that none is sufficiently rigorous to prove acceptable. It is highly unlikely, for example, that many of the documentations submitted to county recorders hitherto have included details of state-of-moult. In consequence, historical reports claimed from the West Midlands Region are not itemised here.

Acknowledgments

References and acknowledgments for the material on this site can be found <here>. However, the text on Lesser Black-backed Gull has proved much more complex than those dealing with most other species and I would like to acknowledge here the particular contributions of Dawn Balmer, Nick Barlow, Lee Johnson, John Oates, John Sirrett and Jim Winsper, who each provided valuable correspondence and responded to my questions (and speculations). Data on age-distribution was supplied to Jim Winsper by Terry Hinett (Bartley), Steve Nuttall (Belvide), Ian Ward (Chasewater) and Tim Marlow (Draycote).

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