Lesser Black-backed Gull (graellsii & intermedius)

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Colony at "quai de la loire" at Calais, NW France. This is a mixed colony of Lesser Black-backed Gulls and Herring Gulls, with Lesser Black-backeds dominating. Theer are now about 200 pairs breeding. Picture from April 14 2014 (Jean-Michel Sauvage).

A historical ecology of two closely related gull species (Laridae): multiple adaptations to a man-made environment

A historical ecology of two closely related gull
species (Laridae): multiple adaptations to a man-made environment

- by: Kees (C.J.) Camphuysen -

Ph.D.-thesis, University of Groningen, Groningen, The Netherlands, June 2013

Full PDF by clicking image.

Appendix 6 ‐ Annual adult survival, recruitment and parental investment

Annual adult survival

While the fledging rates of Herring Gulls (0.88 ± 0.29 fledglings pair-1) found in Kelderhuispolder were considerably higher than those of the sympatric Lesser Black-backed Gulls (0.49 ± 0.17; t
12 = -3.07, P< 0.01, two-tailed), our own colony censuses (2009 - 2011) indicated a continuing decline in breeding numbers of the former and an increase in the latter species (Appendix 3). Part
of the explanation was expected to be found in species-specific differences in annual survival, the age of first breeding and the actual numbers of recruits returning to breed. Data available on
fecundity and adult survival in several seabird populations suggest a negative relationship between the two (Weimerskirch 2002). Devoting more energy per individual parent to the raising of young would come at the expense of adult survival (Clutton-Brock 1988, 1991, Pyle et al. 1997). The shape of this relationship is convex (App Fig. 6.1; repeated below), similar to the classical figure representing the optimisation of the trade-off between survival and fecundity or other vital rates (Cody 1966). Cody (1966) proposed a model in which by the "Principle of Allocation" maximum contribution to future generations would be achieved by those individuals which utilise, to increase K (carrying capacity), some of the energy conserved by reducing r (the reproductive rate; Williams 1966, Nur 1984).

The apparent survival and fecundity of both species is described in Chapter 7. Apparent survival (i.e. survival confounded by permanent emigration), investigated on the basis of the colour-ring programme, amounted to a mean of 79% in female and 86% in male Herring Gulls. Additive year effects rather than sex provided highest model support in Lesser Black-backed Gulls, in which apparent survival for both sexes combined varied between 81% and 100% (mean ≈91%). Given the observed population trends, the lower apparent survival in Herring Gulls than in Lesser Blackbacked Gulls found in this study did not come unexpected. An inverse relationship between annual apparent survival and fecundity was indeed found in Lesser Black-backed Gulls (Chapter 7). Herring Gulls, with their consistently lower survival rates, showed a consistently higher fecundity than Lesser Black-backed Gulls (App Fig. 6.1).

Fig. 6.1 Hypothetical relationship between fecundity and survival in Laridae to achieve a constant population growth rate (λ), a convex curve representing optimisation between survival and fecundity (Cody 1966, Weimerskirch 2002). The plotted values are mean fecundity versus mean apparent annual adult survival for gulls breeding in Kelderhuispolder, 2006-2011.

Recruitment and the Balance Per Annum

Levels of recruitment were thus far rather low in the Kelderhuispolder. Breeding birds with immature plumage characteristics are rare in these colonies, notably in Lesser Black-backed Gulls, suggesting that current recruits are at least 4 years old (5cy). Some estimates of recruitment rates are required to be able to calculate (or even just estimate) the Balance Per Annum (BPA; the difference between the number of chicks per pair surviving to breeding age and the number of adults dying per pair per year; Birkhead & Sears in Perrins 1991).

Between 2006 and 2011, 554 chicks of Lesser Black-backed Gulls and 237 chicks of Herring Gulls were colour-ringed around fledging (c. 40d of age). Of the first three cohorts (2006-2008), 254 and 114 chicks respectively could have reached breeding age (5cy) during the period described in this thesis. In fact, only 20% of the chicks (51 out of 254) of Lesser Black-backed Gulls and 13% of Herring Gulls (15 out of 114) were demonstrated to have reached that age (App Table 6.1). Based on a mean fledging rate of 0.49 fledglings pair-1 (Table 2.1, Appendix 3), the number of Lesser Black-backed Gulls reaching a reproductive status (≥5cy) was thus 0.10 chicks
pair-1. Based on a fledging rate of 0.88 fledglings pair-1 in Herring Gulls, a marginally higher 0.12 chicks pair-1 was found. Considering an annual adult mortality of 0.18 adults pair-1 (Chapter 7) in Lesser Black-backed Gulls, the BPA would be -0.08. With the considerably higher adult mortality of c. 0.35 adults pair-1 (annual survival 82.5%), the BPA for Herring Gulls would be -0.23. Both populations may be considered unstable or declining, but the former mainly as a result of low reproductive success, the latter mainly as a result of lower annual survival. Rather few "recruits" (that includes prospecting birds and only few confirmed breeders) have been demonstrated to have returned to the Kelderhuispolder colonies: 5% of all colour-ringed fledglings of Lesser Blackbacked Gulls and 4% of all Herring Gulls. If recruitment was based only on these birds, the BPA would be considerably lower, but it is unlikely that all recruits are detected. Further evidence showing that chicks raised at Texel recruit at a relatively high age (if at all) is provided in the comparison of re-sightings within the colonies at Texel and at IJmuiden (AppTable 6.2).

Age composition – Though breeding attempts of birds in immature plumage may be fairly common in certain (developing) mainland colonies in The Netherlands, the mean age of first breeding in Herring Gulls is normally probably around 5 or 6 years of age (Vercruijsse 1999). In a dense colony at the Isle of May (Scotland), no 4cy birds (3 yrs of age) were found holding territories or breeding in the 1970s, but some 5cy gulls (4 yrs old) did (Chabrzyk & Coulson 1976). For the Isle of May, it was estimated that the mean age of first breeding was 5.25 years, with 55% of the birds breeding for the first time when five years (6cy) old.
The age composition of the breeding population at Texel is largely unknown, but early recruits (or even immature visitors) in the colony are rare. All Lesser Black-backed Gulls that had been ringed in other colonies and that were found to breed in the Kelderhuispolder in recent years were fairly old (9-17 yr). Despite considerable observer effort, only six Lesser Black-backed Gulls that had fledged between 2006 and 2009 were confirmed to have returned to nest and actually breed in Kelderhuispolder, while there are four birds that have moved and recruited and bred elsewhere. The much higher return rates in IJmuiden of prospecting birds are particularly striking, and prospecting or settling recruits are also younger at this mainland colony and first-year survival was higher. Of 152 ringed chicks of Lesser Black-backed Gulls and 44 chicks of Herring Gulls, at least 38 resp. 35% survived its first year against 29% and 26% at Texel (AppTable 6.1). To compare the returns of potential recruits in the two colonies for the exact same period (ringing effort 2008-11 versus ring-reading in 2009-2012):

Immigration, emigration

With respect to immigration and emigration (or colony dispersal), only anecdotal information is available, again on the basis of ringing data, but this time including some metal-ringed individuals. Within the Kelderhuispolder, adult breeding birds have been encountered that were ringed as fledglings in Orfordness (UK), Zeebrugge (B), Europoort/Maasvlakte and IJmuiden (mainland NL) and from other colonies at Texel. Birds that fledged at Texel in recent years have been found as prospecting individuals at Trischen (Wadden Sea FRG) and Vlieland (Wadden Sea NL), and as confirmed breeders in Zaandam and IJmuiden (mainland NL).

Immigration - Two Lesser Black-backed Gulls ringed as chicks in IJmuiden have recruited in the Kelderhuispolder colonies, 11 individuals originated from Europoort/Maasvlakte, and two from Orfordness (Suffolk, UK) (AppFig. 6.2). All these birds had been ringed as chicks between 1988 and 1997 and were between 9 and 17 years of age (mean 14.3 yrs) when detected as breeding birds at Texel. A recent exception is a bird ringed as chick in Zeebrugge (Belgium) that recruited in the Kelderhuispolder at an age of 5 years, after a prospecting visit one year earlier.

Emigration - Seven Lesser Black-backed Gulls ringed as young birds since 2006 at Texel have been seen prospecting or were demonstrated to breed in other colonies: K.BAU (Trischen, FRG), K.FAU (IJmuiden, Forteiland), K.LAC (Vliehors, Vlieland), K.NAP (Achtersluispolder Zaandam), P.CAC (IJmuiden, Forteiland), P.CBU (IJmuiden, Forteiland), and P.CDV (Vliehors, Vlieland). Of these, at least three have bred: KFAU (2011-12), KNAP (2011-12), and PCBU (in 2011, prospecting in 2012).

Fig. 6.2 Links between de Kelderhuispolder colony and other Lesser Black-backed Gull colonies based ringed and colour-ringed individuals ringed at Texel and elsewhere.

Trends and seasonality in mortality as derived from beached bird surveys

Mortality is an aspect that is notoriously difficult to study during colony work. Birds sometimes disappear, but in the absence of physical evidence, anything could have happened. Since large gulls are coastal seabirds, some insight in seasonal as well as annual patterns and trends in mortality can be evaluated on the basis of beached bird surveys (>50 yrs of data). The data were used to investigate the age composition and the overall densities of dead gulls on the North Sea coastline, in order to check if mortality levels may have changed in recent years. Many fledglings fail to really leave the colony area, but fledglings from coastal colonies (such as those at Texel) often die on the nearby beaches, during attempts to find some food or on the beach roosts where they mix with moulting adults. Beached bird surveys could therefore be a useful tool to quantify seasonal patterns in mortality.
We have seen that the incidence of oil pollution in Herring Gulls has declined significantly over the years (Appendix 2, AppFig. 2.13) and currently, most gulls die from 'natural causes' (disease, starvation), or from entanglements in plastics or fishing gear or other accidents. We have also seen that the annual cycle and the migratory movements in autumn (or post-breeding dispersal) make that the composition of the population of gulls utilising Dutch nearshore water and on our beaches will change (Appendix 4): there are no or few "local" Lesser Black-backed Gulls in winter (because all these birds are in Portugal and Spain). Although many Dutch Herring Gulls remain here in winter (Appendix 5), their numbers will be augmented by birds from a Baltic, Scandinavian or German origin. This should be kept in mind when consulting the results from beached bird surveys over time.
In App Fig. 6.3 the monthly densities of dead gulls are provided for each of the years of study (2006-12) against a background of "historical" densities based on surveys between 1980 and 2005. From the material provided in Appendix 2 it is obvious that the breeding population of Lesser Black-backed Gulls increased by a factor 7.5 between 1980 and 2005, whereas that of Herring Gulls peaked reduced in the same period by 25%. Current densities were therefore much higher than 'historical ones' in Lesser Black-backed Gulls, while the reverse was true for Herring Gulls. Both species have a late summer peak in densities, which was however more prominent in the historical data of Herring Gulls than it is nowadays. In recent years as well as historically, densities of stranded Lesser Black-backed Gulls increased from April to July followed by a decline.
Very low numbers (as expected) are found in winter (Nov-Mar). From April through July, it is mostly adult birds that are found, while juveniles start to predominate in August-October. Over the years, the age composition of the stranded animals remained more or less the same, but current densities are higher (AppFig. 6.4). A similar trend was found in Herring Gulls, except that first year birds remain numerous throughout the winter and into spring (AppFig. 6.5). Juveniles predominated in Aug-Oct, but in June and certainly in July proportionally many adult gulls were found. Recent winters have been relatively mild (AppFig. 2.8), despite some spells of cold weather, and winter mortalities will therefore have been suppressed (AppTable 2.3).

Fig. 6.3 Annual monthly densities of dead Lesser Black-backed Gulls (A) and Herring Gulls (bottom) along the North Sea shoreline, based on beached bird surveys 2006-2012 against the longterm mean densities found during beached bird surveys 1980-2005 (average ± SE); NZG/NSO beached bird surveys, unpubl. data; based on 7741counts covering 45,243 km, retrieving corpses of 1817 Lesser Black-backed Gulls, and 15,842 Herring Gulls.

Fig. 6.4 Monthly densities of dead Lesser Black-backed Gulls (A) and Herring Gulls (B) along the North Sea shoreline, based on beached bird surveys 1980-2005 and 2006-2012 (average ± SE) and the proportion (%) of first year birds found dead; NZG/NSO beached bird surveys, unpubl. data.

Intermittent breeding

Extensive ring-reading activities in the prospecting and early breeding phase resulted in high numbers of re-sightings of birds ringed as breeding birds in previous seasons, with few exceptions at or near the exact same territories as in earlier seasons. However, not all returning birds could subsequently be demonstrated to breed. Some failures to confirm breeding were resulted from difficulties to read rings in vegetated or 'hilly' terrain (steep dunes, esp. in the prime Herring Gull habitat). Confirmed breeding attempts of returning adults were consistently scarcer in Lesser Black-backed Gulls than in Herring Gulls (46% in LBBG, 63% in HG; AppTable 6.3).
"Not breeding" is more difficult to confirm than "breeding", but the difference between the species is considerable in all seasons with sufficient data (AppFig. 6.4). Site-fidelity is such that breeding attempts elsewhere in the colony are unlikely (5 cases have been documented within the colony, 2007-2012; these involved moves from monitored study plots to nearby nesting habitat just beyond the study areas). The percentage (%) of prospecting birds in which breeding was subsequently confirmed is used here to indicate the "breeding incentive", or the drive to breed in a given season. Some birds skipped one, two or even three years and re-appearances as breeding birds were typically very close to or exactly at the original nesting site (certainly in males; divorced females usually moved to another territory). Lesser Blacked-backed Gulls were slightly more frequently found to skip breeding seasons (22% of 208 confirmed re-breeding cases were after one or more years of non-breeding, against 15% in Herring Gull, n= 166; AppTable 6.3). Skipping two or more seasons was fairly rare in Herring Gulls (1%, n= 166), slightly commoner in Lesser Black-backed Gulls (6%, n= 208).
Most of the prospectors that could not be confirmed as breeding birds disappeared from the colony after early May (some returned in July and August). In Lesser Black-backed Gulls (not in Herring Gulls), breeding confirmations (%, AppTable 6.4) and the onset of laying (date, timing, Table 2.1) were negatively correlated: non-breeding (of earlier established breeding birds) tended to be more frequent in delayed seasons (RS = -0.90, n= 5). The data series is too short, however, to draw firm conclusions. A low frequency of breeding confirmations was particularly striking in 2012, a season with a much delayed onset of breeding. Only 25% of the prospecting Lesser Blackbacked Gulls were found breeding later in that season. In Herring Gulls, reproductive success (fledglings pair-1, Table 2.1) and the proportion of confirmed breeding attempts by prospecting individuals were positively correlated (RS = 0.80, n= 5).

As a result of the difficult terrain the Herring Gulls were nesting in, it was easier to read rings of Lesser Black-backed Gulls at their nest. The difference between the species in apparent non-breeding by prospecting adults (AppTable 6.4) may therefore be larger than indicated. At the nearby mainland colony Forteiland in IJmuiden, where nest initiations are earlier than at Texel and where fledging rates are seemingly higher (no concrete data available, just a visual impression), the same colour-rings have been deployed since 2008. Of colour-ringed Lesser Black-backed Gulls returning and prospecting in IJmuiden during 2009-2012, 55.7±10.3% were demonstrated to breed (Texel 2009-2012 46.8±15.8%, AppTable 6.4). Of Herring Gulls returning in IJmuiden, 61.7±12.3% were found breeding (Texel 66.3±11.4%). These values are of the same order of magnitude as those on Texel, even though providing evidence that birds were breeding is far more easy at IJmuiden than at Texel. The results suggest that Lesser Black-backed Gulls at Texel breed on average once every 2.1 years, those in IJmuiden every 1.8 yrs. For Herring Gulls, a highly similar breeding frequency of once every 1.5 (Texel) to 1.6 years (IJmuiden) can be estimated.

Table 6.3. Confirmed breeding in prospecting colour-ringed Lesser Black-backed Gulls (LBBG) and Herring Gulls (HG) in Kelderhuispolder (Texel). Shown are the number of colour-ringed individuals that returned as prospecting birds, the number and percentage in which breeding was subsequently confirmed, and the frequency of season skipping.

Table 6.4. Annual variations in "breeding incentive": the proportion of prospecting colour-ringed Lesser Blackbacked Gulls (LBBG) and Herring Gulls (HG) in Kelderhuispolder (Texel) that were later that season recorded as confirmed breeding birds. Shown are the observer effort (hours in the field prior to 10 May to record prospecting birds and the hours spent during breeding), the ringing cohorts evaluated, and for each species the number of colour-ringed prospecting birds (obs), and the number (br) and the breeding incentive (%).

Parental care and the need for a break - Cody's (1966) model, in which annual survival (and thereby future reproductive success) would be enhanced by those individuals which conserve energy by reducing the current reproductive rate, became again of interest when highly unusual foraging trips were evaluated in the course of a GPS tracking study of Lesser Black-backed Gulls from Kelderhuispolder (Chapter 8). Tracking studies of seabirds occasionally reveal exceptionally long or distant foraging trips. The characteristics, the frequency, and the possible triggers of such unusual trips in comparison to thousands of other, more regular trips were considered in the context of reproductive performance and chick growth. The hypothesis that exceptional trips were conducted exclusively by failed breeders, but not by active breeders during incubation or chick care, had to be rejected. Exceptionally long and distant trips occurred irregularly but annually, in many individual birds and in all phases of breeding, except that in active breeders such trips were relatively rare when the chicks were still young and highly vulnerable (<10d of age). The data seem to suggest that additional time for individual maintenance (e.g. self-provisioning, replenishing exhausted resources) rather than extra effort in chick provisioning (a chick starvation hypothesis) was the most important factor. During a breeding attempt, in other words, apart from giving up, individual birds apparently stepped back and reduce their efforts to enhance their own condition (and fitness). The beached bird survey results suggest that an "untimely death" for an adult in the breeding period is far from unlikely. The individual breaks in chick care were potentially at the expense of clutch or chicks, and compensatory behaviour of the partner was required to bring the breeding attempt to a successful end. The reproductive success in birds that performed exceptional trips was rarely compromised, suggesting that mates compensated indeed for these absences.

Post-fledging chick mortality – Vercruijsse (1999) reported the annual survival and recruitment rates of young Herring Gull at Schouwen (Delta area), following the colour-ringing campaigns in 1986-1988. Of 314 fledglings ringed at Schouwen, 37% were demonstrated to be alive after one year (re-sightings in later years, no corrections for detection probabilities). Using the same technique at Texel in 2006-12, the mean first-year survival of Herring Gull chicks ringed at Texel (2006-2011) was a rather lower 25±6%, that of 554 Lesser Black-backed Gull chicks from Texel amounted to 31±7% (AppTable 6.1). First year mortality of fledglings, based on demonstrated survival ranged from 70-85% in Herring Gulls and 58-79% in Lesser Black-backed Gulls The annual mortality in subsequent years was considerably lower.
Annual survival rates after the first year of life (2-5 yrs of age, just prior to expected recruitment) in Herring Gulls from Schouwen in the late 1980s and early 1990s were high and averaged 87.5±1.0%. Immature Herring Gulls from Texel scored a considerably lower and more variable annual survival of 70.0±5.8%. The annual survival of immature Lesser Black-backed Gulls from Texel (82.5±5.2%; AppFig. 6.5) was more or less similar as that in Herring Gulls from Schouwen some decades earlier.

Fig. 6.5 Percentage decline in the number of Herring Gulls (HG) and Lesser Black-backed Gulls (LBBG), ringed as chicks in Schouwen (Sch1986-Sch1988, Herring Gulls only; data from Vercruijsse 1999) or at Texel (2006-2009) after the first year of life (i.e. after the relatively high losses in the juvenile phase).

End Of Appendix 6 ‐ Annual adult survival, recruitment and parental investment

Below: images of birds from the Netherlands, first week of April 2017.

Set of early April adult Lesser Black-backed Gulls in Leiden, the Netherlands. Roof top breeders here. LBBG graellsii in spring: open wings.

LBBG returning to the colonies in the Netherlands

End of March; many adult LBBG of the Maasvlakte (see map) already return to the breeding grounds and try to find nesting locations in the colonies. Successful adults are very faithful to nesting locations and partners and can be found breeding on exactly the same spot (inch-perfect!) year after year. Adult males, returning early in the colony normally wait for the same partner, establishing dominance over a last-year's successful territory. If the female arrives later in the season, other female try to mate with such dominant males, but once his partner arrives, the bond will soon be established and those (often young) females are kicked out the territory. 
By the end of March, 1.000s of LBBG (probably mostly originating from intermedius populations in Scandinavia) migrate east-northeast along the coast-line of the Netherlands and make short stop-overs at the beach. Well over 90% of the birds are adults and trains of birds glide motionless over the beaches of the Wadden Isles into the Danish Wadden Sea.

LBBG, April 16 2001, Nachtegalenkeet Maasvlakte, the Netherlands. An image showing the difference in grey tones between two adult LBBG, breeding in the Netherlands.

At the Maasvlakte, most major position are taken again in the colony by another type of LBBG: Dutch intergrades. The are slightly darker than typical graellsii from Britain, but not as dark as the average intermedius from Scandinavia. They carry the genetic heritage of both taxa. The exchange of gene pools hasn't stopped yet. Still, in the colonies at Maasvlakte we see foreign ringed birds, breeding together with local Dutch birds. Not surprising, these foreign birds often seem to be females.
In the mean time birds born in colonies at Maasvlakte spread to other colonies abroad as well. The very prosperous colony at the Belgium port of Zeebrugge was initiated by both British and Dutch LBBG, as several colour-ringed birds proved.

Primary Moult Scores in adult LBBG:

Year 2003 we did some preliminary research on primary moult scores in adult LBBG. In total, we scored about 1400 birds from early April to mid October. This research will be used to do some more fundamental research in tear 2003-2004. First results for April are quite simple: we scored 49 birds on April 05 and we scored 45 birds on April 19. All these birds had P1 still present, hence PMS = 0. Nevertheless we did see several adult LBBG in the colony, which had P1 shed by April 19. Some notes: the scorse on April 05 2003 were done at Naaktstrand, on preening, bathing and arriving birds, through 20x-60x telescopes. Active moult would be noticed, but to detect arrested moult in such circumstances is pretty difficult. Next year, we try to start our surveys with catching birds and examine the exact primary stage in the inner primaries by taking photographs.

Tables & Graph

Table below: data for the Netherlands (2000-2010), results for April (aggregated week): number of remaining old primaries in adults.
REM10 = 10 primaries still old (hence, complete moult not started yet). First signs of complete moult commencement in WEEK 18 (starting 30 April).

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Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls. Number of remaining old primaries throughout the year (arrested moult excluded). Sample size is 6.171 birds.

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls. Number of remaining old primaries for April (arrested moult excluded). Sample size for this graph is 879 ringed birds.

Above: Scatter Plot for PMS in adult Lesser Black-backed Gulls. Longest new primary is classified "fully grown". Sample size is 2.969 birds. Birds scored on NEW primaries (arrested moult excluded).

The last scoring days were in November, with a larger sample (133 adults) on 04 November 2010. From that date we have the last birds with retaining old primaries (13 birds with 1, 2 or even 3 old primaries; 120 birds with no old primaries left). From other days in November, only 10 birds were scored (all ringed adults). They all had no old primaries left.

The first tine a new primary (P1) was visible, was on 08 May 2006. Prior to this date, birds were scored missing inner primaries, but the very tip was still not visible (and therefore not scored). As soon as the tip of P1 was visible, this was automatically scored "fully grown" For the other primaries to be scored fully grown, they had to exceed the previous primary in length (so P2 fully grown, only from when it is longer than P1).

LINEAR REGRESSION

Onset of primary moult can best be illustrated by a sinusoid for the first few weeks and last weeks, but there appears to be linear dependency over much of the central moult period.

The earliest date for birds with missing primaries is 30 April 2006, which is not the earliest day in our complete sample. We scored 98 Lesser Black-backed Gulls prior to that date, some already in February. As can be expected, none of these was in primary moult, and this group was excluded in our further analysis as it will only result in a misfit for the onset of moult, using linear regression statistics.

LINEAR REGRESSION FOR OLD REMAINING PRIMARIES

Complete sample: Linear regression (n = 6171, R2= 0.820, sign: P<0,000)
PMS = -0,381* WEEK + 17,967

Ringed adults: Linear regression (n = 2069, R2= 0.800, sign: P<0,000)
PMS = -0,325* WEEK + 16,270

Unringed adult: Linear regression (n = 4102, R2= 0.779, sign: P<0,000)
PMS = -0,513* WEEK + 23,031

LINEAR REGRESSION FOR NEW FULLY GROWN PRIMARIES

Complete sample: Linear regression (n = 2969, R2= 0.930, sign: P<0,000)
PMS = 0,302* WEEK - 5,933

Ringed adults: Linear regression (n = 1209, R2= 0.888, sign: P<0,000)
PMS = 0,252* WEEK - 4,747

Unringed adult: Linear regression (n = 1760, R2= 0.833, sign: P<0,000)
PMS = 0,442* WEEK - 11,906

LBBG in the Netherlands: colour ring projects

For over 10 years we have colour ring projects in the Netherlands. By far the largest project involves LBBG breeding in two colonies near the port of Rotterdam: one large colony at Maasvlakte and the second at Dintelhaven (eastern Maasvlakte). Although the other colony is well visible from either side, only separated by water, birds appear to be very faithful and try to nest in the colony where they were raised. Only limited number of ringed birds can be found in the 'wrong' colony.

First LBBG were ringed with white rings, first with letter digit digit, later with letter letter digit. At the colony 'Maasvlakte', orange rings were used. First years (1991-1992) codes were letter digit digit, later letter letter digit. From then on, all ring codes started with E (Europoort Rotterdam), followed by three letters or digits. 
At the colony of 'Dintelhaven' green rings were used, the same repertoire. In the right column of this page you can find some examples of ringed adult LBBG from both locations.

At the port of IJmuiden, near Amsterdam, Fred Cottaar started another LBBG colour ring project on roof top breeding birds. In this project, red and white rings were used with three different codes: a single letter or single digit or bars.
Such codes can also be found on other colours, originating from the isle of Texel, the Netherlands.
Recently, Fred Cottaar caught adult LBBG on the nest and they received red rings, starting with E and followed by three digits.

Another recently started colour-ring project in the Netherlands can be found in the industrial area of Moerdijk (inland breeding LBBG). This project was started by Roland-Jan Buijs and he used mint green rings with two black letters. Because this population breeds in tall grass, Roland-Jan used tibia rings.

Subsequently, an interesting English project was started by David Sowter in 1997.

Purpose:

Despite regular culling and disturbance the Lesser Black Backed Gull colony at Tarnbrook Fell has continued to thrive.  This study is intended to determine the extent to which movement of Lesser Black Backed Gulls between the three sites, which are inter-visible on a good day, may play a part in the breeding success of the Tarnbrook colony.  It is also hoped to establish whether there are common feeding, roosting and migration sites and what part they play in the movement of gulls between colonies for breeding.

Method: 

In the pilot year, 1997, 200 juveniles were ringed at each of the three sites using green darvic with a single white letter code: R, T, and W.  For each of the five years 1998- 2001, 2500 birds will be ringed at each of the three sites with black darvic rings with yellow site letter plus a three letter/number combination  (left leg) so individual birds can be identified in the field by year and site.  Individual BTO metal rings (right leg) will also identify birds.

To develop and analyze data based on personal observation and records achieved through co-operation with local bird clubs and through the BTO network and the large gull coordinator, Peter Rock.

Sites:

Ribble Estuary Nature Reserve: ( 53 42’ N : 2 55’ W - SD 3924) in co-operation with Mike Gee/Dick Lambert, English Nature Wardens and SW Lancs Ringing Group.  Dick and the SW Lancs Ringing Group have carried out some ringing for most years since the mid-1980’s.  The site is within salt marsh near the mouth of the River Ribble and is subject to periodic flooding by high tides, which decimates the breeding success in some years.

Tarnbrook Fell, Forest of Bowland: (54.01N  2.35W - SD 6258) with permission and sponsorship from Grosvenor Estates, Abbeystead and Mr R Banks the Abbeystead Estate manager;  Mr R Challenor for Mallowdale estates and NW Water for Brennand estates.  Dr John Coulson and associates have been undertaking studies and control measures at this site since at least the mid-1980’s.   The colony, which is within heather, grass and peat-hag moorland at a height of 450-530 metres, has continued to maintain high numbers or expand despite culling and managed disturbance over a number of years.

South Walney Nature Reserve: (54.03N 3.12W - SD 2162) in co-operation with Rob Shaw and Nick Littlewood, the warden of the reserve.  There has been ringing of gulls on this dune site at the south end of Walney Island since at least the early 1960’s.

Colour Ring combinations:  
R is Ribble;  
T is Tarnbrook Fell ; 
W is Walney Island.

1997: GREEN showing WHITE single letter for site.
1998 - 2002: BLACK showing YELLOW 4 letter/number combinations.

Please send details of sightings with date, location, details of gulls and activity to:

David Sowter: 5 The Grove Penwortham Preston PR1 0UU. 01772 749220. davidsowter@freenet.co.uk

All responses will be acknowledged.

Especially these kind of project, with many LBBG colour ringed, may give full insight in variation of LBBG plumages from different locations. See also the Distribution Section.

Set of late March adult Lesser Black-backed Gulls in Leiden, the Netherlands. Roof top breeders here. Pictures of several birds March 17-24 2015 at Leiden, the Netherlands. These will probably breed in this area, sometimes called "Dutch intergrades" or "continental graellsii", and birds are a shade darker grey on upperparts than British graellsii, matching many intermedius from Denmark or S Norway.
Lesser Black-backed Gull EV adult, March 24 2015, Leiden, the Netherlands.	Lesser Black-backed Gull N03J adult, March 24 2015, Leiden, the Netherlands.