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A SURVEY OF ROOSTING GULLS ON
THORNE MOORS IN 1998

 

Birding Site Guide

Bryan P. Wainwright

 

 

THORNE &
HATFIELD
MOORS
CONSERVATION
FORUM
 

 

THMCF Technical Report No. 1

Thorne & Hatfield Moors Conservation Forum
Museum & Art Gallery, Chequer Road, Doncaster, DN1 2AE

ISSN 1468-2087
1999

 

CONTENTS

1. Preface 1
Martin Limbert 2. Introduction             3
3. Identification of gulls
3.1 General aspects             4
3.2 Yellow-legged Gull      5
4. Roost sites         6
5. Movements in 1998        7
6. Statistics from the 1998
survey        8
7. Species accounts 7.1 Mediterranean Gull           10
7.2 Black-headed Gull         10
7.3 Common Gull        10
7.4 Lesser Black-backed Gull         11
7.5 Herring Gull        11
7.6 Yellow-legged Gull            12
7.7 Iceland Gull         12
7.8 Glaucous Gull            12
7.9 Great Black-backed Gull         12
7.10 Unrecorded species           13
8. Mortality and predation            14
9. Evidence from 1999            15
10. Acknowledgements         16

APPENDIX
Species Identification of Immature Gulls
Nomenclature of species named in the report

 

 

2. INTRODUCTION During 1998, an attempt was made to carry out a year long survey of roosting gulls on Thorne Moors. No such survey had ever been undertaken on this site previously, although the breeding Black-headed Gulls1 have been monitored since 1966. However, as gulls have become such a prominent feature of the Moors in recent years, especially with the advent of permanent roosts, it was felt that an intensive survey was necessary. In addition to usage by greater numbers, there have been consequent changes in the status of the commoner gulls, and increasing records of scarcer species. There have been four species of gull added to the Thorne Moors list since 1989. Thus 1998 seemed an appropriate time to conduct a survey, and thereby provide a modern baseline for recording, enabling future changes to be more effectively recognized and studied.

The survey was an attempt to assess the increasing numbers of gulls using Thorne Moors over a twelve month period. Although Black-headed Gulls were known to roost previously and to have breeding colonies, there were few other records of gulls landing on, or otherwise utilizing, Thorne Moors until the peat milling fields north of Fisons' Road started to flood extensively. They were abandoned in stages from 1995, and came under the direct control of English Nature. Field drains were blocked, and intervening peat bunds were created. A fire in September 1995 burned through many of the bunds, and charred the surface. Late in that year, flooding was first apparent, in the area to the west of the Shoulder o' Mutton. It then subsequently spread southwards. However, the flooding was seasonal, being especially prevalent in winter and the following spring. Only Black-headed Gulls in the breeding season were attracted to the flooded workings in consistent numbers, though they did not nest there. It was not until 1996 that larger gulls, using the Moors as a flyway between the Humber and inland feeding areas, started to land with any regularity, and this led to a general increase in gull movements in the area. It was at this time that roosts first became established. During January 1997, English Nature pumped water from Shearburn & Pitts Drain on to the former milled areas to the north, and in general water levels remained high in the following spring.

From the summer of 1997, there were stretches of permanent water for the first time. This proved irresistible in attracting the gulls that flew over, en route to other roosts. They would linger overhead and often land for a while to rest, drink and preen, before moving on. Black-headed Gulls were present as usual, and odd Common Gulls could sometimes be found in with them. However, it was the larger gulls that were to make full use of these large open expanses of shallow water. There followed more use of the Moors not just by transient gulls, which began to delay for a while before moving on to a roost elsewhere, but also by gulls increasingly using the flooded workings for daytime loafing, and/or as an overnight roost site.

1Scientific names employed in this report are given in section 10.

3. IDENTIFICATION OF GULLS 3.1 General aspects The starting point of the 1998 study was simply to get on Thorne Moors in the early mornings and late evenings, and count gulls as they flew over, or landed. Difficulties arose in counting or estimating large flocks of mixed gulls when many of them were immatures, but these were largely overcome {q.v.}. It was important to find established flight paths over the Moors. These varied very little (in the early months of the year at least), so a position from which to count the gulls could be obtained. Their flight paths seemed to alter in adverse weather, and also seasonally. Initially in 1998, virtually all gulls crossing the Moors in an evening heading NE, would land to rest, drink and preen, before some would depart and continue NE to other roosts (therefore probably on to the Humber estuary). A counting method, separating the gulls departing from the Moors from those that remained to roost, had to be established. However, in the later months, most gulls stayed to roost on the Moors overnight.

Gulls are not the easiest group of birds to identify,, even when adult. Immature gulls are far harder, and many birders don't try or give up too soon. Certain immature plumages in some species are more readily identifiable than others. Generally, the more mature a gull becomes, the more recognizable it is. Juvenile plumages are arguably the most difficult, but with good views, patience and practice, these can also be safely dealt with. In large mixed flocks (especially at any distance) errors will be made, but these should be negligible and, in total, should not undermine the validity of the exercise.

The key to identification, when confronted for the first time with a group of mixed immature gulls, is to be systematic. Developing a routine list of features to check or eliminate - a diagnostic key - helps. Provided that the key is always started and followed through in the same way, it will always produce an identification. I developed such a key, but it was only a rough tool which allowed species level identification for the three commonest immature large gulls. Lesser Black-backed, Herring and Great Black-backed (the key is in the Appendix below).

Counting gulls is not an exact science, even with the aid of clickers, but it does not need to be. The most important point in counting gulls is consistency: to be able to have a log of directly comparable results. In the long term, it does not matter if results are under-counted by 2% or 10%, or inadvertently slightly over-estimated, so long as the method of counting is consistent, and as accurate as possible at the time. How are comparable results achieved? Counting can be complicated or even prevented, due to poor visibility, distance, or size of flock. It is always better to err on the side of caution and allow yourself to under-count rather than over-estimate.

I developed what I believe is an acceptable method of counting gulls that would otherwise be unidentifiable. This method can only be used for each separate day count. If the great majority of gulls have been conventionally identified, say 75+, and you are left with a mixed bag of less than 25 unidentified gulls, then it is reasonable to assign these undetermined gulls to the identified totals by proportions. So if 75% of identified gulls were Herring, 75% of the unidentified ones can be assigned to this total, and so on with other species. This method can only be safely used where the majority of gulls have already been recognized, and should not be used when, perhaps, 25+ of gulls are unidentified. Common sense should suggest when to be careful with this method.

For example, if there is a single species colony, then obviously this should be taken into account when using the method. This is the case on Thorne Moors with the Black-headed Gull population, but in 1998 the only colony was particularly small. Learning the commoner species intimately can bring large rewards, not only in watching and amassing data on them, but also in being able to spot that particular gull which is a little bit different: the rarity!

3.2 Yellow-legged Gull In the survey period, an attempt was made to identify landed adult Yellow-legged Gulls. The difficulties of identifying immature Yellow-legged Gulls, and the need to count other species as well, meant that this was the most practical approach to this species. Finding a Yellow-legged Gull required checking through gulls that were at the flooded workings, whether at a roost or not, at the appropriate time of year. This is late July onwards, with the peak month being August. At this period in 1998, most large gulls on Thorne Moors were Lesser Black-backed, then Great Black-backed (c.3), and Herring (!+). There were large numbers of Black-headed Gulls also present, including the few breeding birds and their young.

Initially when checking through adult Herring Gulls, looking for Yellow-legged Gulls, the first feature to look for is a darker grey colour on the upperparts i.e. mantle, back and wings. This is best gauged when adults of other species are alongside. Lighting conditions should preferably be bright, with the sky evenly and lightly overcast, and the strongest light from behind the observer, at a medium high angle. It is also preferable that the suspected gull walks slowly around, so any effect by different lighting angles on the shade of the upperparts can be properly discerned and compared. If the gull does indeed appear to have upperparts of an intermediate shade of grey, between adjacent Herring and Lesser Black-backed, then close inspection of other features should confirm its identity. Wings should be checked for extensive black tips, with small white dot 'mirrors'. A flatter, longer head shape may also be noticed. Leg colour should be checked and compared with nearby gulls with pink legs (Great Black-backed and Herring) and with the yellow legs of Lesser Black-backed, which latter they should match. More detailed features, such as orbital ring colour (redder than Herring Gull), and characteristics of the gonys spot (larger and brighter than Herring Gull), are difficult to check in the field, but should be if viewing conditions are ideal. In flight, the best features to look for on a suspected adult Yellow-legged Gull are darker grey upperparts, more black on the wingtips (especially from below), and small white 'mirrors'. During the survey, many more adult Yellow-legged Gulls were suspected than claimed, and immatures were not even looked for. The status of this species (and to a lesser extent other scarce gull species) may therefore have been under-represented in 1998, but a cautious approach to these gulls, given their identification difficulties, is advisable, or even essential.

4. ROOST SITES Roost sites on Thorne Moors varied in 1998, but all roosting areas were north of Fisons' Road. Here, abandoned milling fields, where peat had been widely skimmed from the surface, but not from depth, left a lowered flat expanse of basal peat, 0.5m deep and intersected by field drains. In this, the remains of paludified tree remains are frequent. These areas, once left, were managed by English Nature, and allowed to flood and begin the long process of re-vegetation. The floodwater consisted mostly of rainwater, and was therefore nutrient deficient. The water level varied, depending on the season and weather. When large areas of open water became permanent from 1995, the water depth could be up to 60cms in places. However, being in general rather shallower, the flood area was prone to fairly rapid shrinkage or expansion due to the prevailing conditions.

With the end of peat milling, the flooded areas were initially devoid of virtually all plant life, but were soon colonised by Sheep's Sorrel, Soft-rush and Hare's-tail Cottongrass. Aquatic plants were absent, except for green algal growths, but after a year or so, tiny patches of Sphagnum became apparent. Past concentrations of birds, such as Black-headed Gulls or wildfowl, may have caused nutrient enrichment from their droppings, the long term ecological effects of which have never been fully investigated. Modern gull roosts and the Black-headed Gull colony are effecting the same, and provide an opportunity for research in this matter.

In 1998, as in earlier years, the gulls chose to roost in open areas with large unbroken stretches of water. They roosted on the water, on linear peat bunds or islands in the water, and on adjacent peat balks. Overall there were three distinct areas for roosts, one often being chosen to the exclusion of the other two. However preference of area could also change, weekly or even from one roost to the next. The reasons for these changes can only be speculated on. Suggestions include changes in the weather, such as wind direction and strength, disturbance by man or predators (most likely Red Foxes), or movements of gulls from further afield determined by season or food availability.

The three roost sites, which were to the north of Fisons' Road, were all areas of flooded workings: (i) West of the Shoulder o' Mutton. (ii) Immediately opposite Mill Drain Marsh. (iii) Immediately opposite Green Belt. The first site to flood extensively in 1995 was also the first to be used for roosting by larger gulls (in 1996), namely the area west of the Shoulder o' Mutton. This site was also the one most frequently used in 1998, and apparently previously. The site immediately north of Fisons' Road, opposite Mill Drain Marsh, was certainly utilized from early 1997, with roosts appearing there from at least the spring of that year. The site opposite Green Belt was the last area to be widely used for roosting, with the first roost noted on 24th January 1998. Before that time, the water was not extensive. In 1998, patterns of gull usage of the different roost sites, and differences in the daily species composition of those roosts, were at least partly related to the change of seasons.

5. MOVEMENTS IN 1998

Gulls gathered to roost after coming in mainly from the west, with the larger species arriving first. Most were present by sunset, but some came in later, though not many after dark. A few flew straight over, but most stopped to drink and preen, and then some of these continued on to other roost sites on fields or on the Humber. The remainder stayed on Thorne Moors until dawn. Dispersal from the moorland roosts was invariably to the west, and started before dawn, with often the smaller species leaving first. In winter, when Great Black-backed Gulls were present, they usually left last, and some would linger long into the morning, unless disturbed.

These movements may relate to scavenging at refuse tips, as gulls were observed leaving Thorne Moors and flying in a straight line to the River Don at Thorne, where they followed it upstream to the west. This is a route to Bootham Lane refuse tip at Hatfield, and beyond to the much larger Warmsworth refuse tip closer to Doncaster. These two sites attracted gulls in large numbers, and many would have used the River Don as a flyway, both in the mornings and evenings. This movement was observed, for instance, in the early morning of 13th February 1998, when large groups of gulls were seen arriving at Warmsworth tip coming from the NE. The closure of Warmsworth tip, towards the end of 1998, may have effected a drop in the number of gulls roosting on Thorne Moors, and in consequence, a lessening in the numbers of gulls heading to, and returning from, that direction. Although no systematic counts have been undertaken in 1999, for comparison, there has been sufficient casual observer coverage to suggest this.

After the closure of Warmsworth tip, it was replaced by Scabba Wood refuse tip, just across and along the River Don. This site differed from the Warmsworth tip, in that it was netted over, and so closed to feeding gulls. More recently, those nets collapsed, and were replaced with very high perimeter nets, so the gulls are now able to gain access. Mitigating the scavenging by gulls, following new operating regulations, the refuse must now be covered with soil almost immediately that it is tipped. The denial of feeding at these tips meant that the gulls had to find alternative feeding areas, and this may have altered their flight paths to and from their roosts. Support for this possibility was obtained on Thorne Moors in the last weeks of 1998. Unusually, gulls would sometimes disperse south from their moorland roosts, as for example on both 24th October and 14th November. Equally unusually, gulls sometimes began to gather from directions other than the west, as on 27th December, when many came in from the north.

6. STATISTICS FROM THE 1998 SURVEY On Thorne Moors during 1998, gulls were counted 149 times on 135 dates. I did 105 of these counts on 91 days; 14 were double counts (morning and evening roosts), and some of these counts were consecutive, either on the same date (morning and evening), or else in the evening and following morning. The remaining 44 counts were done by other observers, but were not necessarily all counts of birds involved in roosts, and none of these were consecutive counts.

The gull roosting areas on Thorne Moors cannot be accessed quickly, with the nearest requiring a 2 mile walk each way, and considering I was working full time as a manual labourer, it could require some effort to get up at 04.00 to count the gulls before work!

Herring Gull was the most frequently recorded species, on 109 of the dates, and was also the commonest species overall. Black-headed Gull was recorded on 101 dates, followed by Great Black-backed (93), Lesser Black-backed (88), Common (58), Yellow-legged (six). Mediterranean (three), and Glaucous and Iceland (a single date each).

In 1998, total roost sizes varied, with the smallest numbering less than 100. The lowest was on 22nd March, when there were only 65 Black-headed Gulls, one Common Gull and 20 Herring Gulls. At the opposite extreme, the largest roost was on 24th January, and totalled 2885 when 131 Black-headed Gulls were present, together with 2302 Herring Gulls, 446 Great Black-backed Gulls and six Common Gulls. For all months the theoretical maximum roost size (each species' highest monthly count combined), and average actual roost size, were: January (3464 and 959), February (1943 and 783), March (1009 and 319), April (777 and 194), May (355 and 77), June (1667 and 702) July (2182 and 1000) August (1953 and 513), September (353 and 131), October (930 and 308), November (1476 and 644) and December (980 and 390). These comprised different percentages for each species, according to month.

In general, roost sizes varied most dramatically in adverse weather, especially during strong westerly winds, when the species composition of the roosts changed significantly, even from one count to the next. Some consecutive counts illustrate this point. On 7th February 1998, there were strong SSW winds, and the following count of gulls was obtained as they left the roost on Thorne Moors the next morning: Herring Gull: 1271, with an additional 95 passing over from other roost sites, totalling 1366. Great Black-backed Gull: 82, with an additional 2 passing over from other roost sites, totalling 84. Black-headed Gull: 30. The weather that day (8th) was clear and sunny, with a light to moderate west wind. That evening, the returning gulls revealed a dramatic drop in numbers: Herring Gull: 309, with an additional 121 passing over to other roost sites, totalling 430. Great Black-backed Gull: 21. Black-headed Gull: 14.

Again, on 7th and 8th November, numbers varied in the Moors roosts, with Black-headed and Common Gull numbers fluctuating in an opposite way to the larger gull species. The weather for 7th was cold, with light SE winds becoming moderate, and 8th was calm. On the morning of 7th counts revealed: Herring Gull: 531. Great Black-backed Gull: 75. Lesser Black-backed Gull: 7. Black-headed Gull: 224. Common Gull: 42. Subsequently, the evening count of 7th comprised: Herring Gull: 1060. Great Black-backed Gull: 179. Lesser Black-backed Gull: 3. Black-headed Gull: 13, excluding 100 flying from fields adjacent to the southern part of Thorne Moors, heading towards the River Don, not the Moors. Common Gull: 6. The roost count on the evening of 8th was as follows: Herring Gull: 426. Great Black-backed Gull: 46. Lesser Black-backed Gull: 2. Black-headed Gull: 113. Common Gull: 19.

7. SPECIES ACCOUNTS 7.1 Mediterranean Gull There were three records during 1998, all of adult birds in winter plumage. On the early morning of 16th August, one was on an open area of peat north of Fisons' Road with a large flock of other gulls, mostly Black-headed. It may have roosted with them, and flew off when all were flushed (BPW, WHP). One flew over Shoulder o' Mutton Tram, circling with Black-headed Gulls before heading SW, during the late morning of 6th September (ML, BPW, AB). One flew over Thorne Colliery, in the company of a few Black-headed Gulls, at dusk on 28th November, the gulls continuing north away from the Moors to other roosts (BPW).

The records in 1998 were the fourth to sixth records for Thorne Moors. The first was an adult in summer plumage, flying and calling at the Black-headed Gull colony at Mill Drain Marsh on 4th June 1994. The second and third records were probably of the same first-winter bird, on 10th and 12th April 1997. This gull flew with Black-headed Gulls, at the flooded workings north of Fisons7 Road near Mill Drain Marsh on 10th, and at Mill Drain Marsh on 12th.

7.2 Black-headed Gull Although large numbers often roosted on the flooded workings in 1998, very few bred there, with possibly no more than five pairs. There were no other breeding colonies on Thorne Moors in 1998. The monthly maximum, average (), and average percentage of all gulls counted, were: January: 700 (125) = 15.82. February: 216 (52) = 6.64. March: 160 (66) = 20.69. April: 350 (70) = 36.08. Commonest species this month. May: 90 (27) = 35.06. June: 60 (48) = 6.84. July: 450 (166) = 16.60. August: 840 (227) = 44.25. Year's maximum count. September: 36 (14) = 10.69. Year's minimum count. October: 97 (54) = 17.53. November: 225 (101) = 15.68. December: 74 (41) = 10.51.

7.3 Common Gull Often could be found on peripheral fields and around Thorne Colliery. This species is the least common of the frequent gull species on Thorne Moors. Of the latter, it was the scarcest overall in 1998, with only Lesser Black-backed Gull marginally scarcer in three winter months.

From a total of 58 day counts for which Common Gull was recorded, the monthly maximum, average (), and average percentage of all gulls counted, were: January: 10 (7) = 0.89. February: 6 (3) = 0.38. March: 5 (4) = 1.25. Scarcest species this month. April: 8 (3) = 1.55. Scarcest species this month. May: 1 (1) = 1.30. Year's minimum count; scarcest species this month. June: 2 (1) = 0.14. Scarcest species this month. July: 4 (2) = 0.20. Scarcest species this month. August: 4 (2) = 0.39. Scarcest species this month. September: 3 (2) = 1.53. Scarcest species this month. October: 5 (3) = 1.97. Scarcest species this month. November: 42 (15) = 2.33. Year's maximum count (on 7th). December: 5 (4) = 1.03. Equal scarcest species this month, with Lesser Black-backed Gull.

7.4 Lesser Black-backed Gull The commonest species in summer (May-September). This was the main species of gull amongst the Black-headed Gull colony. The breeding gulls in the colony accepted them until they were sitting on eggs. By this time, the Lesser Black-backs were tolerated when landed at the edge of the colony, but when flying there, were chased relentlessly out of the immediate vicinity.

From a total of 88 day counts for which Lesser Black-backed Gull was recorded, the monthly maximum, average (), and average percentage of all gulls counted, were: January: 6 (3) = 0.38. Scarcest species this month. February: 4 (2) = 0.26. Year's minimum count; scarcest species this month. March: 7(7) = 2.19. April: 100 (20) = 10.31. May: 214 (34) = 44.16. Commonest species this month. June: 1518 (628) = 89.46. Commonest species this month. July: 1627 (768) = 76.80. Maximum Thorne Moors count (on 5th) from any year (BPW); commonest species this month. August: 1000 (263) = 51.27. Commonest species this month. September: 274 (100) = 76.34. Commonest species this month. October: 25 (13) = 4.22. November: 7 (4) = 0.62. Scarcest species this month. December: 5 (5) = 1.28.

7.5 Herring Gull In 1998, this was the most abundant gull overall on Thorne Moors, and at times one of the two most numerous of all species (along with Wood Pigeon). It was the commonest gull in the winter months (January-March and October-December).

From a total of 109 daily counts for which Herring Gull was recorded, the monthly maximum, average (), and average percentage of all gulls counted, were: January: 2302 (543) = 68.73. Maximum Thorne Moors count (on 24th) from any year (BPW, WHP); commonest species this month. February: 1635 (691) = 88.25. Commonest species this month. Highest monthly average count. March: 827 (237) = 74.29. Commonest species this month. April: 200 (59) = 30.41. May: 40 (9) = 11.69. June: 47 (10) = 1.42. July: 5 (2) = 0.20. Year's minimum count. Equal scarcest species this month, with Common Gull. August: 9 (6) = 1.17. September: 15 (7) = 5.34. October: 432 (133) = 43.18. Commonest species this month. November: 1060 (467) = 72.52. Commonest species this month. December: 763 (273) = 70.00. Commonest species this month.

7.6 Yellow-legged Gull Records refer to Larus cachinnans michahellis only, and all records were of adult birds. First suspected at the end of June but not confirmed until 5th July, when one was down on the flooded workings (BPW). It was immediately next to a second-summer Herring Gull (one of few) and many Lesser Black-backed and a few Great Black-backed. It proceeded to walk through the gulls changing direction as it went; conditions were bright but fully overcast. On 19th July, a landed bird was seen briefly on flooded workings before taking flight (BPW, RJS). At least three were on flooded workings on 2nd August (BPW), constituting a new Thorne Moors maximum. The weather was bright but lightly overcast. Two were standing directly next to Herring Gulls, and Lesser Black-backed were all around, with Great Black-backed also present. One or more were watched on 9th August on the flooded workings (BPW, RJS). On 16th August, one or more were at the flooded workings, when 500+ Black-headed, 700 Lesser .Black-backed, four Herring and one Great Black-backed Gull were also there (BPW, WHP). The final definite record was on 6th September, when one was present, again mainly with Lesser Black-backed Gulls, but also other species, at the flooded workings (BPW, ML).

The 1998 sightings of adult Yellow-legged Gulls on Thorne Moors were the first definite ones from the peat. The two previous confirmed records were obtained from adjacent fields, in 1996 (at least one adult on 26th August) and 1997 (one adult, also on 26th August).

7.7 Iceland Gull Present with 800 gulls (700 Herring and 100 Great Black-backed) leaving a moorland roost on 17th January, going SW. The bird was seen in flight and was in first-winter plumage; it was distinctly smaller than the Herring Gulls (WHP, BPW). Only the second record for Thorne Moors, the first being an adult summer-plumaged bird which flew east in the early morning of 12th April 1997.

7.8 Glaucous Gull The only record was of a second-winter bird flying ENE (towards Thorne Moors) low (c.7m) over Thorne Colliery, on 18th April. It was with about 15 Herring Gulls (BPW, RJS). This is the third record for Thorne Moors, following singles overhead on two previous occasions: an adult on 26th November 1989, and an adult in summer plumage on 9th February 1997.

7.9 Great Black-backed Gull From a total of 93 daily counts for which Great Black-backed Gull was recorded, the monthly maximum, average (), and average percentage of all Gulls counted, were: January: 446 (112) = 14.18. Maximum Thorne Moors count (on 24th) from any year (BPW, WHP). February: 82 (35) = 4.47. March: 10 (5) = 1.57. Year's minimum count. April: 119 (42) = 21.65. May: 10 (6) = 7.79. June: 40 (15) = 2.14. July: 100 (62) = 6.20. August: 100 (15) = 2.92. September: 25 (8) = 6.11. October: 371 (105) = 34.09. November: 179 (57) = 8.85. December: 133 (67) = 17.18.

7.10 Unrecorded species It is perhaps surprising that Little Gull was not recorded during the intense fieldwork of 1998, as there are records from seven previous years. Single first-years were located amongst breeding Black-headed Gulls at the former Shoulder o' Mutton colony on 8th June 1969, 6th-7th June 1970, 20th May 1972 and 7th May 1989, the latter a first-winter present with only four Black-headed Gulls. On 27th June 1993, a moulting first- summer/second-winter flew through the Black-headed Gull colony at Mill Drain Marsh. In 1995, three Little Gulls were noted during the period 30th April to 28th May, their plumages being one first-winter, one transitional first-winter/summer, and one first- summer. They were present at the Mill Drain Marsh gull colony, and on the active milling fields to the east of this, occasionally elsewhere. Finally, on 10th April 1997, a second-summer was associating with the Black-headed Gulls at the flooded workings north of Fisons/ Road.

No Kittiwakes were recorded in 1998, though in 1999 there have been three (to the end of July). The first one was an adult in winter plumage, flying NE over the flooded area north of Green Belt on 7th January. The other two, both summer plumage adults, were at the flooded workings north of Fisons' Road on 20th May, one departing soon after being flushed, the other remaining longer, close to the Black-headed Gull colony. The only previous modern record of this species involves an adult that flew NE over Thorne Moors on 27th August 1980.

8. MORTALITY AND PREDATION Black-headed Gull colonies on Thorne Moors have always suffered some predation, of eggs or unfledged young, by opportunistic larger gulls and Carrion Crows. This threat may have grown, with large gull species being ever more prevalent on Thorne Moors, and with Carrion Crows continuing to roost throughout the year close to the flooded workings. Besides predation by Carrion Crows and other species of gull on Black-headed Gulls, the colonies and roosts are further disturbed and predated on by other mammals and birds. These may take eggs or unfledged young, but sometimes also scavenge or kill Black-headed Gulls that aren’t able to fly. In 1998, there were frequent birds of prey such as Peregrine Falcon (absent from mid-April to mid-August) and Marsh Harrier (present throughout the breeding season). Mammalian predators were also often noted. Red Foxes were encountered around the flooded workings north of Fisons' Road on 16 dates, most often in summer, perhaps largely because of the Black-headed Gull colony, despite its small size. The virtual failure of this colony in 1998 may have been partly due to predators such as Red Foxes and Carrion Crows. The former did not exclusively pursue gulls in this area; other prey was also targeted, for example on 18th October, when one chased a Brown Hare west of the flooded area. On other dates, the gulls seemed strangely oblivious to apparent danger, as on 27th June when a Red Fox was hunting on the flooded workings north of Fisons' Road. Surprisingly, most birds, not just the gulls, seemed unconcerned. The Red Fox found an unidentified item and ate it. One other species of mammalian predator was seen around the flooded working, a Weasel, which was hunting along Shoulder o' Mutton Tram before swimming a dyke there, on 11th January.

Dead gulls were sometimes found at the flooded workings, those intact showing that they had died from causes other than predation. However once dead. Carrion Crows sometimes scavenged the carcasses. Examples were a first-winter Herring Gull on 14th February, and an adult Great Black-backed Gull freshly dead on 30th August. Injured gulls included an unaged Lesser Black-backed Gull which had about 6m of video tape around a wing (suggesting it had fed at a tip) as it came in to roost on 27th June, and an adult Herring Gull with a damaged wing on 29th November.

9. EVIDENCE FROM 1999 After completing the 1998 survey, I perceived a drop in the numbers of roosting gulls (with the exception of Lesser Black-backed Gull) in the early months at least of 1999, although there are no figures to confirm or deny this. For the summer of 1999, there have been three counts of Lesser Black-backed Gulls coming in to roost at the flooded workings. These comprised c.500 on 27th May, 1000+ on 14th June, and 1500+ on 10th July. Over 300 gulls from the latter count came from the south after flying over Hatfield Moors (134 Lesser Black-backed Gulls did the same on the previous night, when they were observed from Hatfield Moors). From the total that landed on Thorne Moors on 10th July, perhaps a third eventually continued to the Humber. These figures are similar to 1998, since one of the counts of Lesser Black-backed Gulls is above average the second below, and the last well above average, compared to the previous year.

To monitor figures more closely for real fluctuations, a comparable second survey would be required; if any variations were found, possible explanations would have to be sought. Reasons might relate to change in feeding sites, or new roost areas (such as Lakeside, Doncaster); the weather could be influential, as might habitat change at the roost site. There are still many questions unanswered about gull movements, and a single survey such as this cannot answer them all. It is my hope that future work will be able to make use of the information I have brought together here, and combine it with new information to make for a fuller picture.

 

10. ACKNOWLEDGEMENTS I would like to thank all those people who took the time to count gulls on Thorne Moors, and for sending the records to augment my own: A. Brown, C. Featherstone, S. Hiner, M. Limbert, W.H. Priestley, P.C. Roworth, J.M. Roworth, A. Scutt, R.J. Sprakes, S. Warrillow.

 

APPENDIX Species Identification of Immature Gulls This is not meant to be an exhaustive, totally authoritative account of gull identification and in certain in-between plumages of gulls it may not even work! What it is intended to express are the more readily obvious field marks for identification of immature gulls for the beginner or the fieldworker who needs to identify mixed gulls quickly so the species may be counted. The three larger common gulls are dealt with first. In Flight, (without size comparisons) for Herring Gull - Larus argentatus, Lesser Black-backed Gull - L. fuscus graellsii, Greater Black-backed Gull - L. marinus.

This is meant to be a general guide, for use in the field for gull counts, and it is in no way meant to be an exhaustive or definitive guide. I found it useful for my own project, being no expert, and thought it may benefit others. I hope this proves to be the case since too often immature gulls are neglected, on local patches where gulls may not be the main focus of attention.

1. Does gull show a pale window, on secondaries, from above and/or below? Yes; It is a Herring Gull. No; Probably not a Herring Gull.

2. If not a Herring Gull, does it show either; a scaly wing pattern WITH black greater coverts and/or tertials; or a uniform none contrasting wing (and back) colour (with a gradual darkening to tips)? Yes; It is a Lesser Black-backed Gull. No; Not Lesser Black-backed Gull.

3. Alternatively if the features for No1 and 2 are not seen, does it instead show either; a scaly pattern WITHOUT black greater coverts and/or tertials; or a none uniform wing? Yes; It is a Greater Black-backed Gull No; Not Greater Black-backed Gull.

4. Additional to above, if gull ever shows a darker back than secondaries, it is a Greater Black-backed Gull (2nd summer).

Identification of Immature Gulls on the Ground or on Water, (with size comparison). 5. Leg colour is a good guide, but should only be used with direct comparison of gulls alongside, preferably of known identity. If gulls are juvenile (scaly) or young, then leg colour should not be used as the only feature. Always bare in mind Yellow-legged Gulls - L. cachinnans could be present (immatures have pink legs, adults yellow). Legs are pink in Herring Gulls, usually yellow in Yellow-legged and Lesser Black-backed; and pink in Greater Black-backed.

6. Size and structure are a good guide, but can be difficult to judge if other gulls (of other species) are not present. The smaller of the group is the Lesser Black-backed Gull, the largest reaching the smallest size of Herring. Greater Black-backed are the largest and Herring can be anywhere between these two. When presented with a mixed group (including some or all immatures), first count all the gulls before identification, then check the smallest gull/s in the group first, if legs are visible and they have yellow legs in comparison to the larger gulls present you have Lesser Black-backed; if only one smaller gull is present, also check plumage, thereby establishing if it is a Lesser Black backed. Count them and deduct from total.

If no Lesser Black-backeds appear to be present, or legs cannot be seen, see if there is any obvious differences in size, note the plumage of smaller landed gulls and of flying birds and identify, and if they are coming in to land, compare size with gulls already present, Lesser Black-backed have longer, narrower wings and this can also be seen on landed birds. If no birds are flying, and no Lesser Black-backed appear to be present, things get difficult. You could still have non juvenile, immature Lesser Black-backed Gulls present with fleshy coloured legs. If the smaller immature gulls legs are not visible and it/they have been observed not to have black greater coverts and/or tertials (whether on landed or flying gulls), you have no juvenile or first winter/summer Lesser Black-backed Gulls, or even no Lesser Black-backed at all. If no older immature or adult Lesser Black-backed Gulls have been identified in flight, you may still have these in birds in the grounded group.

For second summer (or older) smaller gulls in the group, without legs visible it will be necessary to rely on structure, the head and bill are the best place to start. Does the gull have a gentle open faced, more rounded and smaller head than the larger gulls? Is the bill smaller and thinner? Does the head have a obvious two-tone dark and light appearance, with the ear coverts and crown being the darker areas? What is the bill colour? Some features are best observed in flight. Is the back colour as adult Lesser Black-backed or Herring? Is the under-wing uniformly dark or length wise two-tone, or is it white with black tips, and mirrors? (older Herring). Are the wings more uniform in width along the full length, or do they obviously broaden on the inner-wing? Does the gulls structure overall remind you of a Common Gull rather than a Greater Black-backed? Is it generally a darker immature gull than others present?

If your answers to these questions favours the first suggestions in each question you probably have a Lesser Black-backed Gull. If the answers to these questions do not favour the first suggestion you have something else. Next then, you need to determine if you have any Herring or Greater Black-backed Gull/s before considering anything rarer.

Because of its very variable sizes, immature Herring Gulls are generally a trickier affair to identify. Even if there is only one immature Greater Black-backed Gull present with other gulls, it should be easier to pick out than a possibly lone Herring Gull; because the Greater Black-backed will invariably be obviously larger than all the rest (unless you have an entire flock of giant Herring Gulls!).

Now you've picked out the biggest gull of the group (remember to look at general structure also, especially when observing over uneven ground; and compare this and size with gulls at the same distance to allow for any possible 'scope distorting effect, especially in warmer weather). Is this gull and possibly others obviously larger than the others, or does the size seem to alter as the gull/s move around? Try looking away from your chosen gull/s and look at others your sure must be smaller. Go back to your original gull/s, does it/they still seem obviously larger? If your answer is yes; you've probably got a Greater Black-backed Gull/s. If your answer is no; then you've either got all Herring or all Greater Black-backed Gulls; (assuming you've followed elimination of Lesser Black-backed as above); now it's down to plumage and structure.

Bare parts aren't much help, except in that early ages the Herrings' have a bigger paler area at the base of the bill. The bill structure is more help; while the Greater Black-backed doesn't have a proportionally longer bill, it is obviously thicker. The forehead is similarly angled but longer, and the neck is much thicker. The brow is more prominent giving a more aggressive, less open faced appearance; wings are shorter and flight looks heavy. The body is bulkier.

Some features for plumage have already been mentioned, but in brief; if flying look for pale inner wing areas on all ages, and windows above and below, for Herring. On Juvenile or younger gulls, Herrings are less scaly or coarsely marked on upper-parts. On older gulls look for more contrast in the wings and especially a lighter coloured back, these are features of Herring Gulls. Is the under-wing one colour with darker tips, and mirrors (as for Herring), or is trailing edge darker (as for Greater Black-backed)? Herring have a more clear cut tail band, Greater Black-backeds' smudge up the tail.

Now you've established the identity of at least one (in a two species flock), or two (in a three species flock), of gulls, you can start comparisons and counting. By deducting the identified number from the total (which you DID count earlier) you have the remaining number as Herring Gulls, (except where there are only Lesser Black-backed and Greater Black-backed, when the remainder, if you followed this guide, would be Greater Black-backed Gulls). Always go through the features of the remaining gulls, just in case there could be the odd rarity tucked away in there!

Black-headed and Common Gulls are the two remaining common species, and are small species. Black-headed Gull is most readily told from its white leading-edge on the upperwing primaries in all plumages. Common Gull is like a small version of Herring Gull, easy to tell when with other species of gulls, but not so easy in a single species flock at longer range. With any species single birds flying over are particularly difficult to judge the size of.

With Common Gull look at its structure compared to what you now know about Herring Gull. At range it should have a small bill that is not too prominent unlike a Herring’s, the bill should have a greenish tinge, not yellow (except for juvenile and 1st-winter). The head should be small and lacking the aggressive look of larger species, and the chest will be small, down to the lower body the size will be fairly similar and even, not showing a broad and prominent chest. Another good point is when you look at the flying bird, the wings of Common Gull should look too slender for a larger species. Common Gull is well marked having distinctive bold black markings on the tail and wings. After 1st-winter the tail is clean white with the black band disappearing in 1st-summer, leaving a pure white tail. Herring Gull has a much less bold and distinctive pattern, the tail above the distal black band being marked with black and the tail-band itself not disappearing until 3rd-summer. The wings are similarly more neatly marked than Herring.

Beware however because Common Gull can look remarkably similar to other less common or rare species, in some plumages. This is not the place to discus these in detail, but the most likely confusion could be with 1st-winter Mediterranean Gull. The main differences are that Mediterranean Gull has a clean centre to the under-wing and is paler grey (in parts) above. The tail has a neat bold thinner black distal band and the rest of the tail is pure white even at this age unlike Common which will have a broad black distal band and dark spots over the rest of the tail. The other much rarer and more troublesome species with regard to Common Gull is Ring-billed Gull.

One species flocks can be challenging, however the information in this guide should still be enough to identify species. What this guide does not do is identify specific ages or plumages of gulls, and for this a specialist guide is required. Also remember gulls can be very variable, and not only do odd or aberrant individuals need to be considered but also other less common races, before any rarer species are considered.

Finally, don't give up! It won't change the course of British birding if you make a mistake! I still make lots, and anyone who says they don't is either lying or a manic sea-watcher. Don't assume other birders already know what gulls they're looking at, often birders find it easier to just say one of the big five than to check the gull properly; check the features for yourself as best you can, and ask or read up later, if your unsure on a point.

And. Be careful you could soon find yourself addicted!

 

 

NOMENCLATURE OF SPECIES NAMED IN THE REPORT

Birds

Marsh Harrier Circus aeruginosus Peregrine Falcon Falco peregrinus Mediterranean Gull Larus melanocephalus Little Gull L. minutus Black-headed Gull L. ridibundus Common Gull L. canus Lesser Black-backed Gull L. fuscus Herring Gull L. argentatus Yellow-legged Gull L. cachinnans Iceland Gull L. glaucoides Glaucous Gull L. hyperboreus Great Black-backed Gull L. marinus Kittiwake Rissa tridactyla Wood Pigeon Columba palumbus Carrion Crow Corvus corone

Mammals Red Fox Vulpes vulpes Weasel Mustela nivalis Brown Hare Lepus capensis

 

Plants Sphagnum spp., Sheep's Sorrel Rumex acetosella Soft-rush Juncus effusus Hare's-tail Cottongrass Eriophorum vaginatum